D588: win32: use fewer system calls for unlink()
Adrian Buehlmann
adrian at cadifra.com
Fri Sep 1 18:15:10 UTC 2017
On 2017-09-01 08:16, Adrian Buehlmann wrote:
> Ugh. Can I reply to a phabricator notification by email?
>
> Adding gregory.szorc at gmail.com manually, as I'm not sure replaying to
> those nasty phabricator emails is going to work...
>
> On 2017-09-01 00:32, indygreg (Gregory Szorc) wrote:
>> diff --git a/mercurial/win32.py b/mercurial/win32.py
>> --- a/mercurial/win32.py
>> +++ b/mercurial/win32.py
>> @@ -12,6 +12,7 @@
>> import msvcrt
>> import os
>> import random
>> +import stat
>> import subprocess
>>
>> from . import (
>> @@ -522,11 +523,26 @@
>> def unlink(f):
>> '''try to implement POSIX' unlink semantics on Windows'''
>>
>> - if os.path.isdir(f):
>> - # use EPERM because it is POSIX prescribed value, even though
>> - # unlink(2) on directories returns EISDIR on Linux
>> - raise IOError(errno.EPERM,
>> - "Unlinking directory not permitted: '%s'" % f)
>> + # If the path doesn't exist, raise that exception.
>> + # If it is a directory, emulate POSIX behavior.
>> + try:
>> + st = os.stat(f)
>> + if stat.S_ISDIR(st.st_mode):
>> + # use EPERM because it is POSIX prescribed value, even though
>> + # unlink(2) on directories returns EISDIR on Linux
>> + raise IOError(errno.EPERM,
>> + "Unlinking directory not permitted: '%s'" % f)
>> + except OSError as e:
>> + if e.errno == errno.ENOENT:
>> + raise
>> +
>> + # In the common case, a normal unlink will work. Try that first and fall
>> + # back to more complexity if and only if we need to.
>> + try:
>> + os.unlink(f)
>> + return
>> + except (IOError, OSError) as e:
>> + pass
>>
>> # POSIX allows to unlink and rename open files. Windows has serious
>> # problems with doing that:
>
> Do you get an error at all, if a file, which is in open state, is unlinked?
>
> My fear is: You won't get an error, but instead the filename is blocked
> by the file being held in place by the other process, until the other
> process closes it. Which means: You already lost the game.
>
> Which would explain why we didn't do things like you propose here.
>
> See also
>
> https://www.mercurial-scm.org/wiki/UnlinkingFilesOnWindows
>
> (specifically, heading 2)
>
In other words:
The "zombie state" - which the rename dance afterwards tries to avoid -
has already been initiated (by calling unlink) when the rename dance is
started. Which is pointless. You then might as well remove the rename
step: It can't help any more. It will even fail, as you can't rename a
file in zombie state.
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