# HG changeset patch
# User Georges Racinet <georges.racinet@octobus.net>
# Date 1543584957 -3600
# Node ID a89b20a49c13c504ef9c60a8c903905e34723070
# Parent  619ee4039bd47a1b187898edc75df7ae85b14b80
rust-cpython: using MissingAncestors from Python code

As precedently done with LazyAncestors on cpython.rs, we test for the
presence of the 'rustext' module.

incrementalmissingrevs() has two callers within the Mercurial core:
`setdiscovery.partialdiscovery` and the `only()` revset.

This move shows a significant discovery performance improvement
in cases where the baseline is slow: using perfdiscovery on the PyPy
repos, prepared with `contrib/discovery-helper <repo> 50 100`, we
get averaged medians of 403ms with the Rust version vs 742ms without
(about 45% better).

But there are still indications that performance can be worse in cases
the baseline is fast, possibly due to the conversion from Python to
Rust and back becoming the bottleneck. We could measure this on
mozilla-central in cases were the delta is just a few changesets.
This requires confirmation, but if that's the reason, then an
upcoming `partialdiscovery` fully in Rust should solve the problem.

Differential Revision: https://phab.mercurial-scm.org/D5551

diff -r 619ee4039bd4 -r a89b20a49c13 mercurial/revlog.py
--- a/mercurial/revlog.py	Mon Jan 14 17:07:39 2019 +0100
+++ b/mercurial/revlog.py	Fri Nov 30 14:35:57 2018 +0100
@@ -893,6 +893,8 @@
         if common is None:
             common = [nullrev]
 
+        if rustext is not None:
+            return rustext.ancestor.MissingAncestors(self.index, common)
         return ancestor.incrementalmissingancestors(self.parentrevs, common)
 
     def findmissingrevs(self, common=None, heads=None):