From f46f83ac016388213fd650b57007b70c3de473e6 Mon Sep 17 00:00:00 2001
From: Nathan Bossart <bossartn@amazon.com>
Date: Thu, 26 Mar 2020 17:08:26 +0000
Subject: [PATCH v3 1/1] Avoid marking WAL segments as ready-for-archive too
 early.

This is a proof-of-concept patch for relocating the logic that is
responsible for marking a segment as ready-for-archive to a
separate process.
---
 src/backend/access/transam/xlog.c  |  3 ---
 src/backend/postmaster/walwriter.c | 43 ++++++++++++++++++++++++++++++++++++++
 2 files changed, 43 insertions(+), 3 deletions(-)

diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index 7621fc05e2..1c4a4a83de 100644
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@@ -2564,9 +2564,6 @@ XLogWrite(XLogwrtRqst WriteRqst, bool flexible)
 
 				LogwrtResult.Flush = LogwrtResult.Write;	/* end of page */
 
-				if (XLogArchivingActive())
-					XLogArchiveNotifySeg(openLogSegNo);
-
 				XLogCtl->lastSegSwitchTime = (pg_time_t) time(NULL);
 				XLogCtl->lastSegSwitchLSN = LogwrtResult.Flush;
 
diff --git a/src/backend/postmaster/walwriter.c b/src/backend/postmaster/walwriter.c
index 45a2757969..42a21d9130 100644
--- a/src/backend/postmaster/walwriter.c
+++ b/src/backend/postmaster/walwriter.c
@@ -45,6 +45,7 @@
 #include <unistd.h>
 
 #include "access/xlog.h"
+#include "access/xlog_internal.h"
 #include "libpq/pqsignal.h"
 #include "miscadmin.h"
 #include "pgstat.h"
@@ -91,6 +92,9 @@ WalWriterMain(void)
 	MemoryContext walwriter_context;
 	int			left_till_hibernate;
 	bool		hibernating;
+	XLogRecPtr	insert = (XLogRecPtr) 0;
+	bool		shouldGetCurrBytePos = true;
+	XLogSegNo	lastSegno = (XLogSegNo) 0;
 
 	/*
 	 * Properly accept or ignore signals the postmaster might send us
@@ -210,9 +214,21 @@ WalWriterMain(void)
 	/*
 	 * Loop forever
 	 */
+	shouldGetCurrBytePos = true;
 	for (;;)
 	{
 		long		cur_timeout;
+		XLogRecPtr	flush;
+
+		/*
+		 * Retrieve the WAL insert pointer, which is expected to always be on a
+		 * WAL record boundary.
+		 */
+		if (shouldGetCurrBytePos)
+		{
+			insert = GetXLogInsertRecPtr();
+			shouldGetCurrBytePos = false;
+		}
 
 		/*
 		 * Advertise whether we might hibernate in this cycle.  We do this
@@ -243,6 +259,33 @@ WalWriterMain(void)
 		else if (left_till_hibernate > 0)
 			left_till_hibernate--;
 
+		/*
+		 * If the current flush position is greater than or equal to the insert
+		 * position we recorded earlier, it is safe to mark all WAL segments up
+		 * to but excluding the current one as ready for archival.
+		 */
+		flush = GetFlushRecPtr();
+		if (flush >= insert)
+		{
+			XLogSegNo segno;
+
+			/* for proof-of-concept, assume default segment size */
+			XLByteToSeg(insert, segno, DEFAULT_XLOG_SEG_SIZE);
+
+			/* for proof-of-concept, initialize lastSegno to current segment */
+			if (lastSegno == 0)
+				lastSegno = segno;
+
+			if (XLogArchivingActive())
+			{
+				for (XLogSegNo i = lastSegno; i < segno; i++)
+					XLogArchiveNotifySeg(i);
+			}
+
+			lastSegno = segno;
+			shouldGetCurrBytePos = true;
+		}
+
 		/*
 		 * Sleep until we are signaled or WalWriterDelay has elapsed.  If we
 		 * haven't done anything useful for quite some time, lengthen the
-- 
2.16.6