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 017f03b6d8ce35d4170f38fef5cfaeb77f5b5a7f..cbc35454a7e3836268cf06effac0517b38475c74 100644 (file)
--- a/src/backend/access/transam/xact.c
+++ b/src/backend/access/transam/xact.c
@@ -816,6 +816,13 @@ 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