From d49274218577f1687b7e26ed145b60f6b0fe3859 Mon Sep 17 00:00:00 2001
From: alterego655 <824662526@qq.com>
Date: Mon, 2 Feb 2026 11:12:04 +0800
Subject: [PATCH v11] Improve read_local_xlog_page_guts by replacing polling 
 with latch-based waiting.

Replace inefficient polling loops in read_local_xlog_page_guts with facilities developed in
xlogwait module when WAL data is not yet available.  This eliminates CPU-intensive busy
waiting and improves responsiveness by waking processes immediately when their target
LSN becomes available.
---
 src/backend/access/transam/xlogutils.c | 47 ++++++++++++++++++++++----
 src/backend/replication/walsender.c    |  4 ---
 2 files changed, 40 insertions(+), 11 deletions(-)

diff --git a/src/backend/access/transam/xlogutils.c b/src/backend/access/transam/xlogutils.c
index 5fbe39133b8..4303e78280f 100644
--- a/src/backend/access/transam/xlogutils.c
+++ b/src/backend/access/transam/xlogutils.c
@@ -23,6 +23,7 @@
 #include "access/xlogrecovery.h"
 #include "access/xlog_internal.h"
 #include "access/xlogutils.h"
+#include "access/xlogwait.h"
 #include "miscadmin.h"
 #include "storage/fd.h"
 #include "storage/smgr.h"
@@ -880,12 +881,7 @@ read_local_xlog_page_guts(XLogReaderState *state, XLogRecPtr targetPagePtr,
 	loc = targetPagePtr + reqLen;
 
 	/*
-	 * Loop waiting for xlog to be available if necessary
-	 *
-	 * TODO: The walsender has its own version of this function, which uses a
-	 * condition variable to wake up whenever WAL is flushed. We could use the
-	 * same infrastructure here, instead of the check/sleep/repeat style of
-	 * loop.
+	 * Waiting for xlog to be available if necessary.
 	 */
 	while (1)
 	{
@@ -947,7 +943,44 @@ read_local_xlog_page_guts(XLogReaderState *state, XLogRecPtr targetPagePtr,
 			}
 
 			CHECK_FOR_INTERRUPTS();
-			pg_usleep(1000L);
+
+			/*
+			 * Wait for LSN using appropriate method based on server state.
+			 */
+			if (!RecoveryInProgress())
+			{
+				/* Primary: wait for flush */
+				WaitForLSN(WAIT_LSN_TYPE_PRIMARY_FLUSH, loc, -1);
+			}
+			else
+			{
+				/* Standby: wait for replay */
+				WaitLSNResult result = WaitForLSN(WAIT_LSN_TYPE_STANDBY_REPLAY, loc, -1);
+
+				switch (result)
+				{
+					case WAIT_LSN_RESULT_SUCCESS:
+						/* LSN was replayed, loop back to recheck timeline */
+						break;
+
+					case WAIT_LSN_RESULT_NOT_IN_RECOVERY:
+
+						/*
+						 * Promoted while waiting. This is the tricky case.
+						 * We're now a primary, so loop back and use flush
+						 * logic instead of replay logic.
+						 */
+						break;
+
+					default:
+						elog(ERROR, "unexpected wait result");
+				}
+			}
+
+			/*
+			 * Loop back to recheck everything. Timeline might have changed
+			 * during our wait.
+			 */
 		}
 		else
 		{
diff --git a/src/backend/replication/walsender.c b/src/backend/replication/walsender.c
index a0e6a3d200c..105d826c693 100644
--- a/src/backend/replication/walsender.c
+++ b/src/backend/replication/walsender.c
@@ -1035,10 +1035,6 @@ StartReplication(StartReplicationCmd *cmd)
 /*
  * XLogReaderRoutine->page_read callback for logical decoding contexts, as a
  * walsender process.
- *
- * Inside the walsender we can do better than read_local_xlog_page,
- * which has to do a plain sleep/busy loop, because the walsender's latch gets
- * set every time WAL is flushed.
  */
 static int
 logical_read_xlog_page(XLogReaderState *state, XLogRecPtr targetPagePtr, int reqLen,
-- 
2.51.0