Release synchronous replication waiters immediately on configuration changes.

Previously, when synchronous_standby_names was changed (for example,
by reducing the number of required synchronous standbys or modifying
the standby list), backends waiting for synchronous replication were not
released immediately, even if the new configuration no longer required them
to wait. They could remain blocked until additional messages arrived from
standbys and triggered their release.

This commit improves walsender so that backends waiting for synchronous
replication are released as soon as the updated configuration takes effect and
the new settings no longer require them to wait, by calling
SyncRepReleaseWaiters() when configuration changes are processed.

As part of this change, the duplicated code that handles configuration changes
in walsender has been refactored into a new helper function, which is now used
at the three existing call places.

Since this is an improvement rather than a bug fix, it is applied only to
the master branch.

Author: Shinya Kato <shinya11.kato@gmail.com>
Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Reviewed-by: Fujii Masao <masao.fujii@gmail.com>
Reviewed-by: Xuneng Zhou <xunengzhou@gmail.com>
Discussion: https://postgr.es/m/CAOzEurSRii0tEYhu5cePmRcvS=ZrxTLEvxm3Kj0d7_uKGdM23g@mail.gmail.com

diff --git a/src/backend/replication/walsender.c b/src/backend/replication/walsender.c
index a0e6a3d200c9f27cb48693f09d1508936a1794ff..2cde8ebc7296c4a2ea320dc34916a44891a4b5f4 100644 (file)
--- a/src/backend/replication/walsender.c
+++ b/src/backend/replication/walsender.c
@@ -1611,6 +1611,32 @@ WalSndWriteData(LogicalDecodingContext *ctx, XLogRecPtr lsn, TransactionId xid,
        ProcessPendingWrites();
 }
 
+/*
+ * Handle configuration reload.
+ *
+ * Process the pending configuration file reload and reinitializes synchronous
+ * replication settings. Also releases any waiters that may now be satisfied due
+ * to changes in synchronous replication requirements.
+ */
+static void
+WalSndHandleConfigReload(void)
+{
+       if (!ConfigReloadPending)
+               return;
+
+       ConfigReloadPending = false;
+       ProcessConfigFile(PGC_SIGHUP);
+       SyncRepInitConfig();
+
+       /*
+        * Recheck and release any now-satisfied waiters after config reload
+        * changes synchronous replication requirements (e.g., reducing the number
+        * of sync standbys or changing the standby names).
+        */
+       if (!am_cascading_walsender)
+               SyncRepReleaseWaiters();
+}
+
 /*
  * Wait until there is no pending write. Also process replies from the other
  * side and check timeouts during that.
@@ -1646,12 +1672,7 @@ ProcessPendingWrites(void)
                CHECK_FOR_INTERRUPTS();
 
                /* Process any requests or signals received recently */
-               if (ConfigReloadPending)
-               {
-                       ConfigReloadPending = false;
-                       ProcessConfigFile(PGC_SIGHUP);
-                       SyncRepInitConfig();
-               }
+               WalSndHandleConfigReload();
 
                /* Try to flush pending output to the client */
                if (pq_flush_if_writable() != 0)
@@ -1854,12 +1875,7 @@ WalSndWaitForWal(XLogRecPtr loc)
                CHECK_FOR_INTERRUPTS();
 
                /* Process any requests or signals received recently */
-               if (ConfigReloadPending)
-               {
-                       ConfigReloadPending = false;
-                       ProcessConfigFile(PGC_SIGHUP);
-                       SyncRepInitConfig();
-               }
+               WalSndHandleConfigReload();
 
                /* Check for input from the client */
                ProcessRepliesIfAny();
@@ -2899,12 +2915,7 @@ WalSndLoop(WalSndSendDataCallback send_data)
                CHECK_FOR_INTERRUPTS();
 
                /* Process any requests or signals received recently */
-               if (ConfigReloadPending)
-               {
-                       ConfigReloadPending = false;
-                       ProcessConfigFile(PGC_SIGHUP);
-                       SyncRepInitConfig();
-               }
+               WalSndHandleConfigReload();
 
                /* Check for input from the client */
                ProcessRepliesIfAny();