0004-CV-POC.txt

diff --git a/src/backend/access/transam/xlogrecovery.c b/src/backend/access/transam/xlogrecovery.c
index dbe9394762..8a9505a52d 100644
--- a/src/backend/access/transam/xlogrecovery.c
+++ b/src/backend/access/transam/xlogrecovery.c
@@ -358,6 +358,9 @@ typedef struct XLogRecoveryCtlData
 	RecoveryPauseState recoveryPauseState;
 	ConditionVariable recoveryNotPausedCV;
 
+	/* Replay state (see check_for_replay() for more explanation) */
+	ConditionVariable replayedCV;
+
 	slock_t		info_lck;		/* locks shared variables shown above */
 } XLogRecoveryCtlData;
 
@@ -468,6 +471,7 @@ XLogRecoveryShmemInit(void)
 	SpinLockInit(&XLogRecoveryCtl->info_lck);
 	InitSharedLatch(&XLogRecoveryCtl->recoveryWakeupLatch);
 	ConditionVariableInit(&XLogRecoveryCtl->recoveryNotPausedCV);
+	ConditionVariableInit(&XLogRecoveryCtl->replayedCV);
 }
 
 /*
@@ -1935,6 +1939,11 @@ ApplyWalRecord(XLogReaderState *xlogreader, XLogRecord *record, TimeLineID *repl
 	XLogRecoveryCtl->lastReplayedTLI = *replayTLI;
 	SpinLockRelease(&XLogRecoveryCtl->info_lck);
 
+	/*
+	 * wake up walsender(s) used by logical decoding on standby.
+	 */
+	ConditionVariableBroadcast(&XLogRecoveryCtl->replayedCV);
+
 	/*
 	 * If rm_redo called XLogRequestWalReceiverReply, then we wake up the
 	 * receiver so that it notices the updated lastReplayedEndRecPtr and sends
@@ -4942,3 +4951,22 @@ assign_recovery_target_xid(const char *newval, void *extra)
 	else
 		recoveryTarget = RECOVERY_TARGET_UNSET;
 }
+
+/*
+ * Return the ConditionVariable indicating that a replay has been done.
+ *
+ * This is needed for logical decoding on standby. Indeed the "problem" is that
+ * WalSndWaitForWal() waits for the *replay* LSN to increase, but gets woken up
+ * by walreceiver when new WAL has been flushed. Which means that typically
+ * walsenders will get woken up at the same time that the startup process
+ * will be - which means that by the time the logical walsender checks
+ * GetXLogReplayRecPtr() it's unlikely that the startup process already replayed
+ * the record and updated XLogCtl->lastReplayedEndRecPtr.
+ *
+ * The ConditionVariable XLogRecoveryCtl->replayedCV solves this corner case.
+ */
+ConditionVariable *
+check_for_replay(void)
+{
+	return &XLogRecoveryCtl->replayedCV;
+}
diff --git a/src/backend/replication/walsender.c b/src/backend/replication/walsender.c
index 3042e5bd64..05350bb535 100644
--- a/src/backend/replication/walsender.c
+++ b/src/backend/replication/walsender.c
@@ -1552,6 +1552,17 @@ WalSndWaitForWal(XLogRecPtr loc)
 {
 	int			wakeEvents;
 	static XLogRecPtr RecentFlushPtr = InvalidXLogRecPtr;
+	ConditionVariable *replayedCV = check_for_replay();
+
+	/*
+	 * Prepare the replayedCV to sleep. Note that this is enough to be added
+	 * in the wait queue and then waked up (while in WalSndWait() below)
+	 * by ConditionVariableBroadcast() during the WAL replay. Also Note that
+	 * if awakaned by the CV broadcast we'll exit the loop right after due to
+	 * the loc <= RecentFlushPtr test done in the loop. Indeed, CV brodcast
+	 * would mean that a replay occured.
+	 */
+	ConditionVariablePrepareToSleep(replayedCV);
 
 	/*
 	 * Fast path to avoid acquiring the spinlock in case we already know we
@@ -1670,6 +1681,7 @@ WalSndWaitForWal(XLogRecPtr loc)
 		WalSndWait(wakeEvents, sleeptime, WAIT_EVENT_WAL_SENDER_WAIT_WAL);
 	}
 
+	ConditionVariableCancelSleep();
 	/* reactivate latch so WalSndLoop knows to continue */
 	SetLatch(MyLatch);
 	return RecentFlushPtr;
diff --git a/src/include/access/xlogrecovery.h b/src/include/access/xlogrecovery.h
index 47c29350f5..2bfeaaa00f 100644
--- a/src/include/access/xlogrecovery.h
+++ b/src/include/access/xlogrecovery.h
@@ -15,6 +15,7 @@
 #include "catalog/pg_control.h"
 #include "lib/stringinfo.h"
 #include "utils/timestamp.h"
+#include "storage/condition_variable.h"
 
 /*
  * Recovery target type.
@@ -155,4 +156,6 @@ extern void RecoveryRequiresIntParameter(const char *param_name, int currValue,
 
 extern void xlog_outdesc(StringInfo buf, XLogReaderState *record);
 
+extern ConditionVariable *check_for_replay(void);
+
 #endif							/* XLOGRECOVERY_H */
diff --git a/src/include/replication/walsender.h b/src/include/replication/walsender.h
index 52bb3e2aae..2fd745fe72 100644
--- a/src/include/replication/walsender.h
+++ b/src/include/replication/walsender.h
@@ -13,6 +13,7 @@
 #define _WALSENDER_H
 
 #include <signal.h>
+#include "storage/condition_variable.h"
 
 /*
  * What to do with a snapshot in create replication slot command.