Add a function GetLockConflicts() to lock.c to report xacts holding

locks that would conflict with a specified lock request, without
actually trying to get that lock.  Use this instead of the former ad hoc
method of doing the first wait step in CREATE INDEX CONCURRENTLY.
Fixes problem with undetected deadlock and in many cases will allow the
index creation to proceed sooner than it otherwise could've.  Per
discussion with Greg Stark.

1 files changed, 99 insertions, 1 deletions

diff --git a/src/backend/storage/lmgr/lock.c b/src/backend/storage/lmgr/lock.c
index 7fe011a2ba..c59999e20c 100644
--- a/src/backend/storage/lmgr/lock.c
+++ b/src/backend/storage/lmgr/lock.c
@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/storage/lmgr/lock.c,v 1.171 2006/08/19 01:36:28 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/storage/lmgr/lock.c,v 1.172 2006/08/27 19:14:34 tgl Exp $
  *
  * NOTES
  *	  A lock table is a shared memory hash table.  When
@@ -1686,6 +1686,104 @@ LockReassignCurrentOwner(void)
 
 
 /*
+ * GetLockConflicts
+ *		Get a list of TransactionIds of xacts currently holding locks
+ *		that would conflict with the specified lock/lockmode.
+ *		xacts merely awaiting such a lock are NOT reported.
+ *
+ * Of course, the result could be out of date by the time it's returned,
+ * so use of this function has to be thought about carefully.
+ *
+ * Only top-level XIDs are reported.  Note we never include the current xact
+ * in the result list, since an xact never blocks itself.
+ */
+List *
+GetLockConflicts(const LOCKTAG *locktag, LOCKMODE lockmode)
+{
+	List	   *result = NIL;
+	LOCKMETHODID lockmethodid = locktag->locktag_lockmethodid;
+	LockMethod	lockMethodTable;
+	LOCK	   *lock;
+	LOCKMASK	conflictMask;
+	SHM_QUEUE  *procLocks;
+	PROCLOCK   *proclock;
+	uint32		hashcode;
+	LWLockId	partitionLock;
+
+	if (lockmethodid <= 0 || lockmethodid >= lengthof(LockMethods))
+		elog(ERROR, "unrecognized lock method: %d", lockmethodid);
+	lockMethodTable = LockMethods[lockmethodid];
+	if (lockmode <= 0 || lockmode > lockMethodTable->numLockModes)
+		elog(ERROR, "unrecognized lock mode: %d", lockmode);
+
+	/*
+	 * Look up the lock object matching the tag.
+	 */
+	hashcode = LockTagHashCode(locktag);
+	partitionLock = LockHashPartitionLock(hashcode);
+
+	LWLockAcquire(partitionLock, LW_SHARED);
+
+	lock = (LOCK *) hash_search_with_hash_value(LockMethodLockHash,
+												(void *) locktag,
+												hashcode,
+												HASH_FIND,
+												NULL);
+	if (!lock)
+	{
+		/*
+		 * If the lock object doesn't exist, there is nothing holding a
+		 * lock on this lockable object.
+		 */
+		LWLockRelease(partitionLock);
+		return NIL;
+	}
+
+	/*
+	 * Examine each existing holder (or awaiter) of the lock.
+	 */
+	conflictMask = lockMethodTable->conflictTab[lockmode];
+
+	procLocks = &(lock->procLocks);
+
+	proclock = (PROCLOCK *) SHMQueueNext(procLocks, procLocks,
+										 offsetof(PROCLOCK, lockLink));
+
+	while (proclock)
+	{
+		if (conflictMask & proclock->holdMask)
+		{
+			PGPROC *proc = proclock->tag.myProc;
+
+			/* A backend never blocks itself */
+			if (proc != MyProc)
+			{
+				/* Fetch xid just once - see GetNewTransactionId */
+				TransactionId xid = proc->xid;
+
+				/*
+				 * Race condition: during xact commit/abort we zero out
+				 * PGPROC's xid before we mark its locks released.  If we see
+				 * zero in the xid field, assume the xact is in process of
+				 * shutting down and act as though the lock is already
+				 * released.
+				 */
+				if (TransactionIdIsValid(xid))
+					result = lappend_xid(result, xid);
+			}
+		}
+
+		proclock = (PROCLOCK *) SHMQueueNext(procLocks, &proclock->lockLink,
+											 offsetof(PROCLOCK, lockLink));
+	}
+
+	LWLockRelease(partitionLock);
+
+	return result;
+}
+
+
+/*
  * AtPrepare_Locks
  *		Do the preparatory work for a PREPARE: make 2PC state file records
  *		for all locks currently held.