[PATCH 1 of 6 RFC] manifest: introduce an accessor class for manifests

[Yuya Nishihara]

On Thu, 3 Nov 2016 15:27:37 -0700, Durham Goode wrote:
> # HG changeset patch
> # User Durham Goode <durham at fb.com>
> # Date 1478208817 25200
> #      Thu Nov 03 14:33:37 2016 -0700
> # Branch stable
> # Node ID 1788ee9e1df92ac94b9be84eac6d16e3bad903a9
> # Parent  b9f7b0c10027764cee77f9c6d61877fcffea837f
> manifest: introduce an accessor class for manifests
> 
> This introduces a revlogaccessor class which can be used to allow multiple
> objects hold an auto-invalidating reference to a revlog, without having to hold
> a reference to the actual repo object. Future patches will switch repo.manifest
> and repo.manifestlog to access the manifest through this accessor. This will fix
> the circular reference caused by manifestlog and manifestctx holding a reference
> to the repo
> 
> diff --git a/mercurial/localrepo.py b/mercurial/localrepo.py
> --- a/mercurial/localrepo.py
> +++ b/mercurial/localrepo.py
> @@ -514,6 +514,11 @@ class localrepository(object):
>          # manifest creation.
>          return manifest.manifest(self.svfs)
>  
> +    @unfilteredpropertycache
> +    def manifestaccessor(self):
> +        return revlogaccessor('00manifest.i', self.svfs,
> +                              self._constructmanifest)

Honestly I don't get why manifestlog and manifestctxs have to live longer
than the other repo properties. But suppose that is necessary, I agree we'll
need this kind of a wrapper. Maybe we can move the accessor to the manifestlog,
but still the accessor will have to be shared (and updated transparently) by
the manifestlog and its cachable manifestctxs.

> +def revlogaccessor(filename, opener, constructor):
> +    """Creates an accessor that provides cached and invalidated access to a
> +    revlog, via instance.revlog. This is useful for letting multiple objects
> +    hold a reference to the revlog, without having to hold a possibly-circular
> +    reference to the actual repository.  """
> +
> +    # We have to use a runtime type here, because the only way to create a
> +    # property is to put it on a class itself, and the property is dynamically
> +    # defined by the filename parameter.
> +    class accessor(object):

Perhaps we could refactor filecache to avoid dynamically creating a class, but
that would be a minor issue of this RFC series.