From 82029dca028c9427a821ef06e27e28198ce1aba6 Mon Sep 17 00:00:00 2001
From: Imran Zaheer <imran.zhir@gmail.com>
Date: Sat, 15 Aug 2026 23:50:33 +0900
Subject: [PATCH v3] Fix checkpointer restartpoint assertion failure

When recovery starts from a backup without a signal file, pg_subtrans
is not started at the beginning of recovery and remains unstarted
throughout recovery. However, previously, a restartpoint run by
the checkpointer during recovery nevertheless tried to truncate
pg_subtrans, triggering the assertion failure:

    TRAP: failed Assert("TransactionIdIsValid(initial)")

This commit fixes this by tracking whether pg_subtrans has been
started during recovery, and have the checkpointer check this flag
before truncating pg_subtrans at restartpoints.

Backpatch to all supported versions.
---
 src/backend/access/transam/xlog.c         |  7 ++---
 src/backend/access/transam/xlogrecovery.c | 32 +++++++++++++++++++++++
 src/include/access/xlogrecovery.h         |  2 ++
 3 files changed, 38 insertions(+), 3 deletions(-)

diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index f79961633a6..e9508fe9d7b 100644
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@@ -5260,6 +5260,7 @@ StartupXLOG(void)
 			 * during recovery and need not be started yet.
 			 */
 			StartupSUBTRANS(oldestActiveXID);
+			SetRecoverySubtransInitialized();
 
 			/*
 			 * If we're beginning at a shutdown checkpoint, we know that
@@ -7243,10 +7244,10 @@ CreateRestartPoint(int flags)
 	 * Truncate pg_subtrans if possible.  We can throw away all data before
 	 * the oldest XMIN of any running transaction.  No future transaction will
 	 * attempt to reference any pg_subtrans entry older than that (see Asserts
-	 * in subtrans.c).  When hot standby is disabled, though, we mustn't do
-	 * this because StartupSUBTRANS hasn't been called yet.
+	 * in subtrans.c).  During recovery, don't truncate pg_subtrans until hot
+	 * standby initialization has started it.
 	 */
-	if (EnableHotStandby)
+	if (RecoverySubtransInitialized())
 		TruncateSUBTRANS(GetOldestTransactionIdConsideredRunning());
 
 	/* Real work is done; log and update stats. */
diff --git a/src/backend/access/transam/xlogrecovery.c b/src/backend/access/transam/xlogrecovery.c
index b07a54a9216..26738af34ba 100644
--- a/src/backend/access/transam/xlogrecovery.c
+++ b/src/backend/access/transam/xlogrecovery.c
@@ -313,6 +313,12 @@ typedef struct XLogRecoveryCtlData
 	 */
 	bool		SharedPromoteIsTriggered;
 
+	/*
+	 * SharedRecoverySubtransInitialized indicates whether hot standby
+	 * initialization has started pg_subtrans. Protected by info_lck.
+	 */
+	bool		SharedRecoverySubtransInitialized;
+
 	/*
 	 * recoveryWakeupLatch is used to wake up the startup process to continue
 	 * WAL replay, if it is waiting for WAL to arrive or failover trigger file
@@ -4492,6 +4498,32 @@ CheckPromoteSignal(void)
 	return false;
 }
 
+/*
+ * Has hot standby initialization started pg_subtrans?
+ */
+bool
+RecoverySubtransInitialized(void)
+{
+	bool		result;
+
+	SpinLockAcquire(&XLogRecoveryCtl->info_lck);
+	result = XLogRecoveryCtl->SharedRecoverySubtransInitialized;
+	SpinLockRelease(&XLogRecoveryCtl->info_lck);
+
+	return result;
+}
+
+/*
+ * Remember that hot standby initialization has started pg_subtrans.
+ */
+void
+SetRecoverySubtransInitialized(void)
+{
+	SpinLockAcquire(&XLogRecoveryCtl->info_lck);
+	XLogRecoveryCtl->SharedRecoverySubtransInitialized = true;
+	SpinLockRelease(&XLogRecoveryCtl->info_lck);
+}
+
 /*
  * Wake up startup process to replay newly arrived WAL, or to notice that
  * failover has been requested.
diff --git a/src/include/access/xlogrecovery.h b/src/include/access/xlogrecovery.h
index 0aa85d90e89..55ccbe23f0b 100644
--- a/src/include/access/xlogrecovery.h
+++ b/src/include/access/xlogrecovery.h
@@ -145,6 +145,8 @@ extern XLogRecPtr GetCurrentReplayRecPtr(TimeLineID *replayEndTLI);
 
 extern bool PromoteIsTriggered(void);
 extern bool CheckPromoteSignal(void);
+extern bool RecoverySubtransInitialized(void);
+extern void SetRecoverySubtransInitialized(void);
 extern void WakeupRecovery(void);
 
 extern void StartupRequestWalReceiverRestart(void);
-- 
2.55.0