test(nix): MERGE serialization fix with an isolation spec

nix/checks.nix

@@ -277,7 +277,10 @@
               # Concurrency/isolation specs run via pg_isolation_regress (the stock
               # PostgreSQL isolation tester). Specs live in tests/isolation/specs/,
               # expected output in tests/isolation/expected/. Add new spec names here.
-              isolationSpecList = [ "sample_isolation" ];
+              isolationSpecList = [
+                "merge_serialization"
+                "sample_isolation"
+              ];
             in
             pkgs.writeShellApplication rec {
               name = "postgres-${pgpkg.version}-check-harness";

nix/tests/isolation/expected/merge_serialization.out

@@ -0,0 +1,16 @@
+Parsed test spec with 2 sessions
+
+starting permutation: s1_begin s1_snapshot s2_update s1_merge s1_commit
+step s1_begin: BEGIN ISOLATION LEVEL REPEATABLE READ;
+step s1_snapshot: SELECT v FROM merge_target WHERE k = 1;
+v
+-
+0
+(1 row)
+
+step s2_update: UPDATE merge_target SET v = v + 100 WHERE k = 1;
+step s1_merge: MERGE INTO merge_target t
+                     USING (SELECT 1 AS k) s ON t.k = s.k
+                     WHEN MATCHED THEN UPDATE SET v = t.v + 1;
+ERROR:  could not serialize access due to concurrent update
+step s1_commit: COMMIT;

nix/tests/isolation/specs/merge_serialization.spec

@@ -0,0 +1,32 @@
+# Pins the upstream fix (PG 15.16 / 17.8) where MERGE must raise a
+# serialization failure (SQLSTATE 40001) under REPEATABLE READ when its target
+# row was updated and committed by a concurrent transaction after the MERGE
+# transaction took its snapshot. Before the fix this concurrent-update check
+# was silently skipped for MERGE (unlike a plain UPDATE).
+#
+# s1 opens a REPEATABLE READ transaction and takes its snapshot (s1_snapshot);
+# s2 then updates+commits the row; s1's MERGE on that row must fail with 40001.
+
+setup
+{
+  CREATE TABLE merge_target (k int PRIMARY KEY, v int);
+  INSERT INTO merge_target VALUES (1, 0);
+}
+
+teardown
+{
+  DROP TABLE merge_target;
+}
+
+session s1
+step s1_begin    { BEGIN ISOLATION LEVEL REPEATABLE READ; }
+step s1_snapshot { SELECT v FROM merge_target WHERE k = 1; }
+step s1_merge    { MERGE INTO merge_target t
+                     USING (SELECT 1 AS k) s ON t.k = s.k
+                     WHEN MATCHED THEN UPDATE SET v = t.v + 1; }
+step s1_commit   { COMMIT; }
+
+session s2
+step s2_update   { UPDATE merge_target SET v = v + 100 WHERE k = 1; }
+
+permutation s1_begin s1_snapshot s2_update s1_merge s1_commit