From 60aa2e18df060fcc341847585aa8cd4dcddea5ed Mon Sep 17 00:00:00 2001
From: Bharath Rupireddy <rupiredd@amazon.com>
Date: Sun, 14 Sep 2025 04:54:12 +0000
Subject: [PATCH v3] Use WALReadFromBuffers in more places

Commit 91f2cae introduced WALReadFromBuffers but used it only for
physical replication walsenders. There are several other callers
that use the read_local_xlog_page page_read callback, and logical
replication walsenders can also benefit from reading WAL from WAL
buffers using the new function. This commit extends the use of
WALReadFromBuffers to these callers.

Author: Bharath Rupireddy
Reviewed-by: Jingtang Zhang, Nitin Jadhav
Discussion: https://www.postgresql.org/message-id/CALj2ACVfF2Uj9NoFy-5m98HNtjHpuD17EDE9twVeJng-jTAe7A%40mail.gmail.com
---
 src/backend/access/transam/xlogutils.c | 23 +++++++-
 src/backend/replication/walsender.c    | 77 +++++++++++++++++---------
 2 files changed, 70 insertions(+), 30 deletions(-)

diff --git a/src/backend/access/transam/xlogutils.c b/src/backend/access/transam/xlogutils.c
index 38176d9688e..3e5b4f75feb 100644
--- a/src/backend/access/transam/xlogutils.c
+++ b/src/backend/access/transam/xlogutils.c
@@ -876,6 +876,7 @@ read_local_xlog_page_guts(XLogReaderState *state, XLogRecPtr targetPagePtr,
 	int			count;
 	WALReadError errinfo;
 	TimeLineID	currTLI;
+	Size		bytesRead;
 
 	loc = targetPagePtr + reqLen;
 
@@ -995,9 +996,25 @@ read_local_xlog_page_guts(XLogReaderState *state, XLogRecPtr targetPagePtr,
 		count = read_upto - targetPagePtr;
 	}
 
-	if (!WALRead(state, cur_page, targetPagePtr, count, tli,
-				 &errinfo))
-		WALReadRaiseError(&errinfo);
+	/* First attempt to read from WAL buffers */
+	bytesRead = WALReadFromBuffers(cur_page, targetPagePtr, count, currTLI);
+
+	/* If we still have bytes to read, get them from WAL file */
+	if (bytesRead < count)
+	{
+		if (!WALRead(state,
+					 cur_page + bytesRead,
+					 targetPagePtr + bytesRead,
+					 count - bytesRead,
+					 tli,
+					 &errinfo))
+		{
+			WALReadRaiseError(&errinfo);
+		}
+		bytesRead = count;		/* All requested bytes read */
+	}
+
+	Assert(bytesRead == count);
 
 	/* number of valid bytes in the buffer */
 	return count;
diff --git a/src/backend/replication/walsender.c b/src/backend/replication/walsender.c
index 59822f22b8d..937936c3550 100644
--- a/src/backend/replication/walsender.c
+++ b/src/backend/replication/walsender.c
@@ -1036,6 +1036,7 @@ logical_read_xlog_page(XLogReaderState *state, XLogRecPtr targetPagePtr, int req
 	WALReadError errinfo;
 	XLogSegNo	segno;
 	TimeLineID	currTLI;
+	Size		bytesRead;
 
 	/*
 	 * Make sure we have enough WAL available before retrieving the current
@@ -1073,16 +1074,29 @@ logical_read_xlog_page(XLogReaderState *state, XLogRecPtr targetPagePtr, int req
 	else
 		count = flushptr - targetPagePtr;	/* part of the page available */
 
-	/* now actually read the data, we know it's there */
-	if (!WALRead(state,
-				 cur_page,
-				 targetPagePtr,
-				 count,
-				 currTLI,		/* Pass the current TLI because only
+	/* First attempt to read from WAL buffers */
+	bytesRead = WALReadFromBuffers(cur_page, targetPagePtr, count, currTLI);
+
+	targetPagePtr += bytesRead;
+
+	/* If we still have bytes to read, get them from WAL file */
+	if (bytesRead < count)
+	{
+		if (!WALRead(state,
+					 cur_page + bytesRead,
+					 targetPagePtr,
+					 count - bytesRead,
+					 currTLI,	/* Pass the current TLI because only
 								 * WalSndSegmentOpen controls whether new TLI
 								 * is needed. */
-				 &errinfo))
-		WALReadRaiseError(&errinfo);
+					 &errinfo))
+		{
+			WALReadRaiseError(&errinfo);
+		}
+		bytesRead = count;		/* All requested bytes read */
+	}
+
+	Assert(bytesRead == count);
 
 	/*
 	 * After reading into the buffer, check that what we read was valid. We do
@@ -3176,7 +3190,7 @@ XLogSendPhysical(void)
 	Size		nbytes;
 	XLogSegNo	segno;
 	WALReadError errinfo;
-	Size		rbytes;
+	Size		bytesRead;
 
 	/* If requested switch the WAL sender to the stopping state. */
 	if (got_STOPPING)
@@ -3392,24 +3406,33 @@ XLogSendPhysical(void)
 	enlargeStringInfo(&output_message, nbytes);
 
 retry:
-	/* attempt to read WAL from WAL buffers first */
-	rbytes = WALReadFromBuffers(&output_message.data[output_message.len],
-								startptr, nbytes, xlogreader->seg.ws_tli);
-	output_message.len += rbytes;
-	startptr += rbytes;
-	nbytes -= rbytes;
-
-	/* now read the remaining WAL from WAL file */
-	if (nbytes > 0 &&
-		!WALRead(xlogreader,
-				 &output_message.data[output_message.len],
-				 startptr,
-				 nbytes,
-				 xlogreader->seg.ws_tli,	/* Pass the current TLI because
-											 * only WalSndSegmentOpen controls
-											 * whether new TLI is needed. */
-				 &errinfo))
-		WALReadRaiseError(&errinfo);
+	/* First attempt to read from WAL buffers */
+	bytesRead = WALReadFromBuffers(&output_message.data[output_message.len],
+								   startptr,
+								   nbytes,
+								   xlogreader->seg.ws_tli);
+
+	startptr += bytesRead;
+
+	/* If we still have bytes to read, get them from WAL file */
+	if (bytesRead < nbytes)
+	{
+		if (!WALRead(xlogreader,
+					 &output_message.data[output_message.len + bytesRead],
+					 startptr,
+					 nbytes - bytesRead,
+					 xlogreader->seg.ws_tli,	/* Pass the current TLI
+												 * because only
+												 * WalSndSegmentOpen controls
+												 * whether new TLI is needed. */
+					 &errinfo))
+		{
+			WALReadRaiseError(&errinfo);
+		}
+		bytesRead = nbytes;		/* All requested bytes read */
+	}
+
+	Assert(bytesRead == nbytes);
 
 	/* See logical_read_xlog_page(). */
 	XLByteToSeg(startptr, segno, xlogreader->segcxt.ws_segsize);
-- 
2.47.3