From 80d1d01891621b311d1f4b0611926f00cca0201c Mon Sep 17 00:00:00 2001
From: Daniel Marbach <danielmarbach@users.noreply.github.com>
Date: Tue, 30 Jun 2026 11:47:48 +0200
Subject: [PATCH 1/7] Try out pooling with the learning pump

---
 ...IApprovals.ApproveNServiceBus.approved.txt |   8 +
 .../Learning/HeaderSerializerTests.cs         | 184 +++++++++++++++-
 .../Utils/DictionaryPoolTests.cs              | 201 ++++++++++++++++++
 .../Transports/Learning/HeaderSerializer.cs   |  87 +++++++-
 .../Learning/LearningTransportMessagePump.cs  | 124 ++++++-----
 src/NServiceBus.Core/Utils/DictionaryPool.cs  | 139 ++++++++++++
 6 files changed, 685 insertions(+), 58 deletions(-)
 create mode 100644 src/NServiceBus.Core.Tests/Utils/DictionaryPoolTests.cs
 create mode 100644 src/NServiceBus.Core/Utils/DictionaryPool.cs

diff --git a/src/NServiceBus.Core.Tests/ApprovalFiles/APIApprovals.ApproveNServiceBus.approved.txt b/src/NServiceBus.Core.Tests/ApprovalFiles/APIApprovals.ApproveNServiceBus.approved.txt
index 6a55b5f799b..e71e5f8ab37 100644
--- a/src/NServiceBus.Core.Tests/ApprovalFiles/APIApprovals.ApproveNServiceBus.approved.txt
+++ b/src/NServiceBus.Core.Tests/ApprovalFiles/APIApprovals.ApproveNServiceBus.approved.txt
@@ -246,6 +246,14 @@ namespace NServiceBus
         public static void SetDiagnosticsPath(this NServiceBus.EndpointConfiguration config, string path) { }
         public static void WriteDiagnosticsToLog(this NServiceBus.EndpointConfiguration config) { }
     }
+    public sealed class DictionaryPool<TKey, TValue>
+        where TKey :  notnull
+    {
+        public DictionaryPool(int maxPoolSize = -1, int maxRetainedCapacityPerItem = 1024) { }
+        public static NServiceBus.DictionaryPool<TKey, TValue> Shared { get; }
+        public System.Collections.Generic.Dictionary<TKey, TValue> Rent(int minimumCapacity = 0) { }
+        public void Return(System.Collections.Generic.Dictionary<TKey, TValue> dictionary, bool clearDictionary = true) { }
+    }
     public class Discard : NServiceBus.RecoverabilityAction
     {
         public Discard(string reason) { }
diff --git a/src/NServiceBus.Core.Tests/Transports/Learning/HeaderSerializerTests.cs b/src/NServiceBus.Core.Tests/Transports/Learning/HeaderSerializerTests.cs
index 1ac75b358c1..403a3712598 100644
--- a/src/NServiceBus.Core.Tests/Transports/Learning/HeaderSerializerTests.cs
+++ b/src/NServiceBus.Core.Tests/Transports/Learning/HeaderSerializerTests.cs
@@ -1,11 +1,19 @@
-namespace NServiceBus.Core.Tests.Transports.Learning;
+#nullable enable
+
+namespace NServiceBus.Core.Tests.Transports.Learning;
 
 using System.Collections.Generic;
+using System.Text;
+using System.Text.Json;
+using System.Text.Json.Serialization.Metadata;
 using NUnit.Framework;
 using Particular.Approvals;
 
 public class HeaderSerializerTests
 {
+    static readonly JsonTypeInfo<Dictionary<string, string>> SourceGenTypeInfo =
+        HeaderSerializer.HeaderSerializationContext.Default.DictionaryStringString;
+
     [Test]
     public void Can_round_trip_headers()
     {
@@ -26,4 +34,178 @@ public void Can_round_trip_headers()
         var deserialize = HeaderSerializer.Deserialize(serialized);
         Assert.That(deserialize, Is.EquivalentTo(input));
     }
+
+    [Test]
+    public void Can_deserialize_from_utf8_bytes()
+    {
+        var input = new Dictionary<string, string>
+        {
+            { "key 1", "value 1" },
+            { "key 2", "value 2" }
+        };
+        var serialized = HeaderSerializer.Serialize(input);
+        var bytes = Encoding.UTF8.GetBytes(serialized);
+
+        var deserialized = HeaderSerializer.Deserialize(bytes);
+        Assert.That(deserialized, Is.EquivalentTo(input));
+    }
+
+    [Test]
+    public void Deserialize_from_bytes_with_pool_reuses_dictionary()
+    {
+        var pool = new DictionaryPool<string, string>(maxPoolSize: 4);
+        var input = new Dictionary<string, string>
+        {
+            { "key 1", "value 1" },
+            { "key 2", "value 2" }
+        };
+        var serialized = HeaderSerializer.Serialize(input);
+        var bytes = Encoding.UTF8.GetBytes(serialized);
+
+        var first = HeaderSerializer.Deserialize(bytes, pool);
+        pool.Return(first);
+
+        var second = HeaderSerializer.Deserialize(bytes, pool);
+        Assert.That(second, Is.SameAs(first));
+        Assert.That(second, Is.EquivalentTo(input));
+    }
+
+    [Test]
+    public void Round_trip_empty_dictionary()
+    {
+        var input = new Dictionary<string, string>();
+        var serialized = HeaderSerializer.Serialize(input);
+        var deserialized = HeaderSerializer.Deserialize(serialized);
+        Assert.That(deserialized.Count, Is.EqualTo(0));
+    }
+
+    [Test]
+    public void Round_trip_special_characters_in_values()
+    {
+        var input = new Dictionary<string, string>
+        {
+            { "quotes", "value with \"quoted\" text" },
+            { "backslash", "C:\\Program Files\\" },
+            { "newline", "line1\nline2\r\nline3" },
+            { "tab", "a\tb" },
+            { "unicode", "\u00e9\u00e8\u00ea" },
+            { "emoji", "\ud83d\ude00" }
+        };
+        var serialized = HeaderSerializer.Serialize(input);
+        var deserialized = HeaderSerializer.Deserialize(serialized);
+        Assert.That(deserialized, Is.EquivalentTo(input));
+    }
+
+    [Test]
+    public void Round_trip_unicode_keys_and_values()
+    {
+        var input = new Dictionary<string, string>
+        {
+            { "\u00fcnicode-key", "value-with-\u00fcmlaut" },
+            { "\u4e2d\u6587", "\u65e5\u672c\u8a9e" },
+            { "key \u00e0", "value \u00e9" }
+        };
+        var serialized = HeaderSerializer.Serialize(input);
+        var deserialized = HeaderSerializer.Deserialize(serialized);
+        Assert.That(deserialized, Is.EquivalentTo(input));
+    }
+
+    [Test]
+    public void Manual_parser_matches_source_generated_deserializer_for_serialized_output()
+    {
+        // The old Deserialize used JsonSerializer.Deserialize with the source-generated
+        // context (AOT-friendly). The new code uses a manual Utf8JsonReader. Verify
+        // they agree on every input that Serialize can produce.
+        Dictionary<string, string>[] testCases =
+        [
+            new() { { "a", "b" } },
+            new()
+            {
+                { "key 1", "value 1" },
+                { "key 2", "value 2" }
+            },
+            new()
+            {
+                { "special", "\"quotes\" and \\backslashes" },
+                { "multiline", "line1\nline2" }
+            }
+        ];
+
+        foreach (var input in testCases)
+        {
+            var serialized = HeaderSerializer.Serialize(input);
+            var bytes = Encoding.UTF8.GetBytes(serialized);
+
+            // Old path: source-generated AOT-friendly byte-span overload
+            var oldResult = JsonSerializer.Deserialize(bytes, SourceGenTypeInfo);
+
+            // New path: manual Utf8JsonReader parser
+            var newResult = HeaderSerializer.Deserialize(serialized);
+
+            Assert.That(newResult, Is.EquivalentTo(oldResult!),
+                $"Mismatch for input with {input.Count} entries. Serialized: {serialized}");
+        }
+    }
+
+    [Test]
+    public void Deserialize_handles_leading_and_trailing_whitespace()
+    {
+        var input = new Dictionary<string, string> { { "key", "value" } };
+        var serialized = HeaderSerializer.Serialize(input);
+        var padded = "  \n  " + serialized + "  \n  ";
+
+        var deserialized = HeaderSerializer.Deserialize(padded);
+        Assert.That(deserialized, Is.EquivalentTo(input));
+    }
+
+    [TestCase("")]
+    [TestCase("not json")]
+    [TestCase("[]")]
+    [TestCase("123")]
+    [TestCase("\"string\"")]
+    [TestCase("{ \"key\": 123 }")]
+    [TestCase("{ \"key\": [1, 2] }")]
+    [TestCase("{ \"key\": { \"nested\": true } }")]
+    [TestCase("{ \"key\": }")]
+    [TestCase("{ \"key\": \"value\"")]
+    public void Deserialize_throws_for_malformed_json(string malformed)
+    {
+        Assert.That(() => HeaderSerializer.Deserialize(malformed), Throws.InstanceOf<JsonException>());
+    }
+
+    [Test]
+    public void Deserialize_with_pool_returns_dict_that_can_be_returned_to_pool()
+    {
+        var pool = new DictionaryPool<string, string>(maxPoolSize: 4);
+        var input = new Dictionary<string, string>
+        {
+            { "key 1", "value 1" },
+            { "key 2", "value 2" }
+        };
+        var serialized = HeaderSerializer.Serialize(input);
+        var bytes = Encoding.UTF8.GetBytes(serialized);
+
+        var dict = HeaderSerializer.Deserialize(bytes, pool);
+        Assert.That(dict.Count, Is.EqualTo(2));
+
+        // Returning and re-renting should give back the same (cleared) dictionary.
+        pool.Return(dict);
+        var reused = pool.Rent();
+        Assert.That(reused, Is.SameAs(dict));
+        Assert.That(reused.Count, Is.EqualTo(0));
+    }
+
+    [Test]
+    public void Deserialize_from_bytes_with_pool_returns_dict_on_parse_failure()
+    {
+        var pool = new DictionaryPool<string, string>(maxPoolSize: 4);
+        var badJson = Encoding.UTF8.GetBytes("not valid json");
+
+        Assert.That(() => HeaderSerializer.Deserialize(badJson, pool), Throws.InstanceOf<JsonException>());
+
+        // The failed-parse dictionary was returned to the pool (cleared).
+        // A subsequent rent should succeed normally.
+        var dict = pool.Rent();
+        Assert.That(dict.Count, Is.EqualTo(0));
+    }
 }
\ No newline at end of file
diff --git a/src/NServiceBus.Core.Tests/Utils/DictionaryPoolTests.cs b/src/NServiceBus.Core.Tests/Utils/DictionaryPoolTests.cs
new file mode 100644
index 00000000000..c8106855005
--- /dev/null
+++ b/src/NServiceBus.Core.Tests/Utils/DictionaryPoolTests.cs
@@ -0,0 +1,201 @@
+#nullable enable
+
+namespace NServiceBus.Core.Tests.Utils;
+
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Threading.Tasks;
+using NUnit.Framework;
+
+public class DictionaryPoolTests
+{
+    [Test]
+    public void Rent_returns_empty_dictionary()
+    {
+        var pool = new DictionaryPool<string, string>(maxPoolSize: 4);
+        var dict = pool.Rent();
+        Assert.That(dict, Is.Not.Null);
+        Assert.That(dict.Count, Is.EqualTo(0));
+    }
+
+    [Test]
+    public void Returned_dictionary_is_reused_on_next_rent()
+    {
+        var pool = new DictionaryPool<string, string>(maxPoolSize: 4);
+        var dict = pool.Rent();
+        dict["a"] = "1";
+        pool.Return(dict);
+
+        var reused = pool.Rent();
+        Assert.That(reused, Is.SameAs(dict));
+        Assert.That(reused.Count, Is.EqualTo(0));
+    }
+
+    [Test]
+    public void Rent_with_minimum_capacity_prevents_resize()
+    {
+        var pool = new DictionaryPool<string, string>(maxPoolSize: 4);
+        var dict = pool.Rent(minimumCapacity: 100);
+        Assert.That(dict.Count, Is.EqualTo(0));
+
+        for (int i = 0; i < 100; i++)
+        {
+            dict[$"key{i}"] = $"value{i}";
+        }
+        Assert.That(dict.Count, Is.EqualTo(100));
+    }
+
+    [Test]
+    public void Return_preserves_capacity_for_reuse_without_resize()
+    {
+        var pool = new DictionaryPool<string, string>(maxPoolSize: 4);
+
+        // Fill a dictionary with a realistic header count so its internal
+        // Entry[]/buckets grow to accommodate ~50 entries.
+        var first = pool.Rent();
+        for (int i = 0; i < 50; i++)
+        {
+            first[$"header-{i}"] = $"value-{i}";
+        }
+        var capacityAfterFill = first.Capacity;
+        Assert.That(capacityAfterFill, Is.GreaterThanOrEqualTo(50),
+            "Sanity: filling 50 entries should grow capacity to at least 50.");
+
+        // Return → Clear() preserves the internal arrays (Capacity).
+        pool.Return(first);
+
+        // Rent again — should get the same instance with its capacity intact.
+        var reused = pool.Rent();
+        Assert.That(reused, Is.SameAs(first));
+        Assert.That(reused.Count, Is.EqualTo(0), "Cleared dictionary must be empty.");
+        Assert.That(reused.Capacity, Is.EqualTo(capacityAfterFill),
+            "Clear() must preserve Capacity so the next rent avoids resize.");
+
+        // Refill the same number of entries — capacity must not change,
+        // proving no internal reallocation occurred.
+        for (int i = 0; i < 50; i++)
+        {
+            reused[$"header-{i}"] = $"value-{i}";
+        }
+        Assert.That(reused.Capacity, Is.EqualTo(capacityAfterFill),
+            "Refilling with the same count must not trigger a resize.");
+    }
+
+    [Test]
+    public void Oversized_return_trims_capacity()
+    {
+        var pool = new DictionaryPool<string, string>(maxPoolSize: 4, maxRetainedCapacityPerItem: 5);
+        var dict = pool.Rent();
+        for (int i = 0; i < 100; i++)
+        {
+            dict[$"key{i}"] = $"value{i}";
+        }
+        pool.Return(dict); // Count=100 > 5 → Clear + TrimExcess
+
+        var reused = pool.Rent();
+        reused["x"] = "1";
+        Assert.That(reused.Count, Is.EqualTo(1));
+    }
+
+    [Test]
+    public void Soft_cap_drops_excess_returns_beyond_capacity()
+    {
+        var pool = new DictionaryPool<string, string>(maxPoolSize: 2);
+
+        // Rent three distinct dictionaries without returning any,
+        // so the pool is empty and each Rent allocates fresh.
+        var d1 = pool.Rent();
+        var d2 = pool.Rent();
+        var d3 = pool.Rent();
+
+        // Return all three. The first two bring the pool to its cap (2);
+        // the third must be dropped, not retained.
+        pool.Return(d1); // count → 1
+        pool.Return(d2); // count → 2 (cap)
+        pool.Return(d3); // exceeds cap → dropped, count stays 2
+
+        // Rent twice — we should get d1 and d2 back (in LIFO order via ConcurrentBag).
+        var r1 = pool.Rent();
+        var r2 = pool.Rent();
+
+        Assert.That(r1, Is.SameAs(d2).Or.SameAs(d1), "First rent should return a pooled dictionary.");
+        Assert.That(r2, Is.SameAs(d2).Or.SameAs(d1), "Second rent should return a pooled dictionary.");
+        Assert.That(r1, Is.Not.SameAs(r2), "The two rents should return different dictionaries.");
+
+        // The third rent must NOT return d3 — it was dropped by the cap.
+        var r3 = pool.Rent();
+        Assert.That(r3, Is.Not.SameAs(d3), "Dropped dictionary must not be returned from the pool.");
+        Assert.That(r3.Count, Is.EqualTo(0), "Fresh rent must be empty.");
+    }
+
+    [Test]
+    public void Return_null_throws_argument_null_exception()
+    {
+        var pool = new DictionaryPool<string, string>(maxPoolSize: 4);
+        Assert.Throws<ArgumentNullException>(() => pool.Return(null!));
+    }
+
+    [Test]
+    public void Return_without_clear_preserves_data_for_caller_that_cleared()
+    {
+        var pool = new DictionaryPool<string, string>(maxPoolSize: 4);
+        var dict = pool.Rent();
+        dict["a"] = "1";
+
+        // Caller is responsible for clearing when passing clearDictionary: false.
+        dict.Clear();
+        pool.Return(dict, clearDictionary: false);
+
+        var reused = pool.Rent();
+        Assert.That(reused, Is.SameAs(dict));
+        Assert.That(reused.Count, Is.EqualTo(0));
+    }
+
+    [Test]
+    public void Concurrent_rent_return_does_not_corrupt()
+    {
+        var pool = new DictionaryPool<string, string>(maxPoolSize: 64);
+        var iterations = 5000;
+        var failures = new System.Collections.Concurrent.ConcurrentQueue<string>();
+
+        Parallel.For(0, iterations, i =>
+        {
+            var dict = pool.Rent();
+
+            // Every rented dictionary must start empty — a stale entry
+            // means Clear was skipped or a cross-thread race leaked data.
+            if (dict.Count != 0)
+            {
+                failures.Enqueue($"Iteration {i}: rented dict had Count={dict.Count} (expected 0)");
+                return;
+            }
+
+            // Write a unique key per iteration so collisions are detectable.
+            dict[$"key-{i}"] = $"value-{i}";
+
+            pool.Return(dict);
+        });
+
+        Assert.That(failures, Is.Empty, $"{failures.Count} rent-return cycles saw stale data: {string.Join("; ", failures.Take(5))}");
+
+        // The pool's approximate count must never exceed the cap.
+        Assert.That(pool.Count, Is.LessThanOrEqualTo(64),
+            $"Pool count {pool.Count} exceeded maxPoolSize of 64.");
+
+        // Drain the pool by renting until it's empty. Every returned dictionary
+        // must be empty (Return must have cleared it).
+        var drained = 0;
+        while (pool.Count > 0)
+        {
+            var dict = pool.Rent();
+            drained++;
+            if (dict.Count != 0)
+            {
+                Assert.Fail($"Drained dictionary #{drained} had Count={dict.Count} (expected 0 after Return).");
+            }
+        }
+
+        Assert.That(drained, Is.LessThanOrEqualTo(64), $"Pool retained {drained} dictionaries, exceeding maxPoolSize of 64.");
+    }
+}
\ No newline at end of file
diff --git a/src/NServiceBus.Core/Transports/Learning/HeaderSerializer.cs b/src/NServiceBus.Core/Transports/Learning/HeaderSerializer.cs
index 5b04c4913ce..f5e4686f5b4 100644
--- a/src/NServiceBus.Core/Transports/Learning/HeaderSerializer.cs
+++ b/src/NServiceBus.Core/Transports/Learning/HeaderSerializer.cs
@@ -1,6 +1,10 @@
+#nullable enable
+
 namespace NServiceBus;
 
+using System;
 using System.Collections.Generic;
+using System.Text;
 using System.Text.Json;
 using System.Text.Json.Serialization;
 
@@ -10,9 +14,88 @@ public static string Serialize(Dictionary<string, string> dictionary) =>
         JsonSerializer.Serialize(dictionary, HeaderSerializationContext.Default.DictionaryStringString);
 
     public static Dictionary<string, string> Deserialize(string value) =>
-        JsonSerializer.Deserialize(value, HeaderSerializationContext.Default.DictionaryStringString);
+        Deserialize(Encoding.UTF8.GetBytes(value), pool: null);
+
+    /// <summary>
+    /// Deserializes header JSON (UTF-8) into a dictionary, renting from
+    /// <paramref name="pool"/> when provided to avoid allocating a fresh dictionary.
+    /// When <paramref name="pool"/> is null a new dictionary is allocated.
+    /// </summary>
+    /// <remarks>
+    /// Uses a manual <see cref="Utf8JsonReader"/> parse rather than
+    /// <c>JsonSerializer.Deserialize&lt;Dictionary&lt;string,string&gt;&gt;</c>, because
+    /// System.Text.Json always allocates a fresh dictionary and cannot populate a
+    /// rented/pooled one, so the pool would cut zero GC pressure otherwise.
+    /// </remarks>
+    public static Dictionary<string, string> Deserialize(ReadOnlySpan<byte> utf8Json, DictionaryPool<string, string>? pool = null)
+    {
+        var dict = pool?.Rent() ?? [];
+        try
+        {
+            Populate(utf8Json, dict);
+        }
+        catch
+        {
+            // Return the rented dictionary (which clears it) so a parse failure
+            // doesn't waste the pooled instance.
+            pool?.Return(dict);
+            throw;
+        }
+
+        return dict;
+    }
+
+    static void Populate(ReadOnlySpan<byte> utf8Json, Dictionary<string, string> dict)
+    {
+        var reader = new Utf8JsonReader(utf8Json);
+
+        if (!reader.Read())
+        {
+            throw new JsonException("Invalid JSON: no data.");
+        }
+
+        if (reader.TokenType != JsonTokenType.StartObject)
+        {
+            throw new JsonException($"Invalid JSON: expected start of object, got {reader.TokenType}.");
+        }
+
+        while (reader.Read())
+        {
+            if (reader.TokenType == JsonTokenType.EndObject)
+            {
+                return;
+            }
+
+            if (reader.TokenType != JsonTokenType.PropertyName)
+            {
+                throw new JsonException($"Invalid JSON: expected property name, got {reader.TokenType}.");
+            }
+
+            var key = reader.GetString()!;
+
+            if (!reader.Read())
+            {
+                throw new JsonException("Invalid JSON: expected value.");
+            }
+
+            if (reader.TokenType == JsonTokenType.String)
+            {
+                dict[key] = reader.GetString()!;
+            }
+            else if (reader.TokenType == JsonTokenType.Null)
+            {
+                dict[key] = null!;
+            }
+            else
+            {
+                throw new JsonException($"Invalid JSON: expected string value, got {reader.TokenType}.");
+            }
+        }
+
+        throw new JsonException("Invalid JSON: unexpected end of input.");
+    }
 
     [JsonSourceGenerationOptions(WriteIndented = true, IndentSize = 2, NewLine = "\r\n")]
     [JsonSerializable(typeof(Dictionary<string, string>))]
-    sealed partial class HeaderSerializationContext : JsonSerializerContext;
+    internal sealed partial class HeaderSerializationContext : JsonSerializerContext;
 }
\ No newline at end of file
diff --git a/src/NServiceBus.Core/Transports/Learning/LearningTransportMessagePump.cs b/src/NServiceBus.Core/Transports/Learning/LearningTransportMessagePump.cs
index a6a8068aa24..7d9b2d36408 100644
--- a/src/NServiceBus.Core/Transports/Learning/LearningTransportMessagePump.cs
+++ b/src/NServiceBus.Core/Transports/Learning/LearningTransportMessagePump.cs
@@ -11,6 +11,8 @@
 using Logging;
 using Transport;
 
+using HeaderPool = DictionaryPool<string, string>;
+
 class LearningTransportMessagePump : IMessageReceiver
 {
     public LearningTransportMessagePump(string id,
@@ -306,70 +308,50 @@ async Task ProcessFileAndComplete(ILearningTransportTransaction transaction, str
 
     async Task ProcessFile(ILearningTransportTransaction transaction, string messageId, CancellationToken messageProcessingCancellationToken)
     {
-        var message = await AsyncFile.ReadText(transaction.FileToProcess, messageProcessingCancellationToken).ConfigureAwait(false);
+        var messageBytes = await AsyncFile.ReadBytes(transaction.FileToProcess, messageProcessingCancellationToken).ConfigureAwait(false);
 
         var bodyPath = Path.Combine(bodyDir, $"{messageId}{BodyFileSuffix}");
-        var headers = HeaderSerializer.Deserialize(message);
+        var headers = HeaderSerializer.Deserialize(messageBytes, HeaderPool.Shared);
+        Dictionary<string, string> errorHeaders = null;
 
         var fileCreatedAt = File.GetCreationTimeUtc(transaction.FileToProcess);
 
-        if (headers.Remove(LearningTransportHeaders.TimeToBeReceived, out var ttbrString))
+        try
         {
-            var ttbr = TimeSpan.Parse(ttbrString);
-
-            var utcNow = DateTime.UtcNow;
-
-            if (fileCreatedAt + ttbr < utcNow)
+            if (headers.Remove(LearningTransportHeaders.TimeToBeReceived, out var ttbrString))
             {
-                await transaction.Commit(messageProcessingCancellationToken).ConfigureAwait(false);
-                log.InfoFormat("Dropping message '{0}' as the specified TimeToBeReceived of '{1}' expired since sending the message at '{2:O}'. Current UTC time is '{3:O}'", messageId, ttbrString, fileCreatedAt, utcNow);
-                return;
-            }
-        }
-
-        var body = await AsyncFile.ReadBytes(bodyPath, messageProcessingCancellationToken).ConfigureAwait(false);
-
-        var transportTransaction = new TransportTransaction();
-
-        if (transactionMode == TransportTransactionMode.SendsAtomicWithReceive)
-        {
-            transportTransaction.Set(transaction);
-        }
+                var ttbr = TimeSpan.Parse(ttbrString);
 
-        var processingContext = new ContextBag();
-        var receiveProperties = new ReceiveProperties(new Dictionary<string, string>
-        {
-            ["LearningTransport.FileCreatedAt"] = fileCreatedAt.ToString("O")
-        });
+                var utcNow = DateTime.UtcNow;
 
-        var messageContext = new MessageContext(messageId, headers, body, receiveProperties, transportTransaction, ReceiveAddress, processingContext);
+                if (fileCreatedAt + ttbr < utcNow)
+                {
+                    await transaction.Commit(messageProcessingCancellationToken).ConfigureAwait(false);
+                    log.InfoFormat("Dropping message '{0}' as the specified TimeToBeReceived of '{1}' expired since sending the message at '{2:O}'. Current UTC time is '{3:O}'", messageId, ttbrString, fileCreatedAt, utcNow);
+                    return;
+                }
+            }
 
-        try
-        {
-            await onMessage(messageContext, messageProcessingCancellationToken).ConfigureAwait(false);
-        }
-        catch (Exception ex) when (ex.IsCausedBy(messageProcessingCancellationToken))
-        {
-            log.Debug("Message processing canceled. Rolling back transaction.", ex);
-            transaction.Rollback();
-            throw;
-        }
-        catch (Exception exception)
-        {
-            transaction.ClearPendingOutgoingOperations();
+            var body = await AsyncFile.ReadBytes(bodyPath, messageProcessingCancellationToken).ConfigureAwait(false);
 
-            var processingFailures = retryCounts.AddOrUpdate(messageId, id => 1, (id, currentCount) => currentCount + 1);
+            var transportTransaction = new TransportTransaction();
 
-            headers = HeaderSerializer.Deserialize(message);
-            headers.Remove(LearningTransportHeaders.TimeToBeReceived);
+            if (transactionMode == TransportTransactionMode.SendsAtomicWithReceive)
+            {
+                transportTransaction.Set(transaction);
+            }
 
-            var errorContext = new ErrorContext(exception, headers, messageId, body, receiveProperties, transportTransaction, processingFailures, ReceiveAddress, processingContext);
+            var processingContext = new ContextBag();
+            var receiveProperties = new ReceiveProperties(new Dictionary<string, string>
+            {
+                ["LearningTransport.FileCreatedAt"] = fileCreatedAt.ToString("O")
+            });
 
-            ErrorHandleResult result;
+            var messageContext = new MessageContext(messageId, headers, body, receiveProperties, transportTransaction, ReceiveAddress, processingContext);
 
             try
             {
-                result = await onError(errorContext, messageProcessingCancellationToken).ConfigureAwait(false);
+                await onMessage(messageContext, messageProcessingCancellationToken).ConfigureAwait(false);
             }
             catch (Exception ex) when (ex.IsCausedBy(messageProcessingCancellationToken))
             {
@@ -377,20 +359,52 @@ async Task ProcessFile(ILearningTransportTransaction transaction, string message
                 transaction.Rollback();
                 throw;
             }
-            catch (Exception ex)
+            catch (Exception exception)
             {
-                criticalErrorAction($"Failed to execute recoverability policy for message with native ID: `{messageContext.NativeMessageId}`", ex, messageProcessingCancellationToken);
-                result = ErrorHandleResult.RetryRequired;
+                transaction.ClearPendingOutgoingOperations();
+
+                var processingFailures = retryCounts.AddOrUpdate(messageId, id => 1, (id, currentCount) => currentCount + 1);
+
+                errorHeaders = HeaderSerializer.Deserialize(messageBytes, HeaderPool.Shared);
+                errorHeaders.Remove(LearningTransportHeaders.TimeToBeReceived);
+
+                var errorContext = new ErrorContext(exception, errorHeaders, messageId, body, receiveProperties, transportTransaction, processingFailures, ReceiveAddress, processingContext);
+
+                ErrorHandleResult result;
+
+                try
+                {
+                    result = await onError(errorContext, messageProcessingCancellationToken).ConfigureAwait(false);
+                }
+                catch (Exception ex) when (ex.IsCausedBy(messageProcessingCancellationToken))
+                {
+                    log.Debug("Message processing canceled. Rolling back transaction.", ex);
+                    transaction.Rollback();
+                    throw;
+                }
+                catch (Exception ex)
+                {
+                    criticalErrorAction($"Failed to execute recoverability policy for message with native ID: `{messageContext.NativeMessageId}`", ex, messageProcessingCancellationToken);
+                    result = ErrorHandleResult.RetryRequired;
+                }
+
+                if (result == ErrorHandleResult.RetryRequired)
+                {
+                    transaction.Rollback();
+                    return;
+                }
             }
 
-            if (result == ErrorHandleResult.RetryRequired)
+            await transaction.Commit(messageProcessingCancellationToken).ConfigureAwait(false);
+        }
+        finally
+        {
+            if (errorHeaders is not null)
             {
-                transaction.Rollback();
-                return;
+                HeaderPool.Shared.Return(errorHeaders);
             }
+            HeaderPool.Shared.Return(headers);
         }
-
-        await transaction.Commit(messageProcessingCancellationToken).ConfigureAwait(false);
     }
 
     CancellationTokenSource messagePumpCancellationTokenSource;
diff --git a/src/NServiceBus.Core/Utils/DictionaryPool.cs b/src/NServiceBus.Core/Utils/DictionaryPool.cs
new file mode 100644
index 00000000000..5dbbd0f064e
--- /dev/null
+++ b/src/NServiceBus.Core/Utils/DictionaryPool.cs
@@ -0,0 +1,139 @@
+#nullable enable
+
+namespace NServiceBus;
+
+using System;
+using System.Collections.Concurrent;
+using System.Collections.Generic;
+using System.Threading;
+
+/// <summary>
+/// A thread-safe pool of reusable <see cref="Dictionary{TKey, TValue}"/> instances,
+/// modeled on <see cref="System.Buffers.ArrayPool{T}"/>'s Rent/Return pattern.
+/// </summary>
+/// <remarks>
+/// <para>
+/// Backed by a <see cref="ConcurrentBag{T}"/>, which gives per-thread work-stealing
+/// queues: returns are pushed onto the returning thread's local queue, and rents
+/// preferentially take from the current thread's queue (zero contention) before
+/// stealing from others. This mirrors the thread-local-affinity design behind
+/// <see cref="System.Buffers.ArrayPool{T}.Shared"/> without us reimplementing it.
+/// </para>
+/// <para>
+/// A soft capacity cap (enforced via an atomic counter) bounds retention: returns
+/// beyond the cap are dropped and left to the GC, so the pool never grows unbounded
+/// under bursty or pathological load. The cap is just an upper bound on retention;
+/// actual retention self-limits to the number of in-flight dictionaries.
+/// </para>
+/// <para>
+/// <see cref="Return"/> calls <see cref="Dictionary{TKey, TValue}.Clear"/> by default,
+/// which preserves the internal bucket/entry arrays (Capacity). For workloads where
+/// successive usages need roughly the same number of entries (e.g. message headers,
+/// which are uniform across a given endpoint), this means the preserved capacity
+/// serves the next rent without any internal resize. <c>TrimExcess</c>
+/// only fires when a returned dictionary's entry count exceeds
+/// <c>maxRetainedCapacityPerItem</c>, a safety valve so one anomalously large usage
+/// doesn't permanently inflate the pool's footprint. This is not the common path.
+/// </para>
+/// </remarks>
+public sealed class DictionaryPool<TKey, TValue> where TKey : notnull
+{
+    /// <summary>A shared, process-wide pool instance, analogous to <c>ArrayPool&lt;T&gt;.Shared</c>.</summary>
+    public static DictionaryPool<TKey, TValue> Shared { get; } = new();
+
+    readonly ConcurrentBag<Dictionary<TKey, TValue>> bag = [];
+    readonly int maxPoolSize;
+    readonly int maxRetainedCapacityPerItem;
+    int count; // approximate size, maintained via Interlocked
+
+    /// <summary>
+    /// Approximate number of dictionaries currently retained in the pool.
+    /// Thread-safe and safe to read at any time, but may lag behind the actual
+    /// contents under concurrent access. Intended for diagnostics and testing.
+    /// </summary>
+    internal int Count => Interlocked.CompareExchange(ref count, 0, 0);
+
+    /// <param name="maxPoolSize">
+    /// Soft cap on the number of dictionaries retained. Returns beyond this limit
+    /// are dropped rather than growing the pool unbounded. Defaults to a generous
+    /// multiple of processor count so it's never the bottleneck under normal load.
+    /// </param>
+    /// <param name="maxRetainedCapacityPerItem">
+    /// If a returned dictionary's entry count exceeds this, it is trimmed
+    /// (<c>Clear</c> + <c>TrimExcess</c>) before being pooled, so one unusually large
+    /// usage doesn't permanently inflate the pool's memory footprint. Defaults to
+    /// 1024, well above typical header counts, so normal usage always takes the
+    /// no-realloc path.
+    /// </param>
+    public DictionaryPool(int maxPoolSize = -1, int maxRetainedCapacityPerItem = 1024)
+    {
+        this.maxPoolSize = maxPoolSize > 0 ? maxPoolSize : Math.Max(Environment.ProcessorCount * 4, 64);
+        this.maxRetainedCapacityPerItem = maxRetainedCapacityPerItem;
+    }
+
+    /// <summary>
+    /// Rents a dictionary from the pool, or allocates a new one if the pool is
+    /// currently empty. The returned dictionary is always empty.
+    /// </summary>
+    /// <param name="minimumCapacity">
+    /// Optional capacity hint, mirroring <c>ArrayPool&lt;T&gt;.Rent(minimumLength)</c>.
+    /// When provided, the returned dictionary is pre-sized so you can fill it
+    /// without triggering internal resizes.
+    /// </param>
+    public Dictionary<TKey, TValue> Rent(int minimumCapacity = 0)
+    {
+        Dictionary<TKey, TValue> item;
+        if (bag.TryTake(out var taken))
+        {
+            Interlocked.Decrement(ref count);
+            item = taken;
+        }
+        else
+        {
+            item = [];
+        }
+
+        if (minimumCapacity > 0)
+        {
+            item.EnsureCapacity(minimumCapacity);
+        }
+
+        return item;
+    }
+
+    /// <summary>
+    /// Returns a previously-rented dictionary to the pool for reuse.
+    /// </summary>
+    /// <param name="dictionary">The dictionary previously obtained from <see cref="Rent"/>.</param>
+    /// <param name="clearDictionary">
+    /// Whether to clear the dictionary's contents before pooling it. Defaults to true.
+    /// Only pass false if you've already cleared it yourself; otherwise the next
+    /// <see cref="Rent"/> caller sees stale data.
+    /// </param>
+    public void Return(Dictionary<TKey, TValue> dictionary, bool clearDictionary = true)
+    {
+        ArgumentNullException.ThrowIfNull(dictionary);
+
+        bool tooLarge = dictionary.Count > maxRetainedCapacityPerItem;
+
+        if (clearDictionary || tooLarge)
+        {
+            dictionary.Clear();
+        }
+
+        if (tooLarge)
+        {
+            // Release the oversized backing arrays so one outlier usage
+            // doesn't permanently inflate the pool's memory footprint.
+            dictionary.TrimExcess();
+        }
+
+        if (Interlocked.Increment(ref count) > maxPoolSize)
+        {
+            Interlocked.Decrement(ref count);
+            return; // pool is full: drop it and let the GC reclaim it
+        }
+
+        bag.Add(dictionary);
+    }
+}
\ No newline at end of file

From ef252a4a1d5fc9cba8cf35653478c283ba011075 Mon Sep 17 00:00:00 2001
From: Daniel Marbach <danielmarbach@users.noreply.github.com>
Date: Wed, 1 Jul 2026 17:17:27 +0200
Subject: [PATCH 2/7] Pool outgoing message headers

---
 .../Outgoing/ImmediateDispatchTerminator.cs   | 23 +++++-----
 .../Outgoing/RoutingContextExtensions.cs      | 14 +++++-
 .../Outgoing/RoutingToDispatchConnector.cs    |  8 ++++
 .../Unicast/MessageOperations.cs              | 44 ++++++++-----------
 4 files changed, 52 insertions(+), 37 deletions(-)

diff --git a/src/NServiceBus.Core/Pipeline/Outgoing/ImmediateDispatchTerminator.cs b/src/NServiceBus.Core/Pipeline/Outgoing/ImmediateDispatchTerminator.cs
index c6ac3a760bd..f634a402cf6 100644
--- a/src/NServiceBus.Core/Pipeline/Outgoing/ImmediateDispatchTerminator.cs
+++ b/src/NServiceBus.Core/Pipeline/Outgoing/ImmediateDispatchTerminator.cs
@@ -7,19 +7,22 @@ namespace NServiceBus;
 using Pipeline;
 using Transport;
 
-class ImmediateDispatchTerminator : PipelineTerminator<IDispatchContext>
+class ImmediateDispatchTerminator(IMessageDispatcher dispatcher) : PipelineTerminator<IDispatchContext>
 {
-    public ImmediateDispatchTerminator(IMessageDispatcher dispatcher)
-    {
-        this.dispatcher = dispatcher;
-    }
-
-    protected override Task Terminate(IDispatchContext context)
+    protected override async Task Terminate(IDispatchContext context)
     {
         var transaction = context.Extensions.GetOrCreate<TransportTransaction>();
         var operations = context.Operations as TransportOperation[] ?? context.Operations.ToArray();
-        return dispatcher.Dispatch(new TransportOperations(operations), transaction, context.CancellationToken);
+        try
+        {
+            await dispatcher.Dispatch(new TransportOperations(operations), transaction, context.CancellationToken).ConfigureAwait(false);
+        }
+        finally
+        {
+            foreach (var operation in operations)
+            {
+                DictionaryPool<string, string>.Shared.Return(operation.Message.Headers);
+            }
+        }
     }
-
-    readonly IMessageDispatcher dispatcher;
 }
\ No newline at end of file
diff --git a/src/NServiceBus.Core/Pipeline/Outgoing/RoutingContextExtensions.cs b/src/NServiceBus.Core/Pipeline/Outgoing/RoutingContextExtensions.cs
index 21e5d67d522..1be10b44489 100644
--- a/src/NServiceBus.Core/Pipeline/Outgoing/RoutingContextExtensions.cs
+++ b/src/NServiceBus.Core/Pipeline/Outgoing/RoutingContextExtensions.cs
@@ -11,7 +11,19 @@ static class RoutingContextExtensions
 {
     public static TransportOperation ToTransportOperation(this IRoutingContext context, RoutingStrategy strategy, DispatchConsistency dispatchConsistency, bool copySharedMutableMessageState)
     {
-        var headers = copySharedMutableMessageState ? new Dictionary<string, string>(context.Message.Headers) : context.Message.Headers;
+        Dictionary<string, string> headers;
+        if (copySharedMutableMessageState)
+        {
+            headers = DictionaryPool<string, string>.Shared.Rent(context.Message.Headers.Count);
+            foreach (var kvp in context.Message.Headers)
+            {
+                headers[kvp.Key] = kvp.Value;
+            }
+        }
+        else
+        {
+            headers = context.Message.Headers;
+        }
         var dispatchProperties = context.Extensions.TryGet<DispatchProperties>(out var properties)
             ? copySharedMutableMessageState ? new DispatchProperties(properties) : properties
             : [];
diff --git a/src/NServiceBus.Core/Pipeline/Outgoing/RoutingToDispatchConnector.cs b/src/NServiceBus.Core/Pipeline/Outgoing/RoutingToDispatchConnector.cs
index bc4be17db8b..c809729ea90 100644
--- a/src/NServiceBus.Core/Pipeline/Outgoing/RoutingToDispatchConnector.cs
+++ b/src/NServiceBus.Core/Pipeline/Outgoing/RoutingToDispatchConnector.cs
@@ -62,6 +62,14 @@ public override Task Invoke(IRoutingContext context, Func<IDispatchContext, Task
             ActivityDecorator.PromoteHeadersToTags(activity, outgoingMessage.Headers);
         }
 
+        // When copies were made for multicast, the original headers dictionary is no longer
+        // needed by any transport operation. Return it to the pool. When copies were NOT made
+        // (unicast), the original is in the transport operations and will be returned after dispatch.
+        if (copySharedMutableMessageState)
+        {
+            DictionaryPool<string, string>.Shared.Return(outgoingMessage.Headers);
+        }
+
         if (dispatchConsistency == DispatchConsistency.Default && context.Extensions.TryGet<PendingTransportOperations>(out var pendingOperations))
         {
             pendingOperations.AddRange(operations);
diff --git a/src/NServiceBus.Core/Unicast/MessageOperations.cs b/src/NServiceBus.Core/Unicast/MessageOperations.cs
index 99e770a0883..7ec0ef36914 100644
--- a/src/NServiceBus.Core/Unicast/MessageOperations.cs
+++ b/src/NServiceBus.Core/Unicast/MessageOperations.cs
@@ -8,6 +8,7 @@ namespace NServiceBus;
 using MessageInterfaces;
 using Pipeline;
 using Transport;
+using HeaderPool = DictionaryPool<string, string>;
 
 class MessageOperations
 {
@@ -52,10 +53,7 @@ public Task Publish(IBehaviorContext context, object message, PublishOptions opt
     async Task Publish(IBehaviorContext context, Type messageType, object message, PublishOptions options)
     {
         var messageId = options.UserDefinedMessageId ?? CombGuid.Generate().ToString();
-        var headers = new Dictionary<string, string>(options.OutgoingHeaders)
-        {
-            [Headers.MessageId] = messageId
-        };
+        var headers = RentOutgoingHeaders(options.OutgoingHeaders, messageId);
 
         var publishContext = new OutgoingPublishContext(
             new OutgoingLogicalMessage(messageType, message),
@@ -71,10 +69,7 @@ async Task Publish(IBehaviorContext context, Type messageType, object message, P
         await publishPipeline.Invoke(publishContext, activity).ConfigureAwait(false);
     }
 
-    public Task Subscribe(IBehaviorContext context, Type eventType, SubscribeOptions options)
-    {
-        return Subscribe(context, new Type[] { eventType }, options);
-    }
+    public Task Subscribe(IBehaviorContext context, Type eventType, SubscribeOptions options) => Subscribe(context, [eventType], options);
 
     public async Task Subscribe(IBehaviorContext context, Type[] eventTypes, SubscribeOptions options)
     {
@@ -104,10 +99,7 @@ public async Task Unsubscribe(IBehaviorContext context, Type eventType, Unsubscr
         await unsubscribePipeline.Invoke(unsubscribeContext, activity).ConfigureAwait(false);
     }
 
-    public Task Send<[DynamicallyAccessedMembers(IMessageCreator.CreatorMembersRequired)] T>(IBehaviorContext context, Action<T> messageConstructor, SendOptions options)
-    {
-        return SendMessage(context, typeof(T), messageMapper.CreateInstance(messageConstructor), options);
-    }
+    public Task Send<[DynamicallyAccessedMembers(IMessageCreator.CreatorMembersRequired)] T>(IBehaviorContext context, Action<T> messageConstructor, SendOptions options) => SendMessage(context, typeof(T), messageMapper.CreateInstance(messageConstructor), options);
 
     public Task Send(IBehaviorContext context, object message, SendOptions options)
     {
@@ -119,10 +111,7 @@ public Task Send(IBehaviorContext context, object message, SendOptions options)
     async Task SendMessage(IBehaviorContext context, Type messageType, object message, SendOptions options)
     {
         var messageId = options.UserDefinedMessageId ?? CombGuid.Generate().ToString();
-        var headers = new Dictionary<string, string>(options.OutgoingHeaders)
-        {
-            [Headers.MessageId] = messageId
-        };
+        var headers = RentOutgoingHeaders(options.OutgoingHeaders, messageId);
 
         var outgoingContext = new OutgoingSendContext(
             new OutgoingLogicalMessage(messageType, message),
@@ -145,18 +134,12 @@ public Task Reply(IBehaviorContext context, object message, ReplyOptions options
         return ReplyMessage(context, messageType, message, options);
     }
 
-    public Task Reply<[DynamicallyAccessedMembers(IMessageCreator.CreatorMembersRequired)] T>(IBehaviorContext context, Action<T> messageConstructor, ReplyOptions options)
-    {
-        return ReplyMessage(context, typeof(T), messageMapper.CreateInstance(messageConstructor), options);
-    }
+    public Task Reply<[DynamicallyAccessedMembers(IMessageCreator.CreatorMembersRequired)] T>(IBehaviorContext context, Action<T> messageConstructor, ReplyOptions options) => ReplyMessage(context, typeof(T), messageMapper.CreateInstance(messageConstructor), options);
 
     async Task ReplyMessage(IBehaviorContext context, Type messageType, object message, ReplyOptions options)
     {
         var messageId = options.UserDefinedMessageId ?? CombGuid.Generate().ToString();
-        var headers = new Dictionary<string, string>(options.OutgoingHeaders)
-        {
-            [Headers.MessageId] = messageId
-        };
+        var headers = RentOutgoingHeaders(options.OutgoingHeaders, messageId);
 
         var outgoingContext = new OutgoingReplyContext(
             new OutgoingLogicalMessage(messageType, message),
@@ -172,9 +155,18 @@ async Task ReplyMessage(IBehaviorContext context, Type messageType, object messa
         await replyPipeline.Invoke(outgoingContext, activity).ConfigureAwait(false);
     }
 
-    static void MergeDispatchProperties(ContextBag context, DispatchProperties dispatchProperties)
-    {
+    static void MergeDispatchProperties(ContextBag context, DispatchProperties dispatchProperties) =>
         // we can't add the constraints directly to the SendOptions ContextBag as the options can be reused
         context.Set(new DispatchProperties(dispatchProperties));
+
+    static Dictionary<string, string> RentOutgoingHeaders(Dictionary<string, string> outgoingHeaders, string messageId)
+    {
+        var headers = HeaderPool.Shared.Rent(outgoingHeaders.Count);
+        foreach (var kvp in outgoingHeaders)
+        {
+            headers[kvp.Key] = kvp.Value;
+        }
+        headers[Headers.MessageId] = messageId;
+        return headers;
     }
 }
\ No newline at end of file

From e801d86c47b2de87b967410dcfe069f202662f1e Mon Sep 17 00:00:00 2001
From: Daniel Marbach <danielmarbach@users.noreply.github.com>
Date: Wed, 1 Jul 2026 17:46:07 +0200
Subject: [PATCH 3/7] Introduce HeaderPool as a small abstraction

---
 ...IApprovals.ApproveNServiceBus.approved.txt | 26 +++++++++-----
 .../Learning/HeaderSerializerTests.cs         | 16 ++++-----
 .../Utils/DictionaryPoolTests.cs              |  1 +
 .../Audit/InvokeAuditPipelineBehavior.cs      |  6 ++--
 .../Outgoing/ImmediateDispatchTerminator.cs   |  2 +-
 .../Outgoing/RoutingContextExtensions.cs      |  8 ++---
 .../Outgoing/RoutingToDispatchConnector.cs    |  2 +-
 src/NServiceBus.Core/Transports/HeaderPool.cs | 34 +++++++++++++++++++
 .../Transports/Learning/HeaderSerializer.cs   |  2 ++
 .../Learning/LearningTransportMessagePump.cs  |  1 -
 .../Unicast/MessageOperations.cs              |  7 ++--
 src/NServiceBus.Core/Utils/DictionaryPool.cs  |  4 +--
 .../Utils/ReadOnlyDictionaryExtensions.cs     | 23 +++++++++++++
 13 files changed, 98 insertions(+), 34 deletions(-)
 create mode 100644 src/NServiceBus.Core/Transports/HeaderPool.cs
 create mode 100644 src/NServiceBus.Core/Utils/ReadOnlyDictionaryExtensions.cs

diff --git a/src/NServiceBus.Core.Tests/ApprovalFiles/APIApprovals.ApproveNServiceBus.approved.txt b/src/NServiceBus.Core.Tests/ApprovalFiles/APIApprovals.ApproveNServiceBus.approved.txt
index e71e5f8ab37..e5e367c3025 100644
--- a/src/NServiceBus.Core.Tests/ApprovalFiles/APIApprovals.ApproveNServiceBus.approved.txt
+++ b/src/NServiceBus.Core.Tests/ApprovalFiles/APIApprovals.ApproveNServiceBus.approved.txt
@@ -246,14 +246,6 @@ namespace NServiceBus
         public static void SetDiagnosticsPath(this NServiceBus.EndpointConfiguration config, string path) { }
         public static void WriteDiagnosticsToLog(this NServiceBus.EndpointConfiguration config) { }
     }
-    public sealed class DictionaryPool<TKey, TValue>
-        where TKey :  notnull
-    {
-        public DictionaryPool(int maxPoolSize = -1, int maxRetainedCapacityPerItem = 1024) { }
-        public static NServiceBus.DictionaryPool<TKey, TValue> Shared { get; }
-        public System.Collections.Generic.Dictionary<TKey, TValue> Rent(int minimumCapacity = 0) { }
-        public void Return(System.Collections.Generic.Dictionary<TKey, TValue> dictionary, bool clearDictionary = true) { }
-    }
     public class Discard : NServiceBus.RecoverabilityAction
     {
         public Discard(string reason) { }
@@ -2492,6 +2484,11 @@ namespace NServiceBus.Transport
         Handled = 0,
         RetryRequired = 1,
     }
+    public class HeaderPool : NServiceBus.Utils.DictionaryPool<string, string>
+    {
+        public HeaderPool(int maxPoolSize = -1, int maxRetainedCapacityPerItem = 64) { }
+        public new static NServiceBus.Transport.HeaderPool Shared { get; }
+    }
     public class HostSettings
     {
         public HostSettings(string name, string hostDisplayName, NServiceBus.StartupDiagnosticEntries startupDiagnostic, System.Action<string, System.Exception, System.Threading.CancellationToken> criticalErrorAction, bool setupInfrastructure, NServiceBus.Settings.IReadOnlySettings? coreSettings = null) { }
@@ -2796,4 +2793,17 @@ namespace NServiceBus.Utils
         public static System.Guid Create([System.Runtime.CompilerServices.ParamCollection] [System.Runtime.CompilerServices.ScopedRef] System.ReadOnlySpan<string> values) { }
         public static System.Guid Create(string value) { }
     }
+    public class DictionaryPool<TKey, TValue>
+        where TKey :  notnull
+    {
+        public DictionaryPool(int maxPoolSize = -1, int maxRetainedCapacityPerItem = 1024) { }
+        public static NServiceBus.Utils.DictionaryPool<TKey, TValue> Shared { get; }
+        public System.Collections.Generic.Dictionary<TKey, TValue> Rent(int minimumCapacity = 0) { }
+        public void Return(System.Collections.Generic.Dictionary<TKey, TValue> dictionary, bool clearDictionary = true) { }
+    }
+    public static class ReadOnlyDictionaryExtensions
+    {
+        public static void CopyTo<TKey, TValue>(this System.Collections.Generic.IReadOnlyDictionary<TKey, TValue> source, System.Collections.Generic.Dictionary<TKey, TValue> destination)
+            where TKey :  notnull { }
+    }
 }
\ No newline at end of file
diff --git a/src/NServiceBus.Core.Tests/Transports/Learning/HeaderSerializerTests.cs b/src/NServiceBus.Core.Tests/Transports/Learning/HeaderSerializerTests.cs
index 403a3712598..d3c68b5979a 100644
--- a/src/NServiceBus.Core.Tests/Transports/Learning/HeaderSerializerTests.cs
+++ b/src/NServiceBus.Core.Tests/Transports/Learning/HeaderSerializerTests.cs
@@ -8,6 +8,7 @@ namespace NServiceBus.Core.Tests.Transports.Learning;
 using System.Text.Json.Serialization.Metadata;
 using NUnit.Framework;
 using Particular.Approvals;
+using NServiceBus.Utils;
 
 public class HeaderSerializerTests
 {
@@ -76,7 +77,7 @@ public void Round_trip_empty_dictionary()
         var input = new Dictionary<string, string>();
         var serialized = HeaderSerializer.Serialize(input);
         var deserialized = HeaderSerializer.Deserialize(serialized);
-        Assert.That(deserialized.Count, Is.EqualTo(0));
+        Assert.That(deserialized, Is.Empty);
     }
 
     [Test]
@@ -168,10 +169,7 @@ public void Deserialize_handles_leading_and_trailing_whitespace()
     [TestCase("{ \"key\": { \"nested\": true } }")]
     [TestCase("{ \"key\": }")]
     [TestCase("{ \"key\": \"value\"")]
-    public void Deserialize_throws_for_malformed_json(string malformed)
-    {
-        Assert.That(() => HeaderSerializer.Deserialize(malformed), Throws.InstanceOf<JsonException>());
-    }
+    public void Deserialize_throws_for_malformed_json(string malformed) => Assert.That(() => HeaderSerializer.Deserialize(malformed), Throws.InstanceOf<JsonException>());
 
     [Test]
     public void Deserialize_with_pool_returns_dict_that_can_be_returned_to_pool()
@@ -186,26 +184,26 @@ public void Deserialize_with_pool_returns_dict_that_can_be_returned_to_pool()
         var bytes = Encoding.UTF8.GetBytes(serialized);
 
         var dict = HeaderSerializer.Deserialize(bytes, pool);
-        Assert.That(dict.Count, Is.EqualTo(2));
+        Assert.That(dict, Has.Count.EqualTo(2));
 
         // Returning and re-renting should give back the same (cleared) dictionary.
         pool.Return(dict);
         var reused = pool.Rent();
         Assert.That(reused, Is.SameAs(dict));
-        Assert.That(reused.Count, Is.EqualTo(0));
+        Assert.That(reused, Is.Empty);
     }
 
     [Test]
     public void Deserialize_from_bytes_with_pool_returns_dict_on_parse_failure()
     {
         var pool = new DictionaryPool<string, string>(maxPoolSize: 4);
-        var badJson = Encoding.UTF8.GetBytes("not valid json");
+        var badJson = "not valid json"u8.ToArray();
 
         Assert.That(() => HeaderSerializer.Deserialize(badJson, pool), Throws.InstanceOf<JsonException>());
 
         // The failed-parse dictionary was returned to the pool (cleared).
         // A subsequent rent should succeed normally.
         var dict = pool.Rent();
-        Assert.That(dict.Count, Is.EqualTo(0));
+        Assert.That(dict, Is.Empty);
     }
 }
\ No newline at end of file
diff --git a/src/NServiceBus.Core.Tests/Utils/DictionaryPoolTests.cs b/src/NServiceBus.Core.Tests/Utils/DictionaryPoolTests.cs
index c8106855005..db579bd32b8 100644
--- a/src/NServiceBus.Core.Tests/Utils/DictionaryPoolTests.cs
+++ b/src/NServiceBus.Core.Tests/Utils/DictionaryPoolTests.cs
@@ -7,6 +7,7 @@ namespace NServiceBus.Core.Tests.Utils;
 using System.Linq;
 using System.Threading.Tasks;
 using NUnit.Framework;
+using NServiceBus.Utils;
 
 public class DictionaryPoolTests
 {
diff --git a/src/NServiceBus.Core/Audit/InvokeAuditPipelineBehavior.cs b/src/NServiceBus.Core/Audit/InvokeAuditPipelineBehavior.cs
index a7ec2fc198b..a5ed0988821 100644
--- a/src/NServiceBus.Core/Audit/InvokeAuditPipelineBehavior.cs
+++ b/src/NServiceBus.Core/Audit/InvokeAuditPipelineBehavior.cs
@@ -3,10 +3,10 @@
 namespace NServiceBus;
 
 using System;
-using System.Collections.Generic;
 using System.Threading.Tasks;
 using Pipeline;
 using Transport;
+using NServiceBus.Utils;
 
 class InvokeAuditPipelineBehavior : IForkConnector<IIncomingPhysicalMessageContext, IIncomingPhysicalMessageContext, IAuditContext>
 {
@@ -22,7 +22,9 @@ public async Task Invoke(IIncomingPhysicalMessageContext context, Func<IIncoming
 
         context.Message.RevertToOriginalBodyIfNeeded();
 
-        var processedMessage = new OutgoingMessage(context.Message.MessageId, new Dictionary<string, string>(context.Message.Headers), context.Message.Body);
+        var auditHeaders = HeaderPool.Shared.Rent(context.Message.Headers.Count);
+        context.Message.Headers.CopyTo(auditHeaders);
+        var processedMessage = new OutgoingMessage(context.Message.MessageId, auditHeaders, context.Message.Body);
 
         var auditContext = this.CreateAuditContext(processedMessage, auditAddress, timeToBeReceived, context);
 
diff --git a/src/NServiceBus.Core/Pipeline/Outgoing/ImmediateDispatchTerminator.cs b/src/NServiceBus.Core/Pipeline/Outgoing/ImmediateDispatchTerminator.cs
index f634a402cf6..fe469715640 100644
--- a/src/NServiceBus.Core/Pipeline/Outgoing/ImmediateDispatchTerminator.cs
+++ b/src/NServiceBus.Core/Pipeline/Outgoing/ImmediateDispatchTerminator.cs
@@ -21,7 +21,7 @@ protected override async Task Terminate(IDispatchContext context)
         {
             foreach (var operation in operations)
             {
-                DictionaryPool<string, string>.Shared.Return(operation.Message.Headers);
+                HeaderPool.Shared.Return(operation.Message.Headers);
             }
         }
     }
diff --git a/src/NServiceBus.Core/Pipeline/Outgoing/RoutingContextExtensions.cs b/src/NServiceBus.Core/Pipeline/Outgoing/RoutingContextExtensions.cs
index 1be10b44489..f88eeb624d6 100644
--- a/src/NServiceBus.Core/Pipeline/Outgoing/RoutingContextExtensions.cs
+++ b/src/NServiceBus.Core/Pipeline/Outgoing/RoutingContextExtensions.cs
@@ -6,6 +6,7 @@ namespace NServiceBus;
 using Pipeline;
 using Routing;
 using Transport;
+using NServiceBus.Utils;
 
 static class RoutingContextExtensions
 {
@@ -14,11 +15,8 @@ public static TransportOperation ToTransportOperation(this IRoutingContext conte
         Dictionary<string, string> headers;
         if (copySharedMutableMessageState)
         {
-            headers = DictionaryPool<string, string>.Shared.Rent(context.Message.Headers.Count);
-            foreach (var kvp in context.Message.Headers)
-            {
-                headers[kvp.Key] = kvp.Value;
-            }
+            headers = HeaderPool.Shared.Rent(context.Message.Headers.Count);
+            context.Message.Headers.CopyTo(headers);
         }
         else
         {
diff --git a/src/NServiceBus.Core/Pipeline/Outgoing/RoutingToDispatchConnector.cs b/src/NServiceBus.Core/Pipeline/Outgoing/RoutingToDispatchConnector.cs
index c809729ea90..048db29bb70 100644
--- a/src/NServiceBus.Core/Pipeline/Outgoing/RoutingToDispatchConnector.cs
+++ b/src/NServiceBus.Core/Pipeline/Outgoing/RoutingToDispatchConnector.cs
@@ -67,7 +67,7 @@ public override Task Invoke(IRoutingContext context, Func<IDispatchContext, Task
         // (unicast), the original is in the transport operations and will be returned after dispatch.
         if (copySharedMutableMessageState)
         {
-            DictionaryPool<string, string>.Shared.Return(outgoingMessage.Headers);
+            HeaderPool.Shared.Return(outgoingMessage.Headers);
         }
 
         if (dispatchConsistency == DispatchConsistency.Default && context.Extensions.TryGet<PendingTransportOperations>(out var pendingOperations))
diff --git a/src/NServiceBus.Core/Transports/HeaderPool.cs b/src/NServiceBus.Core/Transports/HeaderPool.cs
new file mode 100644
index 00000000000..15e47da87c4
--- /dev/null
+++ b/src/NServiceBus.Core/Transports/HeaderPool.cs
@@ -0,0 +1,34 @@
+#nullable enable
+
+namespace NServiceBus.Transport;
+
+using NServiceBus.Utils;
+
+/// <summary>
+/// A pool of <c>Dictionary&lt;string, string&gt;</c> instances specialized for
+/// message headers, with defaults tuned for typical header counts.
+/// Inherits from <see cref="DictionaryPool{TKey, TValue}"/>.
+/// </summary>
+/// <remarks>
+/// Use <see cref="Shared"/> for the process-wide instance. The retained-capacity
+/// threshold defaults to 64, well above typical header counts, so normal usage
+/// always takes the no-realloc path on reuse.
+/// </remarks>
+public class HeaderPool : DictionaryPool<string, string>
+{
+    /// <summary>A shared, process-wide header pool instance.</summary>
+    public static new HeaderPool Shared { get; } = new();
+
+    /// <param name="maxPoolSize">
+    /// Soft cap on the number of dictionaries retained. Defaults to a generous
+    /// multiple of processor count.
+    /// </param>
+    /// <param name="maxRetainedCapacityPerItem">
+    /// If a returned dictionary's entry count exceeds this, it is trimmed before
+    /// being pooled. Defaults to 64, well above typical header counts.
+    /// </param>
+    public HeaderPool(int maxPoolSize = -1, int maxRetainedCapacityPerItem = 64)
+        : base(maxPoolSize, maxRetainedCapacityPerItem)
+    {
+    }
+}
\ No newline at end of file
diff --git a/src/NServiceBus.Core/Transports/Learning/HeaderSerializer.cs b/src/NServiceBus.Core/Transports/Learning/HeaderSerializer.cs
index f5e4686f5b4..30aa56cbdbe 100644
--- a/src/NServiceBus.Core/Transports/Learning/HeaderSerializer.cs
+++ b/src/NServiceBus.Core/Transports/Learning/HeaderSerializer.cs
@@ -7,12 +7,14 @@ namespace NServiceBus;
 using System.Text;
 using System.Text.Json;
 using System.Text.Json.Serialization;
+using NServiceBus.Utils;
 
 static partial class HeaderSerializer
 {
     public static string Serialize(Dictionary<string, string> dictionary) =>
         JsonSerializer.Serialize(dictionary, HeaderSerializationContext.Default.DictionaryStringString);
 
+    // TODO: Remove since this is only used in tests?
     public static Dictionary<string, string> Deserialize(string value) =>
         Deserialize(Encoding.UTF8.GetBytes(value), pool: null);
 
diff --git a/src/NServiceBus.Core/Transports/Learning/LearningTransportMessagePump.cs b/src/NServiceBus.Core/Transports/Learning/LearningTransportMessagePump.cs
index 7d9b2d36408..fb6c514d2fe 100644
--- a/src/NServiceBus.Core/Transports/Learning/LearningTransportMessagePump.cs
+++ b/src/NServiceBus.Core/Transports/Learning/LearningTransportMessagePump.cs
@@ -11,7 +11,6 @@
 using Logging;
 using Transport;
 
-using HeaderPool = DictionaryPool<string, string>;
 
 class LearningTransportMessagePump : IMessageReceiver
 {
diff --git a/src/NServiceBus.Core/Unicast/MessageOperations.cs b/src/NServiceBus.Core/Unicast/MessageOperations.cs
index 7ec0ef36914..65b3f095031 100644
--- a/src/NServiceBus.Core/Unicast/MessageOperations.cs
+++ b/src/NServiceBus.Core/Unicast/MessageOperations.cs
@@ -5,10 +5,10 @@ namespace NServiceBus;
 using System.Diagnostics.CodeAnalysis;
 using System.Threading.Tasks;
 using Extensibility;
+using Utils;
 using MessageInterfaces;
 using Pipeline;
 using Transport;
-using HeaderPool = DictionaryPool<string, string>;
 
 class MessageOperations
 {
@@ -162,10 +162,7 @@ static void MergeDispatchProperties(ContextBag context, DispatchProperties dispa
     static Dictionary<string, string> RentOutgoingHeaders(Dictionary<string, string> outgoingHeaders, string messageId)
     {
         var headers = HeaderPool.Shared.Rent(outgoingHeaders.Count);
-        foreach (var kvp in outgoingHeaders)
-        {
-            headers[kvp.Key] = kvp.Value;
-        }
+        outgoingHeaders.CopyTo(headers);
         headers[Headers.MessageId] = messageId;
         return headers;
     }
diff --git a/src/NServiceBus.Core/Utils/DictionaryPool.cs b/src/NServiceBus.Core/Utils/DictionaryPool.cs
index 5dbbd0f064e..e589bb4d7f4 100644
--- a/src/NServiceBus.Core/Utils/DictionaryPool.cs
+++ b/src/NServiceBus.Core/Utils/DictionaryPool.cs
@@ -1,6 +1,6 @@
 #nullable enable
 
-namespace NServiceBus;
+namespace NServiceBus.Utils;
 
 using System;
 using System.Collections.Concurrent;
@@ -36,7 +36,7 @@ namespace NServiceBus;
 /// doesn't permanently inflate the pool's footprint. This is not the common path.
 /// </para>
 /// </remarks>
-public sealed class DictionaryPool<TKey, TValue> where TKey : notnull
+public class DictionaryPool<TKey, TValue> where TKey : notnull
 {
     /// <summary>A shared, process-wide pool instance, analogous to <c>ArrayPool&lt;T&gt;.Shared</c>.</summary>
     public static DictionaryPool<TKey, TValue> Shared { get; } = new();
diff --git a/src/NServiceBus.Core/Utils/ReadOnlyDictionaryExtensions.cs b/src/NServiceBus.Core/Utils/ReadOnlyDictionaryExtensions.cs
new file mode 100644
index 00000000000..a1e0683b0e8
--- /dev/null
+++ b/src/NServiceBus.Core/Utils/ReadOnlyDictionaryExtensions.cs
@@ -0,0 +1,23 @@
+#nullable enable
+
+namespace NServiceBus.Utils;
+
+using System.Collections.Generic;
+
+/// <summary>
+/// Extension methods for dictionary operations.
+/// </summary>
+public static class ReadOnlyDictionaryExtensions
+{
+    /// <summary>
+    /// Copies all entries from <paramref name="source"/> into <paramref name="destination"/>,
+    /// overwriting any existing keys. The destination dictionary is not cleared first.
+    /// </summary>
+    public static void CopyTo<TKey, TValue>(this IReadOnlyDictionary<TKey, TValue> source, Dictionary<TKey, TValue> destination) where TKey : notnull
+    {
+        foreach (var kvp in source)
+        {
+            destination[kvp.Key] = kvp.Value;
+        }
+    }
+}
\ No newline at end of file

From 92e6ae1ea100d00c391d4f9e729628b1b062d83b Mon Sep 17 00:00:00 2001
From: Daniel Marbach <danielmarbach@users.noreply.github.com>
Date: Wed, 1 Jul 2026 21:29:46 +0200
Subject: [PATCH 4/7] DeepCopy on get in the acceptance test outbox

---
 .../Outbox/AcceptanceTestingOutboxStorage.cs               | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/src/NServiceBus.AcceptanceTesting/AcceptanceTestingPersistence/Outbox/AcceptanceTestingOutboxStorage.cs b/src/NServiceBus.AcceptanceTesting/AcceptanceTestingPersistence/Outbox/AcceptanceTestingOutboxStorage.cs
index 5218574ad88..968dca980d1 100644
--- a/src/NServiceBus.AcceptanceTesting/AcceptanceTestingPersistence/Outbox/AcceptanceTestingOutboxStorage.cs
+++ b/src/NServiceBus.AcceptanceTesting/AcceptanceTestingPersistence/Outbox/AcceptanceTestingOutboxStorage.cs
@@ -17,7 +17,12 @@ public Task<OutboxMessage> Get(string messageId, ContextBag context, Cancellatio
             return NoOutboxMessageTask!;
         }
 
-        return Task.FromResult(new OutboxMessage(messageId, storedMessage.TransportOperations));
+        // Return deep copies so callers (the dispatch pipeline) own their header dictionaries.
+        // The dispatch pipeline now pools and clears outgoing header dictionaries after dispatch, so a real
+        // outbox must hand out freshly-deserialized dictionaries on every Get. Sharing the stored references
+        // would let dispatch clear/pool the stored dictionaries, corrupting storage and leaking aliased
+        // dictionaries back into the process-wide header pool.
+        return Task.FromResult(new OutboxMessage(messageId, storedMessage.TransportOperations.Select(o => o.DeepCopy()).ToArray()));
     }
 
     public Task<IOutboxTransaction> BeginTransaction(ContextBag context, CancellationToken cancellationToken = default) => Task.FromResult<IOutboxTransaction>(new AcceptanceTestingOutboxTransaction());

From ec45580b9c1a82ae0658b87a540e95984acff726 Mon Sep 17 00:00:00 2001
From: Daniel Marbach <danielmarbach@users.noreply.github.com>
Date: Wed, 1 Jul 2026 21:32:46 +0200
Subject: [PATCH 5/7] Align with non durable persistence

---
 .../Outbox/AcceptanceTestingOutboxStorage.cs  | 33 +++++++++++++++----
 1 file changed, 26 insertions(+), 7 deletions(-)

diff --git a/src/NServiceBus.AcceptanceTesting/AcceptanceTestingPersistence/Outbox/AcceptanceTestingOutboxStorage.cs b/src/NServiceBus.AcceptanceTesting/AcceptanceTestingPersistence/Outbox/AcceptanceTestingOutboxStorage.cs
index 968dca980d1..d50b41fd8fb 100644
--- a/src/NServiceBus.AcceptanceTesting/AcceptanceTestingPersistence/Outbox/AcceptanceTestingOutboxStorage.cs
+++ b/src/NServiceBus.AcceptanceTesting/AcceptanceTestingPersistence/Outbox/AcceptanceTestingOutboxStorage.cs
@@ -2,6 +2,7 @@
 
 using System;
 using System.Collections.Concurrent;
+using System.Collections.Generic;
 using System.Linq;
 using System.Threading;
 using System.Threading.Tasks;
@@ -17,12 +18,11 @@ public Task<OutboxMessage> Get(string messageId, ContextBag context, Cancellatio
             return NoOutboxMessageTask!;
         }
 
-        // Return deep copies so callers (the dispatch pipeline) own their header dictionaries.
-        // The dispatch pipeline now pools and clears outgoing header dictionaries after dispatch, so a real
-        // outbox must hand out freshly-deserialized dictionaries on every Get. Sharing the stored references
-        // would let dispatch clear/pool the stored dictionaries, corrupting storage and leaking aliased
-        // dictionaries back into the process-wide header pool.
-        return Task.FromResult(new OutboxMessage(messageId, storedMessage.TransportOperations.Select(o => o.DeepCopy()).ToArray()));
+        // Return copies so callers (the dispatch pipeline) own their header dictionaries. The dispatch
+        // pipeline pools and clears outgoing header dictionaries after dispatch, so the outbox must hand out
+        // fresh dictionaries on every Get. Sharing the stored references would let dispatch clear/pool the
+        // stored dictionaries, corrupting storage and leaking aliased dictionaries into the header pool.
+        return Task.FromResult(new OutboxMessage(messageId, storedMessage.TransportOperations.Select(CopyOperation).ToArray()));
     }
 
     public Task<IOutboxTransaction> BeginTransaction(ContextBag context, CancellationToken cancellationToken = default) => Task.FromResult<IOutboxTransaction>(new AcceptanceTestingOutboxTransaction());
@@ -32,7 +32,7 @@ public Task Store(OutboxMessage message, IOutboxTransaction transaction, Context
         var tx = (AcceptanceTestingOutboxTransaction)transaction;
         tx.Enlist(() =>
         {
-            if (!storage.TryAdd(message.MessageId, new StoredMessage(message.MessageId, message.TransportOperations.Select(o => o.DeepCopy()).ToArray())))
+            if (!storage.TryAdd(message.MessageId, new StoredMessage(message.MessageId, message.TransportOperations.Select(CopyOperation).ToArray())))
             {
                 throw new Exception($"Outbox message with id '{message.MessageId}' is already present in storage.");
             }
@@ -63,6 +63,25 @@ public void RemoveEntriesOlderThan(DateTime dateTime)
         }
     }
 
+    // Copies headers/options/body into fresh instances so stored data is independent of the live pipeline
+    // dictionaries (which are pooled and cleared after dispatch). Mirrors NonDurableOutboxStorage.CopyOperation.
+    static TransportOperation CopyOperation(TransportOperation operation)
+    {
+        var headers = operation.Headers != null
+            ? new Dictionary<string, string>(operation.Headers)
+            : [];
+
+        var options = operation.Options != null
+            ? new NServiceBus.Transport.DispatchProperties(operation.Options)
+            : [];
+
+        var body = operation.Body.IsEmpty
+            ? []
+            : operation.Body.ToArray();
+
+        return new TransportOperation(operation.MessageId, options, body, headers);
+    }
+
     readonly ConcurrentDictionary<string, StoredMessage> storage = new();
     static readonly Task<OutboxMessage?> NoOutboxMessageTask = Task.FromResult(default(OutboxMessage));
 

From 34821523262729c5099761844661e234549c1066 Mon Sep 17 00:00:00 2001
From: Daniel Marbach <danielmarbach@users.noreply.github.com>
Date: Wed, 1 Jul 2026 23:14:38 +0200
Subject: [PATCH 6/7] Use ConcurrentStack instead

---
 src/NServiceBus.Core/Utils/DictionaryPool.cs | 17 +++++++++--------
 1 file changed, 9 insertions(+), 8 deletions(-)

diff --git a/src/NServiceBus.Core/Utils/DictionaryPool.cs b/src/NServiceBus.Core/Utils/DictionaryPool.cs
index e589bb4d7f4..33c74654d09 100644
--- a/src/NServiceBus.Core/Utils/DictionaryPool.cs
+++ b/src/NServiceBus.Core/Utils/DictionaryPool.cs
@@ -13,11 +13,12 @@ namespace NServiceBus.Utils;
 /// </summary>
 /// <remarks>
 /// <para>
-/// Backed by a <see cref="ConcurrentBag{T}"/>, which gives per-thread work-stealing
-/// queues: returns are pushed onto the returning thread's local queue, and rents
-/// preferentially take from the current thread's queue (zero contention) before
-/// stealing from others. This mirrors the thread-local-affinity design behind
-/// <see cref="System.Buffers.ArrayPool{T}.Shared"/> without us reimplementing it.
+/// Backed by a <see cref="ConcurrentStack{T}"/>, a lock-free (CAS) linked stack:
+/// <see cref="Rent"/> (<c>TryPop</c>) and <see cref="Return"/> (<c>Push</c>) are wait-free
+/// and never take a monitor lock. Header dictionaries are rented and returned across
+/// <c>await</c> boundaries (rent on the pump/handler thread, return on a dispatch
+/// continuation thread), so there is no thread-local locality to exploit; a single
+/// lock-free structure minimizes the unavoidable cross-thread synchronization cost.
 /// </para>
 /// <para>
 /// A soft capacity cap (enforced via an atomic counter) bounds retention: returns
@@ -41,7 +42,7 @@ public class DictionaryPool<TKey, TValue> where TKey : notnull
     /// <summary>A shared, process-wide pool instance, analogous to <c>ArrayPool&lt;T&gt;.Shared</c>.</summary>
     public static DictionaryPool<TKey, TValue> Shared { get; } = new();
 
-    readonly ConcurrentBag<Dictionary<TKey, TValue>> bag = [];
+    readonly ConcurrentStack<Dictionary<TKey, TValue>> stack = [];
     readonly int maxPoolSize;
     readonly int maxRetainedCapacityPerItem;
     int count; // approximate size, maintained via Interlocked
@@ -83,7 +84,7 @@ public DictionaryPool(int maxPoolSize = -1, int maxRetainedCapacityPerItem = 102
     public Dictionary<TKey, TValue> Rent(int minimumCapacity = 0)
     {
         Dictionary<TKey, TValue> item;
-        if (bag.TryTake(out var taken))
+        if (stack.TryPop(out var taken))
         {
             Interlocked.Decrement(ref count);
             item = taken;
@@ -134,6 +135,6 @@ public void Return(Dictionary<TKey, TValue> dictionary, bool clearDictionary = t
             return; // pool is full: drop it and let the GC reclaim it
         }
 
-        bag.Add(dictionary);
+        stack.Push(dictionary);
     }
 }
\ No newline at end of file

From 3987f3c98af58289e9b1e29e1b98321aa217e33c Mon Sep 17 00:00:00 2001
From: Daniel Marbach <danielmarbach@users.noreply.github.com>
Date: Thu, 2 Jul 2026 07:40:55 +0200
Subject: [PATCH 7/7] Forwarding should copy the headers

---
 src/NServiceBus.Core/Unicast/IncomingMessageOperations.cs | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/src/NServiceBus.Core/Unicast/IncomingMessageOperations.cs b/src/NServiceBus.Core/Unicast/IncomingMessageOperations.cs
index d67d491dbb4..e9cc0ccd58d 100644
--- a/src/NServiceBus.Core/Unicast/IncomingMessageOperations.cs
+++ b/src/NServiceBus.Core/Unicast/IncomingMessageOperations.cs
@@ -1,18 +1,23 @@
 namespace NServiceBus;
 
+using System.Collections.Generic;
 using System.Threading.Tasks;
 using Pipeline;
 using Routing;
 using Transport;
+using NServiceBus.Utils;
 
 static class IncomingMessageOperations
 {
     public static Task ForwardCurrentMessageTo(IIncomingContext context, string destination)
     {
         var messageBeingProcessed = context.Extensions.Get<IncomingMessage>();
+        var headers = HeaderPool.Shared.Rent(messageBeingProcessed.Headers.Count);
+        messageBeingProcessed.Headers.CopyTo(headers);
+
         var outgoingMessage = new OutgoingMessage(
                         messageBeingProcessed.MessageId,
-                        messageBeingProcessed.Headers,
+                        headers,
                         messageBeingProcessed.Body);
 
         var routingContext = new RoutingContext(outgoingMessage, new UnicastRoutingStrategy(destination), context);