v2-01-fix-pgstat-sfc.patch

diff --git a/src/backend/utils/activity/pgstat.c b/src/backend/utils/activity/pgstat.c
index b125802b21..cfc6511093 100644
--- a/src/backend/utils/activity/pgstat.c
+++ b/src/backend/utils/activity/pgstat.c
@@ -103,7 +103,7 @@
 #include "storage/lwlock.h"
 #include "storage/pg_shmem.h"
 #include "storage/shmem.h"
-#include "utils/guc.h"
+#include "utils/guc_hooks.h"
 #include "utils/memutils.h"
 #include "utils/pgstat_internal.h"
 #include "utils/timestamp.h"
@@ -187,7 +187,6 @@ static inline bool pgstat_is_kind_valid(int ikind);
 bool		pgstat_track_counts = false;
 int			pgstat_fetch_consistency = PGSTAT_FETCH_CONSISTENCY_CACHE;
 
-
 /* ----------
  * state shared with pgstat_*.c
  * ----------
@@ -227,6 +226,13 @@ static dlist_head pgStatPending = DLIST_STATIC_INIT(pgStatPending);
  */
 static bool pgStatForceNextFlush = false;
 
+/*
+ * Force-clear existing snapshot before next use to prevent in-transaction
+ * change of stats_fetch_consistency from causing cache mechanism issues.
+ */
+static bool	clear_existing_snapshot = false;
+
+
 /*
  * For assertions that check pgstat is not used before initialization / after
  * shutdown.
@@ -787,6 +793,8 @@ pgstat_clear_snapshot(void)
 	 * forward the reset request.
 	 */
 	pgstat_clear_backend_activity_snapshot();
+
+	clear_existing_snapshot = false;
 }
 
 void *
@@ -885,6 +893,9 @@ pgstat_fetch_entry(PgStat_Kind kind, Oid dboid, Oid objoid)
 TimestampTz
 pgstat_get_stat_snapshot_timestamp(bool *have_snapshot)
 {
+	if (clear_existing_snapshot)
+		pgstat_clear_snapshot();
+
 	if (pgStatLocal.snapshot.mode == PGSTAT_FETCH_CONSISTENCY_SNAPSHOT)
 	{
 		*have_snapshot = true;
@@ -929,6 +940,9 @@ pgstat_snapshot_fixed(PgStat_Kind kind)
 static void
 pgstat_prep_snapshot(void)
 {
+	if (clear_existing_snapshot)
+		pgstat_clear_snapshot();
+
 	if (pgstat_fetch_consistency == PGSTAT_FETCH_CONSISTENCY_NONE ||
 		pgStatLocal.snapshot.stats != NULL)
 		return;
@@ -1671,3 +1685,18 @@ pgstat_reset_after_failure(void)
 	/* and drop variable-numbered ones */
 	pgstat_drop_all_entries();
 }
+
+/*
+ * GUC assign_hook for stats_fetch_consistency
+ */
+void
+assign_stats_fetch_consistency(int newval, void *extra)
+{
+	/*
+	 * Changing this value in a transaction may cause snapshot state
+	 * inconsistencies. Clear the current snapshot to avoid issues before the
+	 * next snapshot use.
+	 */
+	if (pgstat_fetch_consistency != newval)
+		clear_existing_snapshot = true;
+}
diff --git a/src/backend/utils/misc/guc_tables.c b/src/backend/utils/misc/guc_tables.c
index 2f42cebaf6..5f90aecd47 100644
--- a/src/backend/utils/misc/guc_tables.c
+++ b/src/backend/utils/misc/guc_tables.c
@@ -4821,7 +4821,7 @@ struct config_enum ConfigureNamesEnum[] =
 		},
 		&pgstat_fetch_consistency,
 		PGSTAT_FETCH_CONSISTENCY_CACHE, stats_fetch_consistency,
-		NULL, NULL, NULL
+		NULL, assign_stats_fetch_consistency, NULL
 	},
 
 	{
diff --git a/src/include/utils/guc_hooks.h b/src/include/utils/guc_hooks.h
index a82a85c940..2ecb9fc086 100644
--- a/src/include/utils/guc_hooks.h
+++ b/src/include/utils/guc_hooks.h
@@ -121,6 +121,7 @@ extern void assign_search_path(const char *newval, void *extra);
 extern bool check_session_authorization(char **newval, void **extra, GucSource source);
 extern void assign_session_authorization(const char *newval, void *extra);
 extern void assign_session_replication_role(int newval, void *extra);
+extern void assign_stats_fetch_consistency(int newval, void *extra);
 extern bool check_ssl(bool *newval, void **extra, GucSource source);
 extern bool check_stage_log_stats(bool *newval, void **extra, GucSource source);
 extern bool check_synchronous_standby_names(char **newval, void **extra,
diff --git a/src/test/regress/expected/stats.out b/src/test/regress/expected/stats.out
index 813d6d39ea..d62a923cac 100644
--- a/src/test/regress/expected/stats.out
+++ b/src/test/regress/expected/stats.out
@@ -985,6 +985,35 @@ SELECT pg_stat_get_snapshot_timestamp();
 
 COMMIT;
 ----
+-- Changing fetch consistency mode inside transaction doesn't cause crash
+----
+BEGIN;
+SET LOCAL stats_fetch_consistency = cache;
+-- filling stats for the cache mode
+SELECT pg_stat_get_function_calls(0);
+ pg_stat_get_function_calls 
+----------------------------
+                           
+(1 row)
+
+SET LOCAL stats_fetch_consistency = snapshot;
+-- verify that there is no failure on accessing pre-existing snapshot
+SELECT pg_stat_get_function_calls(0);
+ pg_stat_get_function_calls 
+----------------------------
+                           
+(1 row)
+
+SET LOCAL stats_fetch_consistency = none;
+-- verify that snapshot is cleared on changing stats_fetch_consistency
+SELECT pg_stat_get_snapshot_timestamp();
+ pg_stat_get_snapshot_timestamp 
+--------------------------------
+ 
+(1 row)
+
+ROLLBACK;
+----
 -- pg_stat_have_stats behavior
 ----
 -- fixed-numbered stats exist
diff --git a/src/test/regress/sql/stats.sql b/src/test/regress/sql/stats.sql
index 99a28bb79c..4ead6b16ca 100644
--- a/src/test/regress/sql/stats.sql
+++ b/src/test/regress/sql/stats.sql
@@ -476,6 +476,21 @@ SELECT pg_stat_clear_snapshot();
 SELECT pg_stat_get_snapshot_timestamp();
 COMMIT;
 
+----
+-- Changing fetch consistency mode inside transaction doesn't cause crash
+----
+BEGIN;
+SET LOCAL stats_fetch_consistency = cache;
+-- filling stats for the cache mode
+SELECT pg_stat_get_function_calls(0);
+SET LOCAL stats_fetch_consistency = snapshot;
+-- verify that there is no failure on accessing pre-existing snapshot
+SELECT pg_stat_get_function_calls(0);
+SET LOCAL stats_fetch_consistency = none;
+-- verify that snapshot is cleared on changing stats_fetch_consistency
+SELECT pg_stat_get_snapshot_timestamp();
+ROLLBACK;
+
 ----
 -- pg_stat_have_stats behavior
 ----