From ec96c1ce86a1c9b43ca273fe62d4f88569d8d74e Mon Sep 17 00:00:00 2001
From: Matthias van de Meent <matthias@neon.tech>
Date: Wed, 11 Jun 2025 19:43:59 +0200
Subject: [PATCH] XLog: Expose WAL replay progress test

This allows extension code to check how far REDO has come, and
potentially wait or change control flow based on REDO progress.
---
 src/backend/access/transam/xlogrecovery.c | 41 ++++++++++++++++-------
 src/include/access/xlogrecovery.h         |  1 +
 2 files changed, 29 insertions(+), 13 deletions(-)

diff --git a/src/backend/access/transam/xlogrecovery.c b/src/backend/access/transam/xlogrecovery.c
index 5abe1eb9afd..4b41ab6986a 100644
--- a/src/backend/access/transam/xlogrecovery.c
+++ b/src/backend/access/transam/xlogrecovery.c
@@ -491,35 +491,50 @@ EnableStandbyMode(void)
 	disable_startup_progress_timeout();
 }
 
+static XLogRecPtr replayRecPtr = InvalidXLogRecPtr;
+
 /*
- * Wait for recovery to complete replaying all WAL up to and including
- * redoEndRecPtr.
+ * Check if a record at given LSN has been replayed yet.
  *
- * This gets woken up for every WAL record replayed, so make sure you're not
- * trying to wait an LSN that is too far in the future.
+ * Always returns TRUE when not in recovery mode.
  */
-void
-XLogWaitForReplayOf(XLogRecPtr redoEndRecPtr)
+bool
+XLogRecordReplayFinished(XLogRecPtr redoEndRecPtr)
 {
-	static XLogRecPtr replayRecPtr = 0;
-
 	if (!RecoveryInProgress())
-		return;
+		return true;
 
 	/*
 	 * Check the backend-local variable first, we may be able to skip accessing
 	 * shared memory (which requires locking)
 	 */
 	if (redoEndRecPtr <= replayRecPtr)
-		return;
+		return true;
 
+	/* update the backend-local cache with more up-to-date values */
 	replayRecPtr = GetXLogReplayRecPtr(NULL);
 
+	return redoEndRecPtr <= replayRecPtr;
+}
+
+/*
+ * Wait for recovery to complete replaying all WAL up to and including
+ * redoEndRecPtr.
+ *
+ * This gets woken up for every WAL record replayed, so make sure you're not
+ * trying to wait an LSN that is too far in the future.
+ */
+void
+XLogWaitForReplayOf(XLogRecPtr redoEndRecPtr)
+{
 	/*
-	 * Check again if we're going to need to wait, now that we've updated
-	 * the local cached variable.
+	 * Check if the record has been replayed yet. This includes up-to-date
+	 * information about current replay state - if it hasn't been replayed,
+	 * we're probably going to have to wait.
+	 *
+	 * This also returns if we're not in recovery mode.
 	 */
-	if (redoEndRecPtr <= replayRecPtr)
+	if (XLogRecordReplayFinished(redoEndRecPtr))
 		return;
 
 	/*
diff --git a/src/include/access/xlogrecovery.h b/src/include/access/xlogrecovery.h
index d1e2ba9b670..aefc30cf83a 100644
--- a/src/include/access/xlogrecovery.h
+++ b/src/include/access/xlogrecovery.h
@@ -137,6 +137,7 @@ extern void ShutdownWalRecovery(void);
 extern void RemovePromoteSignalFiles(void);
 
 extern bool HotStandbyActive(void);
+extern bool XLogRecordReplayFinished(XLogRecPtr redoEndRecPtr);
 extern void XLogWaitForReplayOf(XLogRecPtr redoEndRecPtr);
 extern XLogRecPtr GetXLogReplayRecPtr(TimeLineID *replayTLI);
 extern RecoveryPauseState GetRecoveryPauseState(void);