vacuum fails with "could not open statistics file" "Device orresource busy"

Hi,

buildfarm member lorikeet had an interesting buildfarm failure in the
logical decoding test. The failure looks unrelated to logical decoding,
but might be more widely relevant:

https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=lorikeet&dt=2018-10-19%2011%3A22%3A34
  VACUUM FULL pg_class;
+ WARNING:  could not open statistics file "pg_stat_tmp/global.stat": Device or resource busy

Now that animal is cygwin based, so maybe it's just some random
weirdness. But ISTM it could also indicate a bug of some sort.

Were it native windows, I'd assume we'd keep a file open with the wrong
flags or such...

Greetings,

Andres Freund