Hot Standby feedback for avoidance of cleanup conflicts on standby.

Standby optionally sends back information about oldestXmin of queries
which is then checked and applied to the WALSender's proc->xmin.
GetOldestXmin() is modified slightly to agree with GetSnapshotData(),
so that all backends on primary include WALSender within their snapshots.
Note this does nothing to change the snapshot xmin on either master or
standby. Feedback piggybacks on the standby reply message.
vacuum_defer_cleanup_age is no longer used on standby, though parameter
still exists on primary, since some use cases still exist.

Simon Riggs, review comments from Fujii Masao, Heikki Linnakangas, Robert Haas

1 files changed, 54 insertions, 4 deletions

diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index 6fdaaff9140..3ba1f29197f 100644
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@@ -158,6 +158,11 @@ static XLogRecPtr LastRec;
  * known, need to check the shared state".
  */
 static bool LocalRecoveryInProgress = true;
+/*
+ * Local copy of SharedHotStandbyActive variable. False actually means "not
+ * known, need to check the shared state".
+ */
+static bool LocalHotStandbyActive = false;
 
 /*
  * Local state for XLogInsertAllowed():
@@ -406,6 +411,12 @@ typedef struct XLogCtlData
 	bool		SharedRecoveryInProgress;
 
 	/*
+	 * SharedHotStandbyActive indicates if we're still in crash or archive
+	 * recovery.  Protected by info_lck.
+	 */
+	bool		SharedHotStandbyActive;
+
+	/*
 	 * recoveryWakeupLatch is used to wake up the startup process to
 	 * continue WAL replay, if it is waiting for WAL to arrive or failover
 	 * trigger file to appear.
@@ -4917,6 +4928,7 @@ XLOGShmemInit(void)
 	 */
 	XLogCtl->XLogCacheBlck = XLOGbuffers - 1;
 	XLogCtl->SharedRecoveryInProgress = true;
+	XLogCtl->SharedHotStandbyActive = false;
 	XLogCtl->Insert.currpage = (XLogPageHeader) (XLogCtl->pages);
 	SpinLockInit(&XLogCtl->info_lck);
 	InitSharedLatch(&XLogCtl->recoveryWakeupLatch);
@@ -6790,8 +6802,6 @@ StartupXLOG(void)
 static void
 CheckRecoveryConsistency(void)
 {
-	static bool backendsAllowed = false;
-
 	/*
 	 * Have we passed our safe starting point?
 	 */
@@ -6811,11 +6821,19 @@ CheckRecoveryConsistency(void)
 	 * enabling connections.
 	 */
 	if (standbyState == STANDBY_SNAPSHOT_READY &&
-		!backendsAllowed &&
+		!LocalHotStandbyActive &&
 		reachedMinRecoveryPoint &&
 		IsUnderPostmaster)
 	{
-		backendsAllowed = true;
+		/* use volatile pointer to prevent code rearrangement */
+		volatile XLogCtlData *xlogctl = XLogCtl;
+
+		SpinLockAcquire(&xlogctl->info_lck);
+		xlogctl->SharedHotStandbyActive = true;
+		SpinLockRelease(&xlogctl->info_lck);
+
+		LocalHotStandbyActive = true;
+
 		SendPostmasterSignal(PMSIGNAL_BEGIN_HOT_STANDBY);
 	}
 }
@@ -6863,6 +6881,38 @@ RecoveryInProgress(void)
 }
 
 /*
+ * Is HotStandby active yet? This is only important in special backends
+ * since normal backends won't ever be able to connect until this returns
+ * true. Postmaster knows this by way of signal, not via shared memory.
+ *
+ * Unlike testing standbyState, this works in any process that's connected to
+ * shared memory.
+ */
+bool
+HotStandbyActive(void)
+{
+	/*
+	 * We check shared state each time only until Hot Standby is active. We
+	 * can't de-activate Hot Standby, so there's no need to keep checking after
+	 * the shared variable has once been seen true.
+	 */
+	if (LocalHotStandbyActive)
+		return true;
+	else
+	{
+		/* use volatile pointer to prevent code rearrangement */
+		volatile XLogCtlData *xlogctl = XLogCtl;
+
+		/* spinlock is essential on machines with weak memory ordering! */
+		SpinLockAcquire(&xlogctl->info_lck);
+		LocalHotStandbyActive = xlogctl->SharedHotStandbyActive;
+		SpinLockRelease(&xlogctl->info_lck);
+
+		return LocalHotStandbyActive;
+	}
+}
+
+/*
  * Is this process allowed to insert new WAL records?
  *
  * Ordinarily this is essentially equivalent to !RecoveryInProgress().