From: Robert Haas <rhaas@postgresql.org>
Date: Wed, 27 Oct 2021 16:56:34 +0000 (-0400)
Subject: Keep shared-memory copy of ThisTimeLineID in sync at all times.
X-Git-Url: http://git.postgresql.org/gitweb/static/developers.postgresql.org?a=commitdiff_plain;h=f06ced35cd9efd54056516f03b3680976dfa74bd;p=users%2Frhaas%2Fpostgres.git

Keep shared-memory copy of ThisTimeLineID in sync at all times.

Previously, the startup process set the global variable ThisTimeLineID
for each record replayed, while leaving the corresponding shared-memory
variable uninitialized. Now, XLogCtl->ThisTimeLineID is always
the same as ThisTimeLineID, and XLogCtl->PrevTimeLineID is also
the same unless a promotion has occurred since server startup.
---

diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index f547efd294..c94ddd3756 100644
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@@ -634,9 +634,19 @@ typedef struct XLogCtlData
 
 	/*
 	 * Shared copy of ThisTimeLineID. Does not change after end-of-recovery.
-	 * If we created a new timeline when the system was started up,
-	 * PrevTimeLineID is the old timeline's ID that we forked off from.
-	 * Otherwise it's equal to ThisTimeLineID.
+	 *
+	 * During recovery, ThisTimeLineID and PrevTimeLineID both hold the
+	 * timeline of the last record we replayed; they will always be equal.
+	 *
+	 * At the end of recovery, if we create a new time line, ThisTimeLineID
+	 * will be set to the newly-created timeline, and PrevTimeLineID will be
+	 * the timeline from which it branched off. If we do not create a new
+	 * timeline, ThisTimeLineID will be equal to PrevTimeLineID, and both
+	 * will be equal either to the timeline of the last WAL record we replayed,
+	 * or the timeline in the checkpoint from which we started up.
+	 *
+	 * At any point when it's possible to insert new WAL, ThisTimeLineID is
+	 * always the timeline that should be used to do that.
 	 */
 	TimeLineID	ThisTimeLineID;
 	TimeLineID	PrevTimeLineID;
@@ -6627,7 +6637,6 @@ StartupXLOG(void)
 				checkPointLoc,
 				EndOfLog;
 	TimeLineID	EndOfLogTLI;
-	TimeLineID	PrevTimeLineID;
 	XLogRecord *record;
 	TransactionId oldestActiveXID;
 	bool		backupEndRequired = false;
@@ -7140,9 +7149,11 @@ StartupXLOG(void)
 	/*
 	 * We must replay WAL entries using the same TimeLineID they were created
 	 * under, so temporarily adopt the TLI indicated by the checkpoint (see
-	 * also xlog_redo()).
+	 * also xlog_redo()). Update shared memory, too.
 	 */
 	ThisTimeLineID = checkPoint.ThisTimeLineID;
+	XLogCtl->ThisTimeLineID = ThisTimeLineID;
+	XLogCtl->PrevTimeLineID = ThisTimeLineID;
 
 	/*
 	 * Copy any missing timeline history files between 'now' and the recovery
@@ -7628,6 +7639,8 @@ StartupXLOG(void)
 
 						/* Following WAL records should be run with new TLI */
 						ThisTimeLineID = newTLI;
+						XLogCtl->ThisTimeLineID = ThisTimeLineID;
+						XLogCtl->PrevTimeLineID = ThisTimeLineID;
 						switchedTLI = true;
 					}
 				}
@@ -7917,7 +7930,6 @@ StartupXLOG(void)
 	 *
 	 * In a normal crash recovery, we can just extend the timeline we were in.
 	 */
-	PrevTimeLineID = ThisTimeLineID;
 	if (ArchiveRecoveryRequested)
 	{
 		char	   *reason;
@@ -7926,6 +7938,7 @@ StartupXLOG(void)
 		Assert(InArchiveRecovery);
 
 		ThisTimeLineID = findNewestTimeLine(recoveryTargetTLI) + 1;
+		XLogCtl->ThisTimeLineID = ThisTimeLineID;
 		ereport(LOG,
 				(errmsg("selected new timeline ID: %u", ThisTimeLineID)));
 
@@ -7964,10 +7977,6 @@ StartupXLOG(void)
 		unlink(recoveryPath);	/* ignore any error */
 	}
 
-	/* Save the selected TimeLineID in shared memory, too */
-	XLogCtl->ThisTimeLineID = ThisTimeLineID;
-	XLogCtl->PrevTimeLineID = PrevTimeLineID;
-
 	/*
 	 * Actually, if WAL ended in an incomplete record, skip the parts that
 	 * made it through and start writing after the portion that persisted.