Re: [PATCH/RFC v3 7/9] write_entry(): use fstat() instead of lstat() when file is open

[Junio C Hamano <gitster@pobox.com>]

Johannes Sixt <j.sixt@viscovery.net> writes:

> Kjetil Barvik schrieb:
>> Currently inside write_entry() we do an lstat(path, &st) call on a
>> file which have just been opened inside the exact same function.  It
>> should be better to call fstat(fd, &st) on the file while it is open,
>> and it should be at least as fast as the lstat() method.
> ...
>> @@ -145,6 +146,11 @@ static int write_entry(struct cache_entry *ce, char *path, const struct checkout
>>  		}
>>  
>>  		wrote = write_in_full(fd, new, size);
>> +		/* use fstat() only when path == ce->name */
>> +		if (state->refresh_cache && !to_tempfile && !state->base_dir_len) {
>> +			fstat(fd, &st);
>> +			fstat_done = 1;
>> +		}
>>  		close(fd);
>
> I've a bad gut feeling about this: It may not work as expected on Windows
> because there is this statement in the documentation:
>
>   "The only guarantee about a file timestamp is that the file time is
>    correctly reflected when the handle that makes the change is closed."
>
> (http://msdn.microsoft.com/en-us/library/ms724290(VS.85).aspx)
>
> We are operating on a temporary file. It could happen that the fstat()
> returns the time when the file was created, as opposed to when the
> write_in_full() was completed successfully. The fstat()ed time ends up in
> the index, but it can be different from what later lstat() calls report
> (and the file would be regarded as modified).
>
> I have the suspicion that the gain from this patch is minimal. Would you
> mind playing it safe and drop this patch?

Hmm, write_entry() is actually called once per one path we write out, and
the fstat() is added to the common case (no --tempfile, no --prefix=<dir>
checkout), which would mean that if there were any performance gain from
this change, it was obtained by trading correctness away.  Sad.