Revert IsPrimaryAlive checks (#400)

* Revert "Add comment explaining why it is safe to use FirstNormalTransactionXid for oldestActiveXid while replica startup (#389)"

This reverts commit 1eeab2d.

* Revert "Set wasShutdown=true during hot-standby replica startup only when primary is not alive (#365)"

This reverts commit b9336bc.

Author: skyzh

src/backend/access/transam/xlog.c

@@ -542,18 +542,6 @@ typedef union WALInsertLockPadded
 	char		pad[PG_CACHE_LINE_SIZE];
 } WALInsertLockPadded;
 
-
-/*
- * NEON: check if primary node is running.
- * Correspondent GUC is received from control plane
- */
-static bool
-IsPrimaryAlive()
-{
-	const char* val = GetConfigOption("neon.primary_is_running", true, false);
-	return val != NULL && strcmp(val, "on") == 0;
-}
-
 /*
  * State of an exclusive backup, necessary to control concurrent activities
  * across sessions when working on exclusive backups.
@@ -7038,25 +7026,7 @@ StartupXLOG(void)
 		EndRecPtr = ControlFile->checkPointCopy.redo;
 
 		memcpy(&checkPoint, &ControlFile->checkPointCopy, sizeof(CheckPoint));
-		// When primary Neon compute node is started, we pretend that it started after a clean shutdown and
-		// no recovery is needed. We don't need to do WAL replay, the page server does that on a page-by-page basis.
-		// When a read-only replica is started, PostgreSQL normally waits for a shutdown checkpoint or running-xacts
-		// record before enabling hot standby, to establish which transactions are still running in the primary,
-		// and might still commit later. But if we know that the primary is not running - because the control plane
-		// says so - we can skip that. That avoids having to wait indefinitely if the primary is not running. This is
-		// particularly important for Neon because we don't start recovery from a checkpoint record, so there's
-		// no guarantee on when we'll see the next checkpoint or running-xacts record, if ever. so if we know the primary is
-		// not currently running, also set wasShutdown to 'true'.
-		if (StandbyModeRequested &&
-			PrimaryConnInfo != NULL && *PrimaryConnInfo != '\0')
-		{
-			if (!IsPrimaryAlive())
-				wasShutdown = true;
-			else
-				wasShutdown = false;
-		}
-		else
-			wasShutdown = true;
+		wasShutdown = true;
 
 		/* Initialize expectedTLEs, like ReadRecord() does */
 		expectedTLEs = readTimeLineHistory(checkPoint.ThisTimeLineID);
@@ -7226,24 +7196,6 @@ StartupXLOG(void)
 		ereport(PANIC,
 				(errmsg("invalid next transaction ID")));
 
-	if (ZenithRecoveryRequested)
-	{
-		if (wasShutdown)
-			checkPoint.oldestActiveXid = InvalidTransactionId;
-		else if (!TransactionIdIsValid(checkPoint.oldestActiveXid))
-		{
-			/*
-			 * Pageserver extracts oldestActiveXid from snapshot and running xacts WAL records
-			 * and include it in checkpoint sent in basebackup.
-			 * So oldestActiveXid can be zero only after database initialization when no checkpoints are yet performed
-			 * and not running xacts records was logged.
-			 * In this case it is possible to use FirstNormalTransactionId as safe conservative estimation
-			 * of oldest active transaction XID.
-			 */
-			checkPoint.oldestActiveXid = FirstNormalTransactionId;
-		}
-	}
-
 	/* initialize shared memory variables from the checkpoint record */
 	ShmemVariableCache->nextXid = checkPoint.nextXid;
 	ShmemVariableCache->nextOid = checkPoint.nextOid;