0001-Add-a-hook-for-Checkpoint-processing.patch

From a0e8d75223fa95dbec1e422eacaef336e45c2008 Mon Sep 17 00:00:00 2001
From: "Andrei V. Lepikhov" <lepihov@gmail.com>
Date: Thu, 13 Nov 2025 15:00:43 +0100
Subject: [PATCH 1/2] Add a hook for Checkpoint processing.

There are many situations in which a Postgres plugin may need to maintain its
internal state across restarts or crashes. Sometimes it wants to synchronise
its state on logical replicas or be saved in a backup employing custom RMGR and
WAL records.

For statistical extensions, such as pg_stat_statements, it is okay to save
their state on postmaster shutdown. However, business extensions may want
to maintain more actual state, periodically dumping it to a disk file or using
WAL and xact callbacks to be as close as possible to the current
database state.

Checkpoint is a key moment where the DBMS performs disk synchronisation and
cuts the WAL. It is a good time to do the same thing for a plugin, too.
Moreover, the plugin is sure that nothing important will be lost with
the WAL cut.

Discussion: https://www.postgresql.org/message-id/CANbhV-E4pTWeF-DsdaGsOrjJNFWPaR%2BDstjrnkqvf9JFFgOKKQ%40mail.gmail.com
---
 src/backend/access/transam/xlog.c | 15 +++++++++++++++
 src/include/access/xlog.h         |  4 ++++
 src/tools/pgindent/typedefs.list  |  1 +
 3 files changed, 20 insertions(+)

diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index 22d0a2e8c3a..c7c0b226724 100644
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@@ -157,6 +157,13 @@ int			wal_segment_size = DEFAULT_XLOG_SEG_SIZE;
  */
 int			CheckPointSegments;
 
+/*
+ * Hook for plugins to take control during checkpoint processing. All
+ * preparation procedures have already been done, and only the sync needs
+ * to be done.
+ */
+Checkpoint_hook_type Checkpoint_hook = NULL;
+
 /* Estimated distance between checkpoints, in bytes */
 static double CheckPointDistanceEstimate = 0;
 static double PrevCheckPointDistance = 0;
@@ -7594,6 +7601,14 @@ CheckPointGuts(XLogRecPtr checkPointRedo, int flags)
 	CheckPointPredicate();
 	CheckPointBuffers(flags);
 
+	/*
+	 * Allow a plugin that depends on a custom RMGR to retain its state through
+	 * reboots or crashes by following specific steps, ensuring that essential
+	 * WAL records are not truncated.
+	 */
+	if (Checkpoint_hook)
+		Checkpoint_hook(checkPointRedo, flags);
+
 	/* Perform all queued up fsyncs */
 	TRACE_POSTGRESQL_BUFFER_CHECKPOINT_SYNC_START();
 	CheckpointStats.ckpt_sync_t = GetCurrentTimestamp();
diff --git a/src/include/access/xlog.h b/src/include/access/xlog.h
index 605280ed8fb..5c071974557 100644
--- a/src/include/access/xlog.h
+++ b/src/include/access/xlog.h
@@ -198,6 +198,10 @@ typedef enum WALAvailability
 struct XLogRecData;
 struct XLogReaderState;
 
+/* Hook for plugins to get control at the end of a CheckPoint */
+typedef void (*Checkpoint_hook_type)(XLogRecPtr checkPointRedo, int flags);
+extern PGDLLIMPORT Checkpoint_hook_type Checkpoint_hook;
+
 extern XLogRecPtr XLogInsertRecord(struct XLogRecData *rdata,
 								   XLogRecPtr fpw_lsn,
 								   uint8 flags,
diff --git a/src/tools/pgindent/typedefs.list b/src/tools/pgindent/typedefs.list
index 23bce72ae64..6ca05499081 100644
--- a/src/tools/pgindent/typedefs.list
+++ b/src/tools/pgindent/typedefs.list
@@ -413,6 +413,7 @@ CatalogIndexState
 ChangeVarNodes_callback
 ChangeVarNodes_context
 CheckPoint
+Checkpoint_hook_type
 CheckPointStmt
 CheckpointStatsData
 CheckpointerRequest
-- 
2.51.2