fix(backup): skip stanza-create on standby to enable prefer-standby backups

internal/cnpgi/instance/backup.go

@@ -20,6 +20,7 @@ package instance
 import (
 	"context"
 	"fmt"
+	"path/filepath"
 	"time"
 
 	"github.com/cloudnative-pg/cloudnative-pg/pkg/postgres"
@@ -118,11 +119,32 @@ func (b BackupServiceImplementation) Backup(
 		return nil, err
 	}
 
-	err = backupCmd.CreatePgbackrestStanza(ctx, configuration.Stanza, env)
+	// pgbackrest stanza-create requires a primary PostgreSQL connection: it reads
+	// the live cluster's control data to initialize the stanza metadata in the
+	// repository. On a standby it fails with pgbackrest exit code 56
+	// ("unable to find primary cluster"), which aborts the whole backup. That
+	// makes target: prefer-standby / standby backups unusable (see issue #83).
+	//
+	// Skip stanza-create when this instance is a standby. This is safe: the
+	// stanza is created by the primary (its own backups + WAL archiving's
+	// CheckWalArchiveDestination run against the same repo), so by the time a
+	// standby backup runs the stanza already exists. We detect the role the same
+	// way PostgreSQL and CNPG itself do — the presence of standby.signal in
+	// PGDATA — so we need neither a database connection nor credentials here.
+	isStandby, err := instanceIsStandby(b.PGDataPath)
 	if err != nil {
-		contextLogger.Error(err, "while initializing pgbackrest stanza")
+		contextLogger.Error(err, "while determining if instance is a standby")
 		return nil, err
 	}
+	if isStandby {
+		contextLogger.Info("Instance is a standby; skipping pgbackrest stanza-create " +
+			"(the stanza is initialized by the primary)")
+	} else {
+		if err = backupCmd.CreatePgbackrestStanza(ctx, configuration.Stanza, env); err != nil {
+			contextLogger.Error(err, "while initializing pgbackrest stanza")
+			return nil, err
+		}
+	}
 
 	backupName := fmt.Sprintf("backup-%v", pgTime.ToCompactISO8601(time.Now()))
 
@@ -166,3 +188,13 @@ func (b BackupServiceImplementation) Backup(
 		},
 	}, nil
 }
+
+// instanceIsStandby reports whether the PostgreSQL instance rooted at pgDataPath
+// is a standby (replica). It checks for the standby.signal file in PGDATA, which
+// is the same signal PostgreSQL itself uses to start in standby mode and the same
+// mechanism CloudNativePG uses to distinguish a replica from a primary. Using the
+// signal file means we do not need a database connection or credentials at backup
+// time to determine the role.
+func instanceIsStandby(pgDataPath string) (bool, error) {
+	return fileutils.FileExists(filepath.Join(pgDataPath, "standby.signal"))
+}

internal/cnpgi/instance/backup_test.go

@@ -0,0 +1,58 @@
+/*
+Copyright 2025, Opera Norway AS
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package instance
+
+import (
+	"os"
+	"path/filepath"
+
+	. "github.com/onsi/ginkgo/v2"
+	. "github.com/onsi/gomega"
+)
+
+var _ = Describe("instanceIsStandby", func() {
+	var pgData string
+
+	BeforeEach(func() {
+		pgData = GinkgoT().TempDir()
+	})
+
+	It("reports false for a primary (no standby.signal)", func() {
+		isStandby, err := instanceIsStandby(pgData)
+		Expect(err).NotTo(HaveOccurred())
+		Expect(isStandby).To(BeFalse())
+	})
+
+	It("reports true when standby.signal is present", func() {
+		Expect(
+			os.WriteFile(filepath.Join(pgData, "standby.signal"), []byte{}, 0o600),
+		).To(Succeed())
+
+		isStandby, err := instanceIsStandby(pgData)
+		Expect(err).NotTo(HaveOccurred())
+		Expect(isStandby).To(BeTrue())
+	})
+
+	It("treats a non-existent PGDATA as a primary rather than erroring", func() {
+		// FileExists returns (false, nil) when the directory does not exist, so a
+		// missing/odd PGDATA path falls back to the primary path (stanza-create
+		// runs) instead of aborting the backup.
+		isStandby, err := instanceIsStandby(filepath.Join(pgData, "does-not-exist"))
+		Expect(err).NotTo(HaveOccurred())
+		Expect(isStandby).To(BeFalse())
+	})
+})

internal/cnpgi/instance/suite_test.go

@@ -0,0 +1,29 @@
+/*
+Copyright 2025, Opera Norway AS
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package instance
+
+import (
+	"testing"
+
+	. "github.com/onsi/ginkgo/v2"
+	. "github.com/onsi/gomega"
+)
+
+func TestInstance(t *testing.T) {
+	RegisterFailHandler(Fail)
+	RunSpecs(t, "Instance plugin test suite")
+}