[PATCH] discovery: be more conservative when adjusting the sample size

[Pierre-Yves David]

# HG changeset patch
# User Pierre-Yves David <pierre-yves.david at octobus.net>
# Date 1559726605 -7200
#      Wed Jun 05 11:23:25 2019 +0200
# Node ID 2c67430451fafcdd68770436c520e2d008428986
# Parent  12793787439538411013edffe0f9b98762d38a37
# EXP-Topic discovery-large-undecided
# Available At https://bitbucket.org/octobus/mercurial-devel/
#              hg pull https://bitbucket.org/octobus/mercurial-devel/ -r 2c67430451fa
discovery: be more conservative when adjusting the sample size

Since 5b34972a0094, the discovery will increase the sample size when it detect a
"complex" undecided set. However this detection focussed on the number of roots
only, this could regress discovery performance when the undecided set has many
roots that eventually get merged into a few heads.

To prevent such misbehavior, we adjust the logic to take in account both heads
and roots. The sample size will be increased only if both are especially large.

Performance testing on the same case as 5b34972a0094, does not show a
significant difference.

diff --git a/mercurial/setdiscovery.py b/mercurial/setdiscovery.py
--- a/mercurial/setdiscovery.py
+++ b/mercurial/setdiscovery.py
@@ -236,14 +236,19 @@ class partialdiscovery(object):
         sample = set(repo.revs('heads(%ld)', revs))
         parentrevs = self._parentsgetter()
 
+        ### update from both directions
+        revsroots = set(repo.revs('roots(%ld)', revs))
+        nbroots = len(revsroots)
+        revsheads = sample.copy()
+        nbheads = len(revsheads)
+
+        if not self._respectsize:
+            size = max(size, min(nbroots, nbheads))
+
         # update from heads
-        revsheads = sample.copy()
         _updatesample(revs, revsheads, sample, parentrevs)
 
         # update from roots
-        revsroots = set(repo.revs('roots(%ld)', revs))
-        if not self._respectsize:
-            size = max(size, len(revsroots))
 
         childrenrevs = self._childrengetter()