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

[Adrian Buehlmann]

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)