v3-0002-Add-pg_last_wal_write_lsn-SQL-function.patch

From 9d22e09d378e8f6c52aa95bc4a0e1650f4621a39 Mon Sep 17 00:00:00 2001
From: alterego655 <824662526@qq.com>
Date: Tue, 25 Nov 2025 19:07:52 +0800
Subject: [PATCH v3 2/5] Add pg_last_wal_write_lsn() SQL function

Returns the current WAL write position on a standby server using
GetWalRcvWriteRecPtr(). This enables verification of WAIT FOR LSN MODE WRITE
and operational monitoring of standby WAL write progress.
---
 doc/src/sgml/func/func-admin.sgml      | 22 ++++++++++++++++++++++
 src/backend/access/transam/xlogfuncs.c | 20 ++++++++++++++++++++
 src/include/catalog/pg_proc.dat        |  4 ++++
 3 files changed, 46 insertions(+)

diff --git a/doc/src/sgml/func/func-admin.sgml b/doc/src/sgml/func/func-admin.sgml
index 1b465bc8ba7..9ff196c4be4 100644
--- a/doc/src/sgml/func/func-admin.sgml
+++ b/doc/src/sgml/func/func-admin.sgml
@@ -688,6 +688,28 @@ postgres=# SELECT '0/0'::pg_lsn + pd.segment_number * ps.setting::int + :offset
        </para></entry>
       </row>
 
+      <row>
+       <entry role="func_table_entry"><para role="func_signature">
+        <indexterm>
+         <primary>pg_last_wal_write_lsn</primary>
+        </indexterm>
+        <function>pg_last_wal_write_lsn</function> ()
+        <returnvalue>pg_lsn</returnvalue>
+       </para>
+       <para>
+        Returns the last write-ahead log location that has been received and
+        passed to the operating system by streaming replication, but not
+        necessarily synced to durable storage.  This is faster than
+        <function>pg_last_wal_receive_lsn</function> but provides weaker
+        durability guarantees since the data may still be in OS buffers.
+        While streaming replication is in progress this will increase
+        monotonically. If recovery has completed then this will remain static
+        at the location of the last WAL record written during recovery. If
+        streaming replication is disabled, or if it has not yet started, the
+        function returns <literal>NULL</literal>.
+       </para></entry>
+      </row>
+
       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <indexterm>
diff --git a/src/backend/access/transam/xlogfuncs.c b/src/backend/access/transam/xlogfuncs.c
index 3e45fce43ed..2797b2bf158 100644
--- a/src/backend/access/transam/xlogfuncs.c
+++ b/src/backend/access/transam/xlogfuncs.c
@@ -347,6 +347,26 @@ pg_last_wal_receive_lsn(PG_FUNCTION_ARGS)
 	PG_RETURN_LSN(recptr);
 }
 
+/*
+ * Report the last WAL write location (same format as pg_backup_start etc)
+ *
+ * This is useful for determining how much of WAL has been received and
+ * passed to the operating system by walreceiver.  Unlike pg_last_wal_receive_lsn,
+ * this data may still be in OS buffers and not yet synced to durable storage.
+ */
+Datum
+pg_last_wal_write_lsn(PG_FUNCTION_ARGS)
+{
+	XLogRecPtr	recptr;
+
+	recptr = GetWalRcvWriteRecPtr();
+
+	if (!XLogRecPtrIsValid(recptr))
+		PG_RETURN_NULL();
+
+	PG_RETURN_LSN(recptr);
+}
+
 /*
  * Report the last WAL replay location (same format as pg_backup_start etc)
  *
diff --git a/src/include/catalog/pg_proc.dat b/src/include/catalog/pg_proc.dat
index 66431940700..478e0a8139f 100644
--- a/src/include/catalog/pg_proc.dat
+++ b/src/include/catalog/pg_proc.dat
@@ -6782,6 +6782,10 @@
   proname => 'pg_last_wal_receive_lsn', provolatile => 'v',
   prorettype => 'pg_lsn', proargtypes => '',
   prosrc => 'pg_last_wal_receive_lsn' },
+{ oid => '6434', descr => 'last wal write location on standby',
+  proname => 'pg_last_wal_write_lsn', provolatile => 'v',
+  prorettype => 'pg_lsn', proargtypes => '',
+  prosrc => 'pg_last_wal_write_lsn' },
 { oid => '3821', descr => 'last wal replay location',
   proname => 'pg_last_wal_replay_lsn', provolatile => 'v',
   prorettype => 'pg_lsn', proargtypes => '',
-- 
2.51.0