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

[Matt Mackall]

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.

-- 
Mathematics is the supreme nostalgia of our time.