discovery: new devel.discovery.randomize option

By default, this is True, but setting it to False is a uniform
way to kill all randomness in integration tests such as test-setdiscovery.t

By "uniform" we mean that it can be passed to implementations in other
languages, for which the monkey-patching of random.sample would be
irrelevant.

In the above mentioned test file, we use it right away,
replacing the adhoc extension that had the same purpose, and to derandomize a
case with many round-trips, that we'll need to behave uniformly in the Rust
version.

Differential Revision: https://phab.mercurial-scm.org/D6427

mercurial/configitems.py

@@ -415,6 +415,9 b" coreconfigitem('devel', 'debug.extension"
 coreconfigitem('devel', 'debug.peer-request',
     default=False,
 )
+coreconfigitem('devel', 'discovery.randomize',
+    default=True,
+)
 _registerdiffopts(section='diff')
 coreconfigitem('email', 'bcc',
     default=None,

mercurial/setdiscovery.py

@@ -390,7 +390,9 b' def findcommonheads(ui, local, remote,'
 
     # full blown discovery
 
-    disco = partialdiscovery(local, ownheads, remote.limitedarguments)
+    randomize = ui.configbool('devel', 'discovery.randomize')
+    disco = partialdiscovery(local, ownheads, remote.limitedarguments,
+                             randomize=randomize)
     # treat remote heads (and maybe own heads) as a first implicit sample
     # response
     disco.addcommons(knownsrvheads)

tests/test-setdiscovery.t

@@ -968,7 +968,7 b' One with >200 heads. We now switch to se'
   updating to branch b
   0 files updated, 0 files merged, 0 files removed, 0 files unresolved
 
-  $ hg -R a debugdiscovery b --debug --verbose --config progress.debug=true
+  $ hg -R a debugdiscovery b --debug --verbose --config progress.debug=true --config devel.discovery.randomize=false
   comparing with b
   query 1; heads
   searching for changes
@@ -980,13 +980,14 b' One with >200 heads. We now switch to se'
   query 3; still undecided: 980, sample size is: 200
   sampling from both directions
   searching: 4 queries
-  query 4; still undecided: 435, sample size is: 210 (no-py3 !)
-  query 4; still undecided: 430, sample size is: 210 (py3 !)
+  query 4; still undecided: 497, sample size is: 210
   sampling from both directions
   searching: 5 queries
-  query 5; still undecided: 185, sample size is: 185 (no-py3 !)
-  query 5; still undecided: 187, sample size is: 187 (py3 !)
-  5 total queries in *.????s (glob)
+  query 5; still undecided: 285, sample size is: 220
+  sampling from both directions
+  searching: 6 queries
+  query 6; still undecided: 63, sample size is: 63
+  6 total queries in *.????s (glob)
   elapsed time:  * seconds (glob)
   heads summary:
     total common heads:          1
@@ -1095,16 +1096,9 b" The case where all the 'initialsamplesiz"
 give 'all remote heads known locally' without checking the remaining heads -
 fixed in 86c35b7ae300:
 
-  $ cat >> $TESTTMP/unrandomsample.py << EOF
-  > import random
-  > def sample(population, k):
-  >     return sorted(population)[:k]
-  > random.sample = sample
-  > EOF
-
   $ cat >> r1/.hg/hgrc << EOF
-  > [extensions]
-  > unrandomsample = $TESTTMP/unrandomsample.py
+  > [devel]
+  > discovery.randomize = False
   > EOF
 
   $ hg -R r1 outgoing r2 -T'{rev} ' --config extensions.blackbox= \