Improve table locking behavior in the face of current DDL.

In the previous coding, callers were faced with an awkward choice:
look up the name, do permissions checks, and then lock the table; or
look up the name, lock the table, and then do permissions checks.
The first choice was wrong because the results of the name lookup
and permissions checks might be out-of-date by the time the table
lock was acquired, while the second allowed a user with no privileges
to interfere with access to a table by users who do have privileges
(e.g. if a malicious backend queues up for an AccessExclusiveLock on
a table on which AccessShareLock is already held, further attempts
to access the table will be blocked until the AccessExclusiveLock
is obtained and the malicious backend's transaction rolls back).

To fix, allow callers of RangeVarGetRelid() to pass a callback which
gets executed after performing the name lookup but before acquiring
the relation lock.  If the name lookup is retried (because
invalidation messages are received), the callback will be re-executed
as well, so we get the best of both worlds.  RangeVarGetRelid() is
renamed to RangeVarGetRelidExtended(); callers not wishing to supply
a callback can continue to invoke it as RangeVarGetRelid(), which is
now a macro.  Since the only one caller that uses nowait = true now
passes a callback anyway, the RangeVarGetRelid() macro defaults nowait
as well.  The callback can also be used for supplemental locking - for
example, REINDEX INDEX needs to acquire the table lock before the index
lock to reduce deadlock possibilities.

There's a lot more work to be done here to fix all the cases where this
can be a problem, but this commit provides the general infrastructure
and fixes the following specific cases: REINDEX INDEX, REINDEX TABLE,
LOCK TABLE, and and DROP TABLE/INDEX/SEQUENCE/VIEW/FOREIGN TABLE.

Per discussion with Noah Misch and Alvaro Herrera.

2 files changed, 10 insertions, 2 deletions

diff --git a/src/include/catalog/index.h b/src/include/catalog/index.h
index 8b78b05a2e6..9efd9afed8d 100644
--- a/src/include/catalog/index.h
+++ b/src/include/catalog/index.h
@@ -99,5 +99,6 @@ extern bool reindex_relation(Oid relid, int flags);
 
 extern bool ReindexIsProcessingHeap(Oid heapOid);
 extern bool ReindexIsProcessingIndex(Oid indexOid);
+extern Oid IndexGetRelation(Oid indexId, bool missing_ok);
 
 #endif   /* INDEX_H */
diff --git a/src/include/catalog/namespace.h b/src/include/catalog/namespace.h
index 904c6fd97d8..d537a48b62c 100644
--- a/src/include/catalog/namespace.h
+++ b/src/include/catalog/namespace.h
@@ -47,9 +47,16 @@ typedef struct OverrideSearchPath
 	bool		addTemp;		/* implicitly prepend temp schema? */
 } OverrideSearchPath;
 
+typedef void (*RangeVarGetRelidCallback)(const RangeVar *relation, Oid relId,
+	Oid oldRelId, void *callback_arg);
 
-extern Oid	RangeVarGetRelid(const RangeVar *relation, LOCKMODE lockmode,
-				 bool missing_ok, bool nowait);
+#define RangeVarGetRelid(relation, lockmode, missing_ok) \
+	RangeVarGetRelidExtended(relation, lockmode, missing_ok, false, NULL, NULL)
+
+extern Oid	RangeVarGetRelidExtended(const RangeVar *relation,
+						 LOCKMODE lockmode, bool missing_ok, bool nowait,
+						 RangeVarGetRelidCallback callback,
+						 void *callback_arg);
 extern Oid	RangeVarGetCreationNamespace(const RangeVar *newRelation);
 extern Oid	RangeVarGetAndCheckCreationNamespace(const RangeVar *newRelation);
 extern void RangeVarAdjustRelationPersistence(RangeVar *newRelation, Oid nspid);