I wrote:
> As an investigative measure, I propose that we insert
>       Assert(MyPgXact->xmin == InvalidTransactionId);
> into 9.4's DefineIndex, just after its InvalidateCatalogSnapshot call.

Well, isn't this interesting:

TRAP: FailedAssertion("!(MyPgXact->xmin == ((TransactionId) 0))", File: 
"indexcmds.c", Line: 777)
2018-04-16 02:41:25.814 PDT [5ad46fbd.5412:2] LOG:  server process (PID 21777) 
was terminated by signal 6: Aborted
2018-04-16 02:41:25.814 PDT [5ad46fbd.5412:3] DETAIL:  Failed process was 
running: CREATE INDEX CONCURRENTLY concur_index1 ON concur_heap(f2,f1);


So we can now refine the problem statement to "SnapshotResetXmin isn't
doing what it's supposed to".  No idea why yet.  9.4 is using a simple
RegisteredSnapshots counter which 9.5 has replaced with a pairing heap,
so you'd think the newer code would be *more* likely to have bugs...

                        regards, tom lane

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