[PATCH 01 of 10] revlog: add clone method

[Augie Fackler]

On Sat, Nov 05, 2016 at 09:40:17PM -0700, Gregory Szorc wrote:
> # HG changeset patch
> # User Gregory Szorc <gregory.szorc at gmail.com>
> # Date 1478405835 25200
> #      Sat Nov 05 21:17:15 2016 -0700
> # Node ID ebbd8d975e4bf59b2bdd44736fdf13222988d1a4
> # Parent  f01367faa792635ad2f7a6b175ae3252292b5121
> revlog: add clone method
>
> Upcoming patches will introduce functionality for in-place
> repository/store "upgrades." Copying the contents of a revlog
> feels sufficiently low-level to warrant being in the revlog
> class.
>
> diff --git a/mercurial/revlog.py b/mercurial/revlog.py
> --- a/mercurial/revlog.py
> +++ b/mercurial/revlog.py
> @@ -1818,3 +1818,32 @@ class revlog(object):
>          if not self._inline:
>              res.append(self.datafile)
>          return res
> +
> +    def clone(self, tr, destrevlog, addrevisioncb=None):
> +        """Copy the contents of this revlog to another revlog.
> +
> +        The destination revlog will contain the same revisions and nodes.

It might be worth calling out that this copying is done via repeated
calls to addrevision, so delta recomputation will take place. Which is
to say this is not a cheap clone, but rather a very expensive clone,
as it's doing a full decompress-undelta-delta-compress cycle.

> +        However, it may not be bit-for-bit identical due to e.g. delta encoding
> +        differences.
> +        """
> +        if len(destrevlog):
> +            raise ValueError(_('destination revlog is not empty'))
> +
> +        index = self.index
> +        for rev in self:
> +            entry = index[rev]
> +
> +            # Some classes override linkrev to take filtered revs into
> +            # account. Use raw entry from index.
> +            linkrev = entry[4]
> +            p1 = index[entry[5]][7]
> +            p2 = index[entry[6]][7]
> +            node = entry[7]
> +            # FUTURE we could optionally allow reusing the delta to avoid
> +            # expensive recomputation.
> +            text = self.revision(rev)
> +
> +            destrevlog.addrevision(text, tr, linkrev, p1, p2, node=node)
> +
> +            if addrevisioncb:
> +                addrevisioncb(self, rev, node)
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