Reset pg_stat_activity.xact_start during PREPARE TRANSACTION.

Once we've completed a PREPARE, our session is not running a transaction,
so its entry in pg_stat_activity should show xact_start as null, rather
than leaving the value as the start time of the now-prepared transaction.

I think possibly this oversight was triggered by faulty extrapolation
from the adjacent comment that says PrepareTransaction should not call
AtEOXact_PgStat, so tweak the wording of that comment.

Noted by Andres Freund while considering bug #10123 from Maxim Boguk,
although this error doesn't seem to explain that report.

Back-patch to all active branches.

diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c
index 2b103f4255933ca02cd773b3cc10963bbf8c206b..463a49840477b1259d4b312d75196457265bcd82 100644 (file)
--- a/src/backend/access/transam/xact.c
+++ b/src/backend/access/transam/xact.c
@@ -2247,8 +2247,9 @@ PrepareTransaction(void)
        AtEOXact_Files();
        AtEOXact_ComboCid();
        AtEOXact_HashTables(true);
-       /* don't call AtEOXact_PgStat here */
+       /* don't call AtEOXact_PgStat here; we fixed pgstat state above */
        AtEOXact_Snapshot(true);
+       pgstat_report_xact_timestamp(0);
 
        CurrentResourceOwner = NULL;
        ResourceOwnerDelete(TopTransactionResourceOwner);