pgstat's on-proc-exit hook has to execute after the last transaction commit

or abort within a backend; rearrange InitPostgres processing to make it so.
Revealed by just-added Asserts along with ECPG regression tests (hm, I wonder
why the core regression tests didn't expose it?).  This possibly is another
reason for missing stats updates ...

Author: tglsfdc

src/backend/postmaster/pgstat.c

@@ -13,7 +13,7 @@
  *
  *	Copyright (c) 2001-2007, PostgreSQL Global Development Group
  *
- *	$PostgreSQL: pgsql/src/backend/postmaster/pgstat.c,v 1.156 2007/05/27 03:50:39 tgl Exp $
+ *	$PostgreSQL: pgsql/src/backend/postmaster/pgstat.c,v 1.157 2007/05/27 05:37:49 tgl Exp $
  * ----------
  */
 #include "postgres.h"
@@ -1771,28 +1771,45 @@ CreateSharedBackendStatus(void)
 }
 
 
+/* ----------
+ * pgstat_initialize() -
+ *
+ *	Initialize pgstats state, and set up our on-proc-exit hook.
+ *	Called from InitPostgres.  MyBackendId must be set,
+ *	but we must not have started any transaction yet (since the
+ *	exit hook must run after the last transaction exit).
+ * ----------
+ */
+void
+pgstat_initialize(void)
+{
+	/* Initialize MyBEEntry */
+	Assert(MyBackendId >= 1 && MyBackendId <= MaxBackends);
+	MyBEEntry = &BackendStatusArray[MyBackendId - 1];
+
+	/* Set up a process-exit hook to clean up */
+	on_shmem_exit(pgstat_beshutdown_hook, 0);
+}
+
 /* ----------
  * pgstat_bestart() -
  *
- *	Initialize this backend's entry in the PgBackendStatus array,
- *	and set up an on-proc-exit hook that will clear it again.
- *	Called from InitPostgres.  MyBackendId and MyDatabaseId must be set.
+ *	Initialize this backend's entry in the PgBackendStatus array.
+ *	Called from InitPostgres.  MyDatabaseId and session userid must be set
+ *	(hence, this cannot be combined with pgstat_initialize).
  * ----------
  */
 void
 pgstat_bestart(void)
 {
-	volatile PgBackendStatus *beentry;
 	TimestampTz proc_start_timestamp;
 	Oid			userid;
 	SockAddr	clientaddr;
-
-	Assert(MyBackendId >= 1 && MyBackendId <= MaxBackends);
-	MyBEEntry = &BackendStatusArray[MyBackendId - 1];
+	volatile PgBackendStatus *beentry;
 
 	/*
-	 * To minimize the time spent modifying the entry, fetch all the needed
-	 * data first.
+	 * To minimize the time spent modifying the PgBackendStatus entry,
+	 * fetch all the needed data first.
 	 *
 	 * If we have a MyProcPort, use its session start time (for consistency,
 	 * and to save a kernel call).
@@ -1839,11 +1856,6 @@ pgstat_bestart(void)
 
 	beentry->st_changecount++;
 	Assert((beentry->st_changecount & 1) == 0);
-
-	/*
-	 * Set up a process-exit hook to clean up.
-	 */
-	on_shmem_exit(pgstat_beshutdown_hook, 0);
 }
 
 /*

src/backend/utils/init/postinit.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/utils/init/postinit.c,v 1.175 2007/03/13 00:33:42 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/utils/init/postinit.c,v 1.176 2007/05/27 05:37:49 tgl Exp $
  *
  *
  *-------------------------------------------------------------------------
@@ -435,6 +435,10 @@ InitPostgres(const char *in_dbname, Oid dboid, const char *username,
 	/* Initialize portal manager */
 	EnablePortalManager();
 
+	/* Initialize stats collection --- must happen before first xact */
+	if (!bootstrap)
+		pgstat_initialize();
+
 	/*
 	 * Set up process-exit callback to do pre-shutdown cleanup.  This has to
 	 * be after we've initialized all the low-level modules like the buffer
@@ -587,7 +591,7 @@ InitPostgres(const char *in_dbname, Oid dboid, const char *username,
 	/* initialize client encoding */
 	InitializeClientEncoding();
 
-	/* initialize statistics collection for this backend */
+	/* report this backend in the PgBackendStatus array */
 	if (!bootstrap)
 		pgstat_bestart();
 

src/include/pgstat.h

@@ -5,7 +5,7 @@
  *
  *	Copyright (c) 2001-2007, PostgreSQL Global Development Group
  *
- *	$PostgreSQL: pgsql/src/include/pgstat.h,v 1.59 2007/05/27 03:50:39 tgl Exp $
+ *	$PostgreSQL: pgsql/src/include/pgstat.h,v 1.60 2007/05/27 05:37:50 tgl Exp $
  * ----------
  */
 #ifndef PGSTAT_H
@@ -501,7 +501,9 @@ extern void pgstat_report_analyze(Oid tableoid, bool shared,
 					  PgStat_Counter livetuples,
 					  PgStat_Counter deadtuples);
 
+extern void pgstat_initialize(void);
 extern void pgstat_bestart(void);
+
 extern void pgstat_report_activity(const char *what);
 extern void pgstat_report_txn_timestamp(TimestampTz tstamp);
 extern void pgstat_report_waiting(bool waiting);