Date: Fri, 25 Aug 2023 12:29:49 -0700
From: Nathan Bossart <nathandbossart@gmail.com>
To: Ian Lawrence Barwick <barwick@gmail.com>
Cc: pgsql-hackers <pgsql-hackers@postgresql.org>, robertmhaas@gmail.com
Subject: Re: pg_stat_get_backend_subxact() and backend IDs?
Message-ID: <20230825192949.GA1917227@nathanxps13>
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On Fri, Aug 25, 2023 at 08:32:51AM -0700, Nathan Bossart wrote:
> On second thought, renaming these exported functions so close to release is
> probably not a great idea.  I should probably skip back-patching that one.
> Or I could have the existing functions call the new ones in v16 for
> backward compatibility...

Here is a new version of the patch that avoids changing the names of the
existing functions.  I'm not thrilled about the name
(pgstat_fetch_stat_local_beentry_by_backend_id), so I am open to
suggestions.  In any case, I'd like to rename all three of the
pgstat_fetch_stat_* functions in v17.

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

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	filename="v3-0001-fix-pg_stat_get_backend_subxact-to-use-real-backe.patch"

From e1ce8bf67f1b713294ef8d38dbee26bfc7ef16f4 Mon Sep 17 00:00:00 2001
From: Nathan Bossart <nathan@postgresql.org>
Date: Thu, 24 Aug 2023 07:58:01 -0700
Subject: [PATCH v3 1/1] fix pg_stat_get_backend_subxact to use real backend id

---
 src/backend/utils/activity/backend_status.c | 39 ++++++++++++++++-----
 src/backend/utils/adt/pgstatfuncs.c         |  2 +-
 src/include/utils/backend_status.h          |  1 +
 3 files changed, 32 insertions(+), 10 deletions(-)

diff --git a/src/backend/utils/activity/backend_status.c b/src/backend/utils/activity/backend_status.c
index 38f91a495b..231cc5dd9a 100644
--- a/src/backend/utils/activity/backend_status.c
+++ b/src/backend/utils/activity/backend_status.c
@@ -1089,9 +1089,34 @@ cmp_lbestatus(const void *a, const void *b)
 PgBackendStatus *
 pgstat_fetch_stat_beentry(BackendId beid)
 {
-	LocalPgBackendStatus key;
 	LocalPgBackendStatus *ret;
 
+	ret = pgstat_fetch_stat_local_beentry_by_backend_id(beid);
+	if (ret)
+		return &ret->backendStatus;
+
+	return NULL;
+}
+
+
+/* ----------
+ * pgstat_fetch_stat_local_beentry_by_backend_id() -
+ *
+ *	Like pgstat_fetch_stat_beentry() but with locally computed additions
+ *	(like xid and xmin values of the backend)
+ *
+ *	The beid argument is the BackendId of the desired session
+ *	(note that this is unlike pgstat_fetch_stat_local_beentry()).
+ *
+ *	NB: caller is responsible for checking if the user is permitted to see this
+ *	info (especially the querystring).
+ * ----------
+ */
+LocalPgBackendStatus *
+pgstat_fetch_stat_local_beentry_by_backend_id(BackendId beid)
+{
+	LocalPgBackendStatus key;
+
 	pgstat_read_current_status();
 
 	/*
@@ -1099,14 +1124,10 @@ pgstat_fetch_stat_beentry(BackendId beid)
 	 * bsearch() to search it efficiently.
 	 */
 	key.backend_id = beid;
-	ret = (LocalPgBackendStatus *) bsearch(&key, localBackendStatusTable,
-										   localNumBackends,
-										   sizeof(LocalPgBackendStatus),
-										   cmp_lbestatus);
-	if (ret)
-		return &ret->backendStatus;
-
-	return NULL;
+	return (LocalPgBackendStatus *) bsearch(&key, localBackendStatusTable,
+											localNumBackends,
+											sizeof(LocalPgBackendStatus),
+											cmp_lbestatus);
 }
 
 
diff --git a/src/backend/utils/adt/pgstatfuncs.c b/src/backend/utils/adt/pgstatfuncs.c
index 2b9742ad21..68ae044399 100644
--- a/src/backend/utils/adt/pgstatfuncs.c
+++ b/src/backend/utils/adt/pgstatfuncs.c
@@ -727,7 +727,7 @@ pg_stat_get_backend_subxact(PG_FUNCTION_ARGS)
 
 	BlessTupleDesc(tupdesc);
 
-	if ((local_beentry = pgstat_fetch_stat_local_beentry(beid)) != NULL)
+	if ((local_beentry = pgstat_fetch_stat_local_beentry_by_backend_id(beid)) != NULL)
 	{
 		/* Fill values and NULLs */
 		values[0] = Int32GetDatum(local_beentry->backend_subxact_count);
diff --git a/src/include/utils/backend_status.h b/src/include/utils/backend_status.h
index 77939a0aed..060431a39b 100644
--- a/src/include/utils/backend_status.h
+++ b/src/include/utils/backend_status.h
@@ -335,6 +335,7 @@ extern uint64 pgstat_get_my_query_id(void);
 extern int	pgstat_fetch_stat_numbackends(void);
 extern PgBackendStatus *pgstat_fetch_stat_beentry(BackendId beid);
 extern LocalPgBackendStatus *pgstat_fetch_stat_local_beentry(int beid);
+extern LocalPgBackendStatus *pgstat_fetch_stat_local_beentry_by_backend_id(BackendId beid);
 extern char *pgstat_clip_activity(const char *raw_activity);
 
 
-- 
2.25.1


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