[PATCH 02 of 11] scmutil: add filecache, a smart property-like decorator that compares stat info

[Adrian Buehlmann]

On 2011-07-18 23:29, Matt Mackall wrote:
> On Mon, 2011-07-18 at 22:32 +0200, Adrian Buehlmann wrote:
>> On 2011-07-18 22:12, Matt Mackall wrote:
>>> On Sat, 2011-07-16 at 18:03 +0200, Adrian Buehlmann wrote:
>>>> On 2011-07-16 16:34, Idan Kamara wrote:
>>>>> # HG changeset patch
>>>>> # User Idan Kamara <idankk86 at gmail.com>
>>>>> # Date 1310227619 -10800
>>>>> # Node ID b99305dd59279aec962e23da2a362e0d8b785965
>>>>> # Parent  d36f5aec2f9e4214fafe048bccd0bb47ac5f9c16
>>>>> scmutil: add filecache, a smart property-like decorator that compares stat info
>>>>>
>>>>> The idea is being able to associate a file with a property, and watch
>>>>> that file stat info for modifications when we decide it's important for it to
>>>>> be up-to-date. Once it changes, we recreate the object.
>>>>>
>>>>> As a consequence, localrepo.invalidate() will become much less expensive in the
>>>>> case where nothing changed on-disk.
>>>>>
>>>>> diff -r d36f5aec2f9e -r b99305dd5927 mercurial/scmutil.py
>>>>> --- a/mercurial/scmutil.py	Sat Jul 16 15:30:43 2011 +0300
>>>>> +++ b/mercurial/scmutil.py	Sat Jul 09 19:06:59 2011 +0300
>>>>> @@ -709,3 +709,41 @@
>>>>>          raise error.RequirementError(_("unknown repository format: "
>>>>>              "requires features '%s' (upgrade Mercurial)") % "', '".join(missings))
>>>>>      return requirements
>>>>> +
>>>>> +class filecache(object):
>>>>> +    '''A property like decorator that tracks a file under .hg/ for updates.
>>>>> +
>>>>> +    Records stat info when called in _invalidatecache.
>>>>> +
>>>>> +    On subsequent calls, compares old stat info with new info, and recreates
>>>>> +    the object when needed, updating the new stat info in _invalidatecache.'''
>>>>> +    def __init__(self, path, instore=False):
>>>>> +        self.path = path
>>>>> +        self.instore = instore
>>>>> +
>>>>> +    def __call__(self, func):
>>>>> +        self.func = func
>>>>> +        self.name = func.__name__
>>>>> +        return self
>>>>> +
>>>>> +    def __get__(self, obj, type=None):
>>>>> +        path = self.instore and obj.sjoin(self.path) or obj.join(self.path)
>>>>> +
>>>>> +        if self.name in obj._invalidatecache:
>>>>> +            cacheentry = obj._invalidatecache[self.name]
>>>>> +            stat = util.stat(path)
>>>>> +
>>>>> +            if stat != cacheentry[1]:
>>>>> +                cacheentry[1] = stat
>>>>> +                result = cacheentry[0] = self.func(obj)
>>>>> +            else:
>>>>> +                result = cacheentry[0]
>>>>> +        else:
>>>>> +            # stat -before- reading so our cache doesn't lie if someone
>>>>> +            # modifies between the time we read+stat it
>>>>> +            stat = util.stat(path)
>>>>> +            result = self.func(obj)
>>>>> +            obj._invalidatecache[self.name] = [result, stat, path]
>>>>> +
>>>>> +        setattr(obj, self.name, result)
>>>>> +        return result
>>>>
>>>> What happens if the file changed its contents without changing mtime nor
>>>> size?
>>>
>>> Excellent question. Answer: we lose.
>>
>> ..
>>
>>> We need to cache and compare -the whole stat result-. There's absolutely
>>> no reason not to here.
>>
>> How does that solve the problem of missing a file change that changes
>> file contents without changing size nor mtime? (and thus failing to call
>> func again)
> 
> We've got three buckets we can dump filesystems into:
> 
> have subsecond timestamps (eg NTFS, Btrfs, ext4..):
>   changes are detected by comparing timestamps
> have inodes (ext3, HFS+):
>   changes made by non-append operations are made atomic rename 
>   and result in timestamp changes
> neither (eg VFAT):
>   similar issues (and solutions) to dirstate apply

And good luck with Windows shares.

Anyway, I don't think this will work. This code is trying to be too clever.

And discovering all the cases where it fails will be very hard.