Fix race condition in enum value comparisons.

When (re) loading the typcache comparison cache for an enum type's values,
use an up-to-date MVCC snapshot, not the transaction's existing snapshot.
This avoids problems if we encounter an enum OID that was created since our
transaction started.  Per report from Andres Freund and diagnosis by Robert
Haas.

To ensure this is safe even if enum comparison manages to get invoked
before we've set a transaction snapshot, tweak GetLatestSnapshot to
redirect to GetTransactionSnapshot instead of throwing error when
FirstSnapshotSet is false.  The existing uses of GetLatestSnapshot (in
ri_triggers.c) don't care since they couldn't be invoked except in a
transaction that's already done some work --- but it seems just conceivable
that this might not be true of enums, especially if we ever choose to use
enums in system catalogs.

Note that the comparable coding in enum_endpoint and enum_range_internal
remains GetTransactionSnapshot; this is perhaps debatable, but if we
changed it those functions would have to be marked volatile, which doesn't
seem attractive.

Back-patch to 9.1 where ALTER TYPE ADD VALUE was added.

diff --git a/src/backend/utils/cache/typcache.c b/src/backend/utils/cache/typcache.c
index 72798ef9effbd6cbeedd78aa60768f2490f01757..e6f66df9a139d4693083e1dfd84bc70e8b0a2d61 100644 (file)
--- a/src/backend/utils/cache/typcache.c
+++ b/src/backend/utils/cache/typcache.c
@@ -1089,7 +1089,7 @@ load_enum_cache_data(TypeCacheEntry *tcache)
        enum_rel = heap_open(EnumRelationId, AccessShareLock);
        enum_scan = systable_beginscan(enum_rel,
                                                                   EnumTypIdLabelIndexId,
-                                                                  true, GetTransactionSnapshot(),
+                                                                  true, GetLatestSnapshot(),
                                                                   1, &skey);
 
        while (HeapTupleIsValid(enum_tuple = systable_getnext(enum_scan)))

diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c
index 7187ca7c98d7da16ec7fdadb8d839ca995056b58..4652915c4e3854fe8e9a08c58801cc1985bcfe81 100644 (file)
--- a/src/backend/utils/time/snapmgr.c
+++ b/src/backend/utils/time/snapmgr.c
@@ -196,9 +196,9 @@ GetTransactionSnapshot(void)
 Snapshot
 GetLatestSnapshot(void)
 {
-       /* Should not be first call in transaction */
+       /* If first call in transaction, go ahead and set the xact snapshot */
        if (!FirstSnapshotSet)
-               elog(ERROR, "no snapshot has been set");
+               return GetTransactionSnapshot();
 
        SecondarySnapshot = GetSnapshotData(&SecondarySnapshotData);