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commit    cd312adc561a0de056bbc0b0087d275bd3e9f282
Author:   Michael Paquier <michael@paquier.xyz>
Date:     Wed Jun 05 23:47:40 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 an
    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 889e86ac5ac..da11b867445 100644
--- a/src/backend/utils/activity/pgstat_replslot.c
+++ b/src/backend/utils/activity/pgstat_replslot.c
@@ -157,8 +157,9 @@ pgstat_drop_replslot(ReplicationSlot *slot)
 {
 	Assert(LWLockHeldByMeInMode(ReplicationSlotAllocationLock, LW_EXCLUSIVE));
 
-	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 91591da3958..4a4b69891d3 100644
--- a/src/backend/utils/activity/pgstat_shmem.c
+++ b/src/backend/utils/activity/pgstat_shmem.c
@@ -855,6 +855,17 @@ 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: 8111e80c5d39]