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commit    bfc44da2478954330233a8df8a146ee920400947
Author:   Michael Paquier <michael@paquier.xyz>
Date:     Wed Jun 05 23:48:21 2024 +0000
First released in: 17.0, 15.8, 16.4

    Prevent inconsistent use of stats entry for replication slots
    
    Concurrent activity around replication slot creation and drop could
    cause a replication slot to use a stats entry it should not have used
    when created, triggering an assertion failure when retrieving this
    inconsistent entry from the dshash table used by the stats facility.
    
    The issue is that pgstat_drop_replslot() calls pgstat_drop_entry()
    without checking the result.  If pgstat_drop_entry() cannot free the
    entry related to the object dropped, pgstat_request_entry_refs_gc()
    should be called.  AtEOXact_PgStat_DroppedStats() and surrounding
    routines dropping stats entries already do that.
    
    This is documented in pgstat_internal.h, but let's add a comment at the
    top of pgstat_drop_entry() as that can be easy to miss.
    
    Reported-by: Alexander Lakhin
    Author: Floris Van Nee
    Analyzed-by: Andres Freund
    Discussion: https://postgr.es/m/17947-b9554521ad963c9c@postgresql.org
    Backpatch-through: 15


 src/backend/utils/activity/pgstat_replslot.c |  5 +++--
 src/backend/utils/activity/pgstat_shmem.c    | 11 +++++++++++
 2 files changed, 14 insertions(+), 2 deletions(-)

diff --git a/src/backend/utils/activity/pgstat_replslot.c b/src/backend/utils/activity/pgstat_replslot.c
index 1a8473bcd45..4faf06045c5 100644
--- a/src/backend/utils/activity/pgstat_replslot.c
+++ b/src/backend/utils/activity/pgstat_replslot.c
@@ -153,8 +153,9 @@ pgstat_acquire_replslot(ReplicationSlot *slot)
 void
 pgstat_drop_replslot(ReplicationSlot *slot)
 {
-	pgstat_drop_entry(PGSTAT_KIND_REPLSLOT, InvalidOid,
-					  ReplicationSlotIndex(slot));
+	if (!pgstat_drop_entry(PGSTAT_KIND_REPLSLOT, InvalidOid,
+						   ReplicationSlotIndex(slot)))
+		pgstat_request_entry_refs_gc();
 }
 
 /*
diff --git a/src/backend/utils/activity/pgstat_shmem.c b/src/backend/utils/activity/pgstat_shmem.c
index 9a4f037959e..9e4ea2e0428 100644
--- a/src/backend/utils/activity/pgstat_shmem.c
+++ b/src/backend/utils/activity/pgstat_shmem.c
@@ -850,6 +850,17 @@ pgstat_drop_database_and_contents(Oid dboid)
 		pgstat_request_entry_refs_gc();
 }
 
+/*
+ * Drop a single stats entry.
+ *
+ * This routine returns false if the stats entry of the dropped object could
+ * not be freed, true otherwise.
+ *
+ * The callers of this function should call pgstat_request_entry_refs_gc()
+ * if the stats entry could not be freed, to ensure that this entry's memory
+ * can be reclaimed later by a different backend calling
+ * pgstat_gc_entry_refs().
+ */
 bool
 pgstat_drop_entry(PgStat_Kind kind, Oid dboid, Oid objoid)
 {


[parent: bb8425491cb7]