pgruntime: protect against concurrent migrations

2 files changed, 22 insertions, 1 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 77b3972..e7bfdf6 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -3,6 +3,12 @@ SPDX-FileCopyrightText: 2026 Stefan Majewsky <majewsky@gmx.net>
 SPDX-License-Identifier: Apache-2.0
 -->
 
+# v1.13.2 (TBD)
+
+Changes:
+
+- pgruntime: Add transaction locking to protect against concurrent migrations.
+
 # v1.13.1 (2026-08-05)
 
 Changes:
diff --git a/pgruntime/behavior.go b/pgruntime/behavior.go
index 3ffff1e..03ff338 100644
--- a/pgruntime/behavior.go
+++ b/pgruntime/behavior.go
@@ -110,7 +110,22 @@ func applyMigrations(ctx context.Context, db gsql.ConnectionHandle, migrations m
 	// apply migrations
 	for _, version := range pendingVersions {
 		err := db.GSQLTransact(ctx, func(tx gsql.Handle) error {
-			_, err := execQuery(ctx, db, migrations[version], nil)
+			// ensure that nobody else is migrating until we are done
+			var actualVersion int64
+			err := queryRow(ctx, db, `SELECT version FROM schema_migrations FOR UPDATE`, nil, []any{&actualVersion})
+			if err != nil {
+				return fmt.Errorf("could not obtain lock for schema migration: %w", err)
+			}
+
+			// ensure that nobody else migrated just before we started our transaction block
+			if version <= actualVersion {
+				return fmt.Errorf("tried to perform migration to version %d, but schema is already at version %d (multiple migrations might be running concurrently)",
+					version, actualVersion,
+				)
+			}
+
+			// perform the next migration
+			_, err = execQuery(ctx, db, migrations[version], nil)
 			if err != nil {
 				return fmt.Errorf("could not execute schema migration: %w", err)
 			}