Better fix for deadlock hazard in CREATE INDEX CONCURRENTLY.

Commit 54eff5311 did not account for the possibility that we'd have
a transaction snapshot due to default_transaction_isolation being
set high enough to require one.  The transaction snapshot is enough
to hold back our advertised xmin and thus risk deadlock anyway.
The only way to get rid of that snap is to start a new transaction,
so let's do that instead.  Also throw in an assert checking that we
really have gotten to a state where no xmin is being advertised.

Back-patch to 9.4, like the previous commit.

Discussion: https://postgr.es/m/CAMkU=1ztk3TpQdcUNbxq93pc80FrXUjpDWLGMeVBDx71GHNwZQ@mail.gmail.com

src/backend/commands/indexcmds.c

@@ -1197,14 +1197,25 @@ DefineIndex(Oid relationId,
 	 * 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().
-	 *
-	 * Our catalog snapshot could have the same effect, so drop that one too.
 	 */
 	limitXmin = snapshot->xmin;
 
 	PopActiveSnapshot();
 	UnregisterSnapshot(snapshot);
-	InvalidateCatalogSnapshot();
+
+	/*
+	 * The snapshot subsystem could still contain registered snapshots that
+	 * are holding back our process's advertised xmin; in particular, if
+	 * default_transaction_isolation = serializable, there is a transaction
+	 * snapshot that is still active.  The CatalogSnapshot is likewise a
+	 * hazard.  To ensure no deadlocks, we must commit and start yet another
+	 * transaction, and do our wait before any snapshot has been taken in it.
+	 */
+	CommitTransactionCommand();
+	StartTransactionCommand();
+
+	/* We should now definitely not be advertising any xmin. */
+	Assert(MyPgXact->xmin == InvalidTransactionId);
 
 	/*
 	 * The index is now valid in the sense that it contains all currently