[Patch] Check file type before calling AllocateFile() for files outof pg data directory to avoid potential issues (e.g. hang).

Hello, Postgres hackers.

I happened to see a hang issue when running a simple copy query. The root cause and repro way are quite simple.

mkfifo /tmp/a

run sql:

copy (select generate_series(1, 10)) to '/tmp/a';

It hangs at AllocateFile()->fopen() because that file was previously created as a fifo file, and it is not ctrl+c cancellable (on Linux).

This is, in theory, not a 100% bug, but it is probably not unusual to see conflicts of files between postgresql sqls and other applications on the same node so I think the fix is needed. I checked all code that calls AllocateFile() and wrote a simple patch to do sanity check (if the file exists it must be a regular file) for those files which are probably out of the postgres data directories which we probably want to ignore. This is actually not a perfect fix since it is not atomic (check and open), but it should fix most of the scenarios. To be perfect, we might want to refactor AllocateFile() to allow atomic check&create using either 'x' in fopen() or O_EXCL in open(), also it seems that we might not want to create temp file for AllocateFile() with fixed filenames. This is beyond of this patch of course.

Thanks.