Date: Tue, 10 Jan 2023 17:08:36 -0800
From: Nathan Bossart <nathandbossart@gmail.com>
To: pgsql-hackers@postgresql.org
Subject: delay starting WAL receiver
Message-ID: <20230111010836.GA1550875@nathanxps13>
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I discussed this a bit in a different thread [0], but I thought it deserved
its own thread.

After setting wal_retrieve_retry_interval to 1ms in the tests, I noticed
that the recovery tests consistently take much longer.  Upon further
inspection, it looks like a similar race condition to the one described in
e5d494d's commit message.  With some added debug logs, I see that all of
the callers of MaybeStartWalReceiver() complete before SIGCHLD is
processed, so ServerLoop() waits for a minute before starting the WAL
receiver.

The attached patch fixes this by adjusting DetermineSleepTime() to limit
the sleep to at most 100ms when WalReceiverRequested is set, similar to how
the sleep is limited when background workers must be restarted.

[0] https://postgr.es/m/20221215224721.GA694065%40nathanxps13

-- 
Nathan Bossart
Amazon Web Services: https://aws.amazon.com

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	filename="v1-0001-handle-race-condition-when-restarting-wal-receive.patch"

From 6f32238f119236322dfb16262a88c3a9c5141413 Mon Sep 17 00:00:00 2001
From: Nathan Bossart <nathandbossart@gmail.com>
Date: Thu, 15 Dec 2022 14:11:43 -0800
Subject: [PATCH v1 1/1] handle race condition when restarting wal receivers

---
 src/backend/postmaster/postmaster.c | 28 ++++++++++++++++++++++------
 1 file changed, 22 insertions(+), 6 deletions(-)

diff --git a/src/backend/postmaster/postmaster.c b/src/backend/postmaster/postmaster.c
index eac3450774..9d6f58e0b3 100644
--- a/src/backend/postmaster/postmaster.c
+++ b/src/backend/postmaster/postmaster.c
@@ -1601,9 +1601,9 @@ checkControlFile(void)
  *
  * In normal conditions we wait at most one minute, to ensure that the other
  * background tasks handled by ServerLoop get done even when no requests are
- * arriving.  However, if there are background workers waiting to be started,
- * we don't actually sleep so that they are quickly serviced.  Other exception
- * cases are as shown in the code.
+ * arriving.  However, if there are background workers or a WAL receiver
+ * waiting to be started, we make sure they are quickly serviced.  Other
+ * exception cases are as shown in the code.
  */
 static void
 DetermineSleepTime(struct timeval *timeout)
@@ -1611,11 +1611,12 @@ DetermineSleepTime(struct timeval *timeout)
 	TimestampTz next_wakeup = 0;
 
 	/*
-	 * Normal case: either there are no background workers at all, or we're in
-	 * a shutdown sequence (during which we ignore bgworkers altogether).
+	 * Normal case: either there are no background workers and no WAL receiver
+	 * at all, or we're in a shutdown sequence (during which we ignore
+	 * bgworkers and the WAL receiver altogether).
 	 */
 	if (Shutdown > NoShutdown ||
-		(!StartWorkerNeeded && !HaveCrashedWorker))
+		(!StartWorkerNeeded && !HaveCrashedWorker && !WalReceiverRequested))
 	{
 		if (AbortStartTime != 0)
 		{
@@ -1674,6 +1675,21 @@ DetermineSleepTime(struct timeval *timeout)
 		}
 	}
 
+	/*
+	 * If WalReceiverRequested is set, we're probably waiting on a SIGCHLD to
+	 * arrive to clear WalReceiverPID before starting the new WAL receiver.  We
+	 * don't expect that to take long, so limit the sleep to 100ms so that we
+	 * start the new WAL receiver promptly.
+	 */
+	if (WalReceiverRequested)
+	{
+		TimestampTz walrcv_wakeup;
+
+		walrcv_wakeup = TimestampTzPlusMilliseconds(GetCurrentTimestamp(), 100);
+		if (next_wakeup == 0 || walrcv_wakeup < next_wakeup)
+			next_wakeup = walrcv_wakeup;
+	}
+
 	if (next_wakeup != 0)
 	{
 		long		secs;
-- 
2.25.1


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