changeset 51577:b5d494f7d28a

push: rework the computation of fallbackheads to be correct

The previous computation tried to be smart but ended up being wrong. This was
caught by phase movement test while reworking the phase discovery logic to be
faster.

The previous logic was failing to catch case where the pushed set was not based
on a common heads (i.e. when the discovery seemed to have "over discovered"
content, outside the pushed set)

In the following graph, `e` is a common head and we `hg push -r f`. We need to
detect `c` as a fallback heads and we previous failed to do so::

  e
  |
  d f
  |/
  c
  |
  b
  |
  a

The performance impact of the change seems minimal. On the most impacted
repository at hand (mozilla-try), the slowdown seems mostly mixed in the
overall noise `hg push` but seems to be in the hundred of milliseconds order of
magnitude. When using rust, we seems to be a bit faster, probably because we
leverage more accelaratd internals.

I added a couple of performance related common for further investigation later
on.

--- a/mercurial/exchange.py	Fri Apr 05 11:05:54 2024 +0200
+++ b/mercurial/exchange.py	Tue Apr 09 02:54:12 2024 +0200
@@ -345,32 +345,56 @@
             # not target to push, all common are relevant
             return self.outgoing.commonheads
         unfi = self.repo.unfiltered()
-        # I want cheads = heads(::ancestorsof and ::commonheads)
-        # (ancestorsof is revs with secret changeset filtered out)
+        # I want cheads = heads(::push_heads and ::commonheads)
+        #
+        # To push, we already computed
+        #     common = (::commonheads)
+        #     missing = ((commonheads::push_heads) - commonheads)
+        #
+        # So we basically search
         #
-        # This can be expressed as:
-        #     cheads = ( (ancestorsof and ::commonheads)
-        #              + (commonheads and ::ancestorsof))"
-        #              )
+        #     almost_heads = heads((parents(missing) + push_heads) & common)
         #
-        # while trying to push we already computed the following:
-        #     common = (::commonheads)
-        #     missing = ((commonheads::ancestorsof) - commonheads)
+        # We use "almost" here as this can return revision that are ancestors
+        # of other in the set and we need to explicitly turn it into an
+        # antichain later. We can do so using:
+        #
+        #     cheads = heads(almost_heads::almost_heads)
         #
-        # We can pick:
-        # * ancestorsof part of common (::commonheads)
+        # In pratice the code is a bit more convulted to avoid some extra
+        # computation. It aims at doing the same computation as highlighted
+        # above however.
         common = self.outgoing.common
-        rev = self.repo.changelog.index.rev
-        cheads = [node for node in self.revs if rev(node) in common]
-        # and
-        # * commonheads parents on missing
-        revset = unfi.set(
-            b'%ln and parents(roots(%ln))',
-            self.outgoing.commonheads,
-            self.outgoing.missing,
-        )
-        cheads.extend(c.node() for c in revset)
-        return cheads
+        unfi = self.repo.unfiltered()
+        cl = unfi.changelog
+        to_rev = cl.index.rev
+        to_node = cl.node
+        parent_revs = cl.parentrevs
+        unselected = []
+        cheads = set()
+        # XXX-perf: `self.revs` and `outgoing.missing` could hold revs directly
+        for n in self.revs:
+            r = to_rev(n)
+            if r in common:
+                cheads.add(r)
+            else:
+                unselected.append(r)
+        known_non_heads = cl.ancestors(cheads, inclusive=True)
+        if unselected:
+            missing_revs = {to_rev(n) for n in self.outgoing.missing}
+            missing_revs.add(nullrev)
+            root_points = set()
+            for r in missing_revs:
+                p1, p2 = parent_revs(r)
+                if p1 not in missing_revs and p1 not in known_non_heads:
+                    root_points.add(p1)
+                if p2 not in missing_revs and p2 not in known_non_heads:
+                    root_points.add(p2)
+            if root_points:
+                heads = unfi.revs('heads(%ld::%ld)', root_points, root_points)
+                cheads.update(heads)
+        # XXX-perf: could this be a set of revision?
+        return [to_node(r) for r in sorted(cheads)]
 
     @property
     def commonheads(self):