subscriptionCheck failures on nightjar

nightjar just did this:

https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=nightjar&dt=2019-02-11%2004%3A33%3A07

The critical bit seems to be that the publisher side of the
010_truncate.pl test failed like so:

2019-02-10 23:55:58.765 EST [40771] sub3 LOG: statement: BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ
2019-02-10 23:55:58.765 EST [40771] sub3 LOG: received replication command: CREATE_REPLICATION_SLOT "sub3_16414_sync_16394" TEMPORARY LOGICAL pgoutput USE_SNAPSHOT
2019-02-10 23:55:58.798 EST [40728] sub1 PANIC: could not open file "pg_logical/snapshots/0-160B578.snap": No such file or directory
2019-02-10 23:55:58.800 EST [40771] sub3 LOG: logical decoding found consistent point at 0/160B578
2019-02-10 23:55:58.800 EST [40771] sub3 DETAIL: There are no running transactions.

I'm not sure what to make of that, but I notice that nightjar has
failed subscriptionCheck seven times since mid-December, and every one
of those shows this same PANIC. Meanwhile, no other buildfarm member
has produced such a failure. It smells like a race condition with
a rather tight window, but that's just a guess.

So: (1) what's causing the failure? (2) could we respond with
something less than take-down-the-whole-database when a failure
happens in this area?

regards, tom lane