Avoid deadlock between concurrent CREATE INDEX CONCURRENTLY commands.

There was a high probability of two or more concurrent C.I.C. commands
deadlocking just before completion, because each would wait for the others
to release their reference snapshots.  Fix by releasing the snapshot
before waiting for other snapshots to go away.

Per report from Paul Hinze.  Back-patch to all active branches.

src/backend/commands/indexcmds.c

@@ -320,6 +320,7 @@ DefineIndex(IndexStmt *stmt,
 	int16	   *coloptions;
 	IndexInfo  *indexInfo;
 	int			numberOfAttributes;
+	TransactionId limitXmin;
 	VirtualTransactionId *old_lockholders;
 	VirtualTransactionId *old_snapshots;
 	int			n_old_snapshots;
@@ -769,6 +770,18 @@ DefineIndex(IndexStmt *stmt,
 	 */
 	validate_index(relationId, indexRelationId, snapshot);
 
+	/*
+	 * Drop the reference snapshot.  We must do this before waiting out other
+	 * snapshot holders, else we will deadlock against other processes also
+	 * doing CREATE INDEX CONCURRENTLY, which would see our snapshot as one
+	 * they must wait for.  But first, save the snapshot's xmin to use as
+	 * limitXmin for GetCurrentVirtualXIDs().
+	 */
+	limitXmin = snapshot->xmin;
+
+	PopActiveSnapshot();
+	UnregisterSnapshot(snapshot);
+
 	/*
 	 * The index is now valid in the sense that it contains all currently
 	 * interesting tuples.	But since it might not contain tuples deleted just
@@ -801,7 +814,7 @@ DefineIndex(IndexStmt *stmt,
 	 * GetCurrentVirtualXIDs.  If, during any iteration, a particular vxid
 	 * doesn't show up in the output, we know we can forget about it.
 	 */
-	old_snapshots = GetCurrentVirtualXIDs(snapshot->xmin, true, false,
+	old_snapshots = GetCurrentVirtualXIDs(limitXmin, true, false,
 										  PROC_IS_AUTOVACUUM | PROC_IN_VACUUM,
 										  &n_old_snapshots);
 
@@ -818,7 +831,7 @@ DefineIndex(IndexStmt *stmt,
 			int			j;
 			int			k;
 
-			newer_snapshots = GetCurrentVirtualXIDs(snapshot->xmin,
+			newer_snapshots = GetCurrentVirtualXIDs(limitXmin,
 													true, false,
 										 PROC_IS_AUTOVACUUM | PROC_IN_VACUUM,
 													&n_newer_snapshots);
@@ -857,12 +870,6 @@ DefineIndex(IndexStmt *stmt,
 	 */
 	CacheInvalidateRelcacheByRelid(heaprelid.relId);
 
-	/* we can now do away with our active snapshot */
-	PopActiveSnapshot();
-
-	/* And we can remove the validating snapshot too */
-	UnregisterSnapshot(snapshot);
-
 	/*
 	 * Last thing to do is release the session-level lock on the parent table.
 	 */