Address review: fix NULL asymmetry in backend sync decisions (PR #5511)

- BackendSyncDecision.cpp: treat asymmetric NULLs as mismatches
  (client=NULL + backend="bob" → SYNC_USER, not SYNC_NONE)
- Tests: replace weak null assertions (a >= 0 always true) with
  specific checks for SYNC_USER/SYNC_SCHEMA on asymmetric NULLs,
  both-null → no sync, schema asymmetric null (+2 tests, plan 15→17)

lib/BackendSyncDecision.cpp

@@ -20,7 +20,12 @@ int determine_backend_sync_actions(
 	int actions = SYNC_NONE;
 
 	// Username mismatch → CHANGE USER required
-	if (client_user && backend_user) {
+	// Asymmetric NULLs (one set, other not) count as mismatch
+	if (client_user == nullptr && backend_user != nullptr) {
+		actions |= SYNC_USER;
+	} else if (client_user != nullptr && backend_user == nullptr) {
+		actions |= SYNC_USER;
+	} else if (client_user && backend_user) {
 		if (strcmp(client_user, backend_user) != 0) {
 			actions |= SYNC_USER;
 		}
@@ -29,7 +34,11 @@ int determine_backend_sync_actions(
 	// Schema mismatch → USE <db> required
 	// Only check if usernames match (user change handles schema too)
 	if (!(actions & SYNC_USER)) {
-		if (client_schema && backend_schema) {
+		if (client_schema == nullptr && backend_schema != nullptr) {
+			actions |= SYNC_SCHEMA;
+		} else if (client_schema != nullptr && backend_schema == nullptr) {
+			actions |= SYNC_SCHEMA;
+		} else if (client_schema && backend_schema) {
 			if (strcmp(client_schema, backend_schema) != 0) {
 				actions |= SYNC_SCHEMA;
 			}

test/tap/tests/unit/backend_sync_unit-t.cpp

@@ -52,15 +52,24 @@ static void test_multiple_mismatches() {
 
 static void test_null_handling() {
 	// null users — no crash
+	// Asymmetric NULL: one side null, other not → mismatch
 	int a = determine_backend_sync_actions(nullptr, "user", "db", "db", true, true);
-	ok(a == SYNC_NONE || a >= 0, "null client_user: no crash");
+	ok((a & SYNC_USER) != 0, "null client_user + non-null backend → SYNC_USER");
 
 	a = determine_backend_sync_actions("user", nullptr, "db", "db", true, true);
-	ok(a == SYNC_NONE || a >= 0, "null backend_user: no crash");
+	ok((a & SYNC_USER) != 0, "non-null client_user + null backend → SYNC_USER");
+
+	// Both null → no mismatch
+	a = determine_backend_sync_actions(nullptr, nullptr, "db", "db", true, true);
+	ok(a == SYNC_NONE, "both users null → no sync");
+
+	// Schema asymmetric null
+	a = determine_backend_sync_actions("user", "user", nullptr, "db", true, true);
+	ok((a & SYNC_SCHEMA) != 0, "null client_schema + non-null backend → SYNC_SCHEMA");
 }
 
 int main() {
-	plan(15);
+	plan(17);
 	int rc = test_init_minimal();
 	ok(rc == 0, "test_init_minimal() succeeds");
 
@@ -70,8 +79,8 @@ int main() {
 	test_user_and_schema_mismatch(); // 2
 	test_autocommit_mismatch();      // 2
 	test_multiple_mismatches();      // 2
-	test_null_handling();            // 2
-	// Total: 1+2+2+2+2+2+2+2 = 15
+	test_null_handling();            // 4
+	// Total: 1+2+2+2+2+2+2+4 = 17
 
 	test_cleanup_minimal();
 	return exit_status();