From 7d430024297f3687d2a59f3cf8f6464009b5f321 Mon Sep 17 00:00:00 2001
From: Peter Wagenet <peter.wagenet@gmail.com>
Date: Mon, 10 Aug 2026 20:48:36 -0700
Subject: [PATCH] fix: give CEF a per-instance cache directory on native runs

Only one Strom instance per machine can use `cefsrc` (HTML sources, DSK
graphics) when running natively. The second instance fails to start any
cefsrc element:

    GstCefSrc:c: gst_base_src_start (): Failed to start
    -> HTTP 500 "Element failed to change its state"

Chromium keys its process singleton on the cache/profile directory. The
strom-full Docker image sets GST_CEF_CACHE_LOCATION=/tmp/cef-cache in its
entrypoint, so containers are fine, but native runs never set it and get
Chromium's default -- which CEF itself warns about at startup:

    Please customize CefSettings.root_cache_path for your application. Use
    of the default value may lead to unintended process singleton behavior.

Default the variable to a directory derived from the data directory, so
instances with different data directories are isolated. An explicitly
configured GST_CEF_CACHE_LOCATION still wins.

Measured on macOS arm64, one headless instance starting a cefsrc flow:

    default (in-memory)     7.1s
    per-instance, cold     22.9s   (one-off, builds an 11MB profile)
    per-instance, warm      0.9s

So the cost is a one-time first-run penalty on a fresh data directory, and
repeat starts get faster. Two concurrent instances with distinct data
directories both start successfully; forced to share one cache directory,
the second still fails as before.
---
 backend/src/main.rs | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/backend/src/main.rs b/backend/src/main.rs
index c21e0d96..8da51afc 100644
--- a/backend/src/main.rs
+++ b/backend/src/main.rs
@@ -299,6 +299,26 @@ fn main() -> anyhow::Result<()> {
         std::process::exit(1);
     });
 
+    // Give CEF (the `cefsrc` element behind HTML sources and DSK graphics) a
+    // per-instance cache directory. The strom-full Docker image sets
+    // GST_CEF_CACHE_LOCATION in its entrypoint, but native runs get Chromium's
+    // default, which warns
+    //
+    //   Please customize CefSettings.root_cache_path for your application. Use
+    //   of the default value may lead to unintended process singleton behavior.
+    //
+    // and makes Chromium's process singleton shared: a second Strom instance on
+    // the same machine cannot start any cefsrc element, failing in
+    // gst_base_src_start() so the flow start returns 500 "Element failed to
+    // change its state". Deriving the directory from the data directory keeps
+    // instances isolated, and gives CEF a warm on-disk profile, which also cuts
+    // repeat flow-start latency. An explicitly configured value always wins.
+    if std::env::var_os("GST_CEF_CACHE_LOCATION").is_none() {
+        if let Some(data_dir) = config.flows_path.parent() {
+            std::env::set_var("GST_CEF_CACHE_LOCATION", data_dir.join("cef-cache"));
+        }
+    }
+
     // Initialize logging with optional file output and log level
     let (log_reload_handle, default_log_filter) =
         init_logging(config.log_file.as_ref(), config.log_level.as_ref()).unwrap_or_else(|e| {