phases: large rewrite on retract boundary

The new code is still pure Python, so we still have room to going significantly
faster. However its complexity of the complex part is `O(|[min_new_draft, tip]|)` instead of
`O(|[min_draft, tip]|` which should help tremendously one repository with old
draft (like mercurial-devel or mozilla-try).

This is especially useful as the most common "retract boundary" operation
happens when we commit/rewrite new drafts or when we push new draft to a
non-publishing server. In this case, the smallest new_revs is very close to the
tip and there is very few work to do.

A few smaller optimisation could be done for these cases and will be introduced in
later changesets.

We still have iterate over large sets of roots, but this is already a great
improvement for a very small amount of work. We gather information on the
affected changeset as we go as we can put it to use in the next changesets.
This extra data collection might slowdown the `register_new` case a bit, however
for register_new, it should not really matters. The set of new nodes is either
small, so the impact is negligible, or the set of new nodes is large, and the
amount of work to do to had them will dominate the overhead the collecting
information in `changed_revs`.

As this new code compute the changes on the fly, it unlock other interesting
improvement to be done in later changeset.

mercurial/phases.py

@@ -677,15 +677,7 b' class phasecache:'
 
     def _retractboundary(self, repo, tr, targetphase, nodes=None, revs=None):
         if targetphase == public:
-            return False
-        # Be careful to preserve shallow-copied values: do not update
-        # phaseroots values, replace them.
-        if revs is None:
-            revs = []
-        if nodes is None:
-            nodes = []
-        if not revs and not nodes:
-            return False
+            return {}
         if (
             targetphase == internal
             and not supportinternal(repo)
@@ -695,34 +687,103 b' class phasecache:'
             name = phasenames[targetphase]
             msg = b'this repository does not support the %s phase' % name
             raise error.ProgrammingError(msg)
+        assert repo.filtername is None
+        cl = repo.changelog
+        torev = cl.index.rev
+        new_revs = set()
+        if revs is not None:
+            new_revs.update(revs)
+        if nodes is not None:
+            new_revs.update(torev(node) for node in nodes)
+        if not new_revs:  # bail out early to avoid the loadphaserevs call
+            return {}  # note: why do people call retractboundary with nothing ?
 
-        torev = repo.changelog.index.rev
+        if nullrev in new_revs:
+            raise error.Abort(_(b'cannot change null revision phase'))
+
+        # Compute change in phase roots by walking the graph
+        #
+        # note: If we had a cheap parent → children mapping we could do
+        # something even cheaper/more-bounded
+        #
+        # The idea would be to walk from item in new_revs stopping at
+        # descendant with phases >= target_phase.
+        #
+        # 1) This detect new_revs that are not new_roots (either already >=
+        #    target_phase or reachable though another new_revs
+        # 2) This detect replaced current_roots as we reach them
+        # 3) This can avoid walking to the tip if we retract over a small
+        #    branch.
+        #
+        # So instead, we do a variation of this, we walk from the smaller new
+        # revision to the tip to avoid missing any potential children.
+        #
+        # The following code would be a good candidate for native code… if only
+        # we could knew the phase of a changeset efficiently in native code.
+        parents = cl.parentrevs
+        phase = self.phase
+        new_roots = set()  # roots added by this phases
+        changed_revs = {}  # revision affected by this call
+        replaced_roots = set()  # older roots replaced by this call
         currentroots = self._phaseroots[targetphase]
-        finalroots = oldroots = set(currentroots)
-        newroots = [torev(node) for node in nodes] + [r for r in revs]
-        newroots = [
-            rev for rev in newroots if self.phase(repo, rev) < targetphase
-        ]
+        start = min(new_revs)
+        end = len(cl)
+        rev_phases = [None] * (end - start)
+        for r in range(start, end):
 
-        if newroots:
-            if nullrev in newroots:
-                raise error.Abort(_(b'cannot change null revision phase'))
-            # do not break the CoW assumption of the shallow copy
-            currentroots = currentroots.copy()
-            currentroots.update(newroots)
+            # gather information about the current_rev
+            r_phase = phase(repo, r)
+            p_phase = None  # phase inherited from parents
+            p1, p2 = parents(r)
+            if p1 >= start:
+                p1_phase = rev_phases[p1 - start]
+                if p1_phase is not None:
+                    p_phase = p1_phase
+            if p2 >= start:
+                p2_phase = rev_phases[p2 - start]
+                if p2_phase is not None:
+                    if p_phase is not None:
+                        p_phase = max(p_phase, p2_phase)
+                    else:
+                        p_phase = p2_phase
 
-            # Only compute new roots for revs above the roots that are being
-            # retracted.
-            minnewroot = min(newroots)
-            aboveroots = [rev for rev in currentroots if rev >= minnewroot]
-            updatedroots = repo.revs(b'roots(%ld::)', aboveroots)
+            # assess the situation
+            if r in new_revs and r_phase < targetphase:
+                if p_phase is None or p_phase < targetphase:
+                    new_roots.add(r)
+                rev_phases[r - start] = targetphase
+                changed_revs[r] = r_phase
+            elif p_phase is None:
+                rev_phases[r - start] = r_phase
+            else:
+                if p_phase > r_phase:
+                    rev_phases[r - start] = p_phase
+                else:
+                    rev_phases[r - start] = r_phase
+                if p_phase == targetphase:
+                    if p_phase > r_phase:
+                        changed_revs[r] = r_phase
+                    elif r in currentroots:
+                        replaced_roots.add(r)
 
-            finalroots = {rev for rev in currentroots if rev < minnewroot}
-            finalroots.update(updatedroots)
-        if finalroots != oldroots:
-            self._updateroots(repo, targetphase, finalroots, tr)
-            return True
-        return False
+        if new_roots:
+            assert changed_revs
+            final_roots = new_roots | currentroots - replaced_roots
+            self._updateroots(repo, targetphase, final_roots, tr)
+            if targetphase > 1:
+                retracted = set(changed_revs)
+                for lower_phase in range(1, targetphase):
+                    lower_roots = self._phaseroots.get(lower_phase)
+                    if lower_roots is None:
+                        continue
+                    if lower_roots & retracted:
+                        simpler_roots = lower_roots - retracted
+                        self._updateroots(repo, lower_phase, simpler_roots, tr)
+            return changed_revs
+        else:
+            assert not changed_revs
+            assert not replaced_roots
+            return {}
 
     def register_strip(
         self,