pg_stat_activity: show NULL stmt start time for walsenders

Returning a non-NULL time is pointless, sinc a walsender is not a
process that would be running normal transactions anyway, but the code
was unintentionally exposing the process start time intermittently,
which was not only bogus but it also confused monitoring systems looking
for idle transactions.  Fix by avoiding all updates in walsenders.

Backpatch to 11, where walsenders started appearing in pg_stat_activity.

Reported-by: Tomas Vondra
Discussion: https://postgr.es/m/20191209234409.exe7osmyalwkt5j4@development

diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c
index bd396f0f08fa1e70a7a66b844fa0294d9038149e..075b243183378b33311ffdab3e09666cdac8a087 100644 (file)
--- a/src/backend/access/transam/xact.c
+++ b/src/backend/access/transam/xact.c
@@ -817,6 +817,13 @@ GetCurrentTransactionStopTimestamp(void)
 void
 SetCurrentStatementStartTimestamp(void)
 {
+   /*
+    * Skip if on a walsender; this is not needed, and it confuses monitoring
+    * if we publish non-NULL values.
+    */
+   if (am_walsender)
+       return;
+
    if (!IsParallelWorker())
        stmtStartTimestamp = GetCurrentTimestamp();
    else