Re: Pinned files at Windows

On Mon, May 27, 2019 at 05:52:13PM +0300, Konstantin Knizhnik wrote:
> Postgres is opening file with FILE_SHARE_DELETE  flag which makes it
> possible to unlink opened file.
> But unlike Unixes, the file is not actually deleted. You can see it using
> "dir" command.
> And stat() function also doesn't return error in this case:
>
> https://stackoverflow.com/questions/27270374/deletefile-or-unlink-calls-succeed-but-doesnt-remove-file
>
> So first check in  pgwin32_safestat (r < 0) is not working at all: stat()
> returns 0, but subsequent call of GetFileAttributesEx
> returns 5 (ERROR_ACCESS_DENIED).

So you would basically hijack the result of GetFileAttributesEx() so
as any errors returned by this function complain with ENOENT for
everything seen. Why would that be a sane idea? What if say a
permission or another error is legit, but instead ENOENT is returned
as you propose, then the caller would be confused by an incorrect
status.

As you mention, what we did as of 9951741 may not be completely right,
and the reason why it was done this way comes from here:
https://www.postgresql.org/message-id/20160712083220.1426.58667@wrigleys.postgresql.org

Could we instead come up with a reliable way to detect if a file is in
a deletion pending state? Mapping blindly EACCES to ENOENT is not a
solution I think we can rely on (perhaps we could check only after
ERROR_ACCESS_DENIED using GetLastError() and map back to ENOENT in
this case still this can be triggered if a virus scanner holds the
file for read, no?). stat() returning 0 for a file pending for
deletion which will go away physically once the handles still keeping
the file around are closed is not something I would have imagined is
sane, but that's what we need to deal with... Windows has a long
history of keeping things compatible, sometimes in their own weird
way, and it seems that we have one here so I cannot imagine that this
behavior is going to change.

Looking around, I have found out about NtCreateFile() which could be
able to report a proper pending deletion status, still that's only
available in kernel mode. Perhaps others have ideas?
--
Michael