pg_ctl on windows can't open postmaster.pid: Permission denied

Hi,

Buildfarm animal thrips just failed with a curious error:
https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=thrips&dt=2017-12-13%2002%3A27%3A27

============== shutting down postmaster ==============
pg_ctl: could not open PID file "C:/buildfarm/buildenv/HEAD/pgsql.build/src/test/regress/./tmp_check/data/postmaster.pid": Permission denied

otherwise there were no failures.

I wonder if we're not opening the file with the right options to allow
us to open it while it's opened for writes in another backend? In the
backend we do so via FILE_SHARE_READ pgwin32_open which backs open and
fopen in backend code.

I've not seen this error previously, so maybe this is just a random
one-off.

Greetings,

Andres Freund