pgstat: Acquire lock when reading variable-numbered stats

Somewhere during the development of the patch acquiring a lock during read
access to variable-numbered stats got lost. The missing lock acquisition won't
cause corruption, but can lead to reading torn values when accessing
stats. Add the missing lock acquisitions.

Reported-by: Greg Stark <stark@mit.edu>
Reviewed-by: "Drouvot, Bertrand" <bdrouvot@amazon.com>
Reviewed-by: Andres Freund <andres@anarazel.de>
Author: Kyotaro Horiguchi <horikyota.ntt@gmail.com>
Discussion: https://postgr.es/m/CAM-w4HMYkM_DkYhWtUGV+qE_rrBxKOzOF0+5faozxO3vXrc9wA@mail.gmail.com
Backpatch: 15-

src/backend/utils/activity/pgstat.c

@@ -844,9 +844,12 @@ pgstat_fetch_entry(PgStat_Kind kind, Oid dboid, Oid objoid)
 	else
 		stats_data = MemoryContextAlloc(pgStatLocal.snapshot.context,
 										kind_info->shared_data_len);
+
+	pgstat_lock_entry_shared(entry_ref, false);
 	memcpy(stats_data,
 		   pgstat_get_entry_data(kind, entry_ref->shared_stats),
 		   kind_info->shared_data_len);
+	pgstat_unlock_entry(entry_ref);
 
 	if (pgstat_fetch_consistency > PGSTAT_FETCH_CONSISTENCY_NONE)
 	{
@@ -983,9 +986,15 @@ pgstat_build_snapshot(void)
 
 		entry->data = MemoryContextAlloc(pgStatLocal.snapshot.context,
 										 kind_info->shared_size);
+		/*
+		 * Acquire the LWLock directly instead of using
+		 * pg_stat_lock_entry_shared() which requires a reference.
+		 */
+		LWLockAcquire(&stats_data->lock, LW_SHARED);
 		memcpy(entry->data,
 			   pgstat_get_entry_data(kind, stats_data),
 			   kind_info->shared_size);
+		LWLockRelease(&stats_data->lock);
 	}
 	dshash_seq_term(&hstat);
 

src/backend/utils/activity/pgstat_shmem.c

@@ -579,6 +579,22 @@ pgstat_lock_entry(PgStat_EntryRef *entry_ref, bool nowait)
 	return true;
 }
 
+/*
+ * Separate from pgstat_lock_entry() as most callers will need to lock
+ * exclusively.
+ */
+bool
+pgstat_lock_entry_shared(PgStat_EntryRef *entry_ref, bool nowait)
+{
+	LWLock	   *lock = &entry_ref->shared_stats->lock;
+
+	if (nowait)
+		return LWLockConditionalAcquire(lock, LW_SHARED);
+
+	LWLockAcquire(lock, LW_SHARED);
+	return true;
+}
+
 void
 pgstat_unlock_entry(PgStat_EntryRef *entry_ref)
 {

src/include/utils/pgstat_internal.h

@@ -581,6 +581,7 @@ extern void pgstat_detach_shmem(void);
 extern PgStat_EntryRef *pgstat_get_entry_ref(PgStat_Kind kind, Oid dboid, Oid objoid,
 											 bool create, bool *found);
 extern bool pgstat_lock_entry(PgStat_EntryRef *entry_ref, bool nowait);
+extern bool pgstat_lock_entry_shared(PgStat_EntryRef *entry_ref, bool nowait);
 extern void pgstat_unlock_entry(PgStat_EntryRef *entry_ref);
 extern bool pgstat_drop_entry(PgStat_Kind kind, Oid dboid, Oid objoid);
 extern void pgstat_drop_all_entries(void);