[BACKPORT 2.16][#19509] YSQL: Fix YB does not release LWLocks on Abor…

…tTransaction

Summary:
Original commit: 7e22b28 / D29194

This revision backports the fix for releasing LWLocks on AbortTransaction. It is imported from the replication slot support revision: https://phorge.dev.yugabyte.com/D29194.

The issue was introduced in 3f536a8 / D5518
Jira: DB-8305

Test Plan:
```
./yb_build.sh --cxx-test pgwrapper_pg_libpq-test --gtest_filter PgLibPqTest.TestLWPgBackendErrorAfterLWLockAcquire
```

Reviewers: telgersma

Reviewed By: telgersma

Subscribers: yql

Tags: #jenkins-ready

Differential Revision: https://phorge.dev.yugabyte.com/D29996

src/postgres/contrib/pg_stat_statements/pg_stat_statements.c

@@ -2313,6 +2313,25 @@ yb_lwlock_crash_after_acquire_pg_stat_statements_reset()
 
 }
 
+/*
+ * Function that caches environmental variable
+ * FLAGS_TEST_yb_lwlock_error_after_acquire_pg_stat_statements_reset.
+ *
+ * This avoids the process of checking the value of the environmental variable
+ * time and again.
+ */
+bool
+yb_lwlock_error_after_acquire_pg_stat_statements_reset()
+{
+	static int cached_value = -1;
+	if (cached_value == -1)
+	{
+		cached_value = YBCIsEnvVarTrue(
+			"FLAGS_TEST_yb_lwlock_error_after_acquire_pg_stat_statements_reset");
+	}
+	return cached_value;
+}
+
 /*
  * Release all entries.
  */
@@ -2333,6 +2352,14 @@ entry_reset(void)
 
 	if (yb_lwlock_crash_after_acquire_pg_stat_statements_reset())
 	  kill(getpid(), 9);
+
+	if (yb_lwlock_error_after_acquire_pg_stat_statements_reset())
+		ereport(ERROR,
+				(errcode(ERRCODE_INTERNAL_ERROR),
+				 errmsg("Forced error since "
+				 		"yb_lwlock_error_after_acquire_pg_stat_statements_reset"
+						" is set")));
+
 	/*
 	 * Write new empty query file, perhaps even creating a new one to recover
 	 * if the file was missing.

src/postgres/src/backend/access/transam/xact.c

@@ -2678,15 +2678,13 @@ AbortTransaction(void)
 	AtAbort_Memory();
 	AtAbort_ResourceOwner();
 
-	if (YBIsPgLockingEnabled()) {
-		/*
-		* Release any LW locks we might be holding as quickly as possible.
-		* (Regular locks, however, must be held till we finish aborting.)
-		* Releasing LW locks is critical since we might try to grab them again
-		* while cleaning up!
-		*/
-		LWLockReleaseAll();
-	}
+	/*
+	 * Release any LW locks we might be holding as quickly as possible.
+	 * (Regular locks, however, must be held till we finish aborting.)
+	 * Releasing LW locks is critical since we might try to grab them again
+	 * while cleaning up!
+	 */
+	LWLockReleaseAll();
 
 	/* Clear wait information and command progress indicator */
 	pgstat_report_wait_end();

src/yb/yql/pggate/pggate_flags.cc

@@ -139,5 +139,9 @@ DEFINE_bool(yb_enable_read_committed_isolation, false,
 DEFINE_test_flag(bool, yb_lwlock_crash_after_acquire_pg_stat_statements_reset, false,
              "Issue sigkill for crash test after acquiring a LWLock in pg_stat_statements reset.");
 
+DEFINE_test_flag(bool, yb_lwlock_error_after_acquire_pg_stat_statements_reset, false,
+                 "Issue ereport(ERROR) for error test after acquiring a LWLock"
+                 " in pg_stat_statements reset.");
+
 DEFINE_test_flag(bool, yb_test_fail_matview_refresh_after_creation, false,
                  "Fail a refresh on a matview after the creation of a new relation.");

src/yb/yql/pggate/pggate_flags.h

@@ -44,6 +44,7 @@ DECLARE_int32(ysql_max_write_restart_attempts);
 DECLARE_bool(ysql_sleep_before_retry_on_txn_conflict);
 DECLARE_bool(ysql_disable_portal_run_context);
 DECLARE_bool(TEST_yb_lwlock_crash_after_acquire_pg_stat_statements_reset);
+DECLARE_bool(TEST_yb_lwlock_error_after_acquire_pg_stat_statements_reset);
 DECLARE_bool(TEST_yb_test_fail_matview_refresh_after_creation);
 DECLARE_bool(ysql_enable_read_request_caching);
 #endif  // YB_YQL_PGGATE_PGGATE_FLAGS_H

src/yb/yql/pgwrapper/pg_libpq-test.cc

@@ -2873,6 +2873,32 @@ TEST_F_EX(PgLibPqTest,
 }
 #endif
 
+// The motive of this test is to prove that when a postgres backend errors out
+// while possessing an LWLock, the lock is released.
+// TEST_yb_lwlock_error_after_acquire_pg_stat_statements_reset when set true
+// will error out a postgres backend after acquiring a LWLock. Specifically in
+// this example, when pg_stat_statements_reset() function is called when this
+// flag is set, it errors out after acquiring a lock on pgss->lock.
+// We verify that future commands on the same connection do not deadlock as the
+// lock should have been released after error.
+class PgLibPqYSQLBackendError: public PgLibPqTest {
+ public:
+  void UpdateMiniClusterOptions(ExternalMiniClusterOptions* options) override {
+    options->extra_tserver_flags.push_back(
+        Format("--TEST_yb_lwlock_error_after_acquire_pg_stat_statements_reset=true"));
+  }
+};
+
+TEST_F_EX(PgLibPqTest,
+          YB_DISABLE_TEST_IN_TSAN(TestLWPgBackendErrorAfterLWLockAcquire),
+          PgLibPqYSQLBackendError) {
+  auto conn = ASSERT_RESULT(Connect());
+  ASSERT_NOK(conn.FetchFormat("SELECT pg_stat_statements_reset()"));
+
+  // Verify that future commands on the same connection works.
+  EXPECT_OK(conn.FetchFormat("SELECT 1"));
+}
+
 class PgLibPqRefreshMatviewFailure: public PgLibPqTest {
  public:
   void UpdateMiniClusterOptions(ExternalMiniClusterOptions* options) override {