On 29.05.2019 22:20, Michael Paquier wrote:
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.
If access to the file is prohibited by lack of permissions, then stat()
should fail with error
and this error is returned by pgwin32_safestat function.
If call of stat() is succeed, then my assumption is that the only reason
of GetFileAttributesEx
failure is that file is deleted and returning ENOENT error code in this
case is correct behavior.
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/20160712083220.1426.58...@wrigleys.postgresql.org
Yes, this is the same reason, but handling STATUS_DELETE_PENDING is not
correct.
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?
Sorry, I do not know better solution.
I have written small test reproducing the problem which proves that
if file is opened with FILE_SHARE_DELETE flag, then
it is possible to delete it using unlink() - no error is returned and
call stat() for it - also succeed.
By any attempt to open this file for reading/writing or performing
GetFileAttributesEx
are failed with ERROR_ACCESS_DENIED (not with ERROR_DELETE_PENDING
which is hidden by Win32 API).
--
Konstantin Knizhnik
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company
