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. So this commit implements that functionality.

Because full delta recomputation can be *very* expensive (we're
talking several hours on the Firefox repository), we support
multiple modes of execution with regards to delta (re)use. This
will allow repository upgrades to choose the "level" of
processing/optimization they wish to perform when converting
revlogs.

It's not obvious from this commit, but "addrevisioncb" will be
used for progress reporting.

mercurial/revlog.py

@@ -1948,3 +1948,117 b' class revlog(object):'
         if not self._inline:
             res.append(self.datafile)
         return res
+
+    DELTAREUSEALWAYS = 'always'
+    DELTAREUSESAMEREVS = 'samerevs'
+    DELTAREUSENEVER = 'never'
+
+    DELTAREUSEALL = set(['always', 'samerevs', 'never'])
+
+    def clone(self, tr, destrevlog, addrevisioncb=None,
+              deltareuse=DELTAREUSESAMEREVS, aggressivemergedeltas=None):
+        """Copy this revlog to another, possibly with format changes.
+
+        The destination revlog will contain the same revisions and nodes.
+        However, it may not be bit-for-bit identical due to e.g. delta encoding
+        differences.
+
+        The ``deltareuse`` argument control how deltas from the existing revlog
+        are preserved in the destination revlog. The argument can have the
+        following values:
+
+        DELTAREUSEALWAYS
+           Deltas will always be reused (if possible), even if the destination
+           revlog would not select the same revisions for the delta. This is the
+           fastest mode of operation.
+        DELTAREUSESAMEREVS
+           Deltas will be reused if the destination revlog would pick the same
+           revisions for the delta. This mode strikes a balance between speed
+           and optimization.
+        DELTAREUSENEVER
+           Deltas will never be reused. This is the slowest mode of execution.
+           This mode can be used to recompute deltas (e.g. if the diff/delta
+           algorithm changes).
+
+        Delta computation can be slow, so the choice of delta reuse policy can
+        significantly affect run time.
+
+        The default policy (``DELTAREUSESAMEREVS``) strikes a balance between
+        two extremes. Deltas will be reused if they are appropriate. But if the
+        delta could choose a better revision, it will do so. This means if you
+        are converting a non-generaldelta revlog to a generaldelta revlog,
+        deltas will be recomputed if the delta's parent isn't a parent of the
+        revision.
+
+        In addition to the delta policy, the ``aggressivemergedeltas`` argument
+        controls whether to compute deltas against both parents for merges.
+        By default, the current default is used.
+        """
+        if deltareuse not in self.DELTAREUSEALL:
+            raise ValueError(_('value for deltareuse invalid: %s') % deltareuse)
+
+        if len(destrevlog):
+            raise ValueError(_('destination revlog is not empty'))
+
+        if getattr(self, 'filteredrevs', None):
+            raise ValueError(_('source revlog has filtered revisions'))
+        if getattr(destrevlog, 'filteredrevs', None):
+            raise ValueError(_('destination revlog has filtered revisions'))
+
+        # lazydeltabase controls whether to reuse a cached delta, if possible.
+        oldlazydeltabase = destrevlog._lazydeltabase
+        oldamd = destrevlog._aggressivemergedeltas
+
+        try:
+            if deltareuse == self.DELTAREUSEALWAYS:
+                destrevlog._lazydeltabase = True
+            elif deltareuse == self.DELTAREUSESAMEREVS:
+                destrevlog._lazydeltabase = False
+
+            destrevlog._aggressivemergedeltas = aggressivemergedeltas or oldamd
+
+            populatecachedelta = deltareuse in (self.DELTAREUSEALWAYS,
+                                                self.DELTAREUSESAMEREVS)
+
+            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.
+                flags = entry[0] & 0xffff
+                linkrev = entry[4]
+                p1 = index[entry[5]][7]
+                p2 = index[entry[6]][7]
+                node = entry[7]
+
+                # (Possibly) reuse the delta from the revlog if allowed and
+                # the revlog chunk is a delta.
+                cachedelta = None
+                text = None
+                if populatecachedelta:
+                    dp = self.deltaparent(rev)
+                    if dp != nullrev:
+                        cachedelta = (dp, str(self._chunk(rev)))
+
+                if not cachedelta:
+                    text = self.revision(rev)
+
+                ifh = destrevlog.opener(destrevlog.indexfile, 'a+',
+                                        checkambig=False)
+                dfh = None
+                if not destrevlog._inline:
+                    dfh = destrevlog.opener(destrevlog.datafile, 'a+')
+                try:
+                    destrevlog._addrevision(node, text, tr, linkrev, p1, p2,
+                                            flags, cachedelta, ifh, dfh)
+                finally:
+                    if dfh:
+                        dfh.close()
+                    ifh.close()
+
+                if addrevisioncb:
+                    addrevisioncb(self, rev, node)
+        finally:
+            destrevlog._lazydeltabase = oldlazydeltabase
+            destrevlog._aggressivemergedeltas = oldamd