From c1500a1ba7e16ac9e98c6baf792f0be64a48e839 Mon Sep 17 00:00:00 2001
From: Alexander Korotkov <akorotkov@postgresql.org>
Date: Thu, 24 Oct 2024 14:34:16 +0300
Subject: [PATCH] Update header comment for lookup_type_cache()

Describe the way we handle concurrent invalidation messages.

Discussion: https://postgr.es/m/CAPpHfdsQhwUrnB3of862j9RgHoJM--eRbifvBMvtQxpC57dxCA%40mail.gmail.com
Reviewed-by: Andrei Lepikhov, Artur Zakirov, Pavel Borisov
---
 src/backend/utils/cache/typcache.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/src/backend/utils/cache/typcache.c b/src/backend/utils/cache/typcache.c
index 2ec136b7d30..f142624ad2e 100644
--- a/src/backend/utils/cache/typcache.c
+++ b/src/backend/utils/cache/typcache.c
@@ -351,6 +351,15 @@ type_cache_syshash(const void *key, Size keysize)
  * invalid.  Note however that we may fail to find one or more of the
  * values requested by 'flags'; the caller needs to check whether the fields
  * are InvalidOid or not.
+ *
+ * Note that while filling TypeCacheEntry we might process concurrent
+ * invalidation messages, causing our not-yet-filled TypeCacheEntry to be
+ * invalidated.  In this case, we typically only clear flags while values are
+ * still available for the caller.  It's expected that the caller holds
+ * enough locks on type-depending objects that the values are still relevant.
+ * It's also important that the tupdesc is filled after all other
+ * TypeCacheEntry items for TYPTYPE_COMPOSITE.  So, tupdesc can't get
+ * invalidated during the lookup_type_cache() call.
  */
 TypeCacheEntry *
 lookup_type_cache(Oid type_id, int flags)