discovery: add a devel.discovery.exchange-heads

Currently all discovery start with testing local and remote heads. For analysis
purpose we make it possible to disable that initial "handshake" and start
discovery with the whole repository as undecided.

Differential Revision: https://phab.mercurial-scm.org/D9801

mercurial/configitems.py

@@ -702,6 +702,13 b' coreconfigitem('
     b'debug.peer-request',
     default=False,
 )
+# If discovery.exchange-heads is False, the discovery will not start with
+# remote head fetching and local head querying.
+coreconfigitem(
+    b'devel',
+    b'discovery.exchange-heads',
+    default=True,
+)
 # If discovery.grow-sample is False, the sample size used in set discovery will
 # not be increased through the process
 coreconfigitem(

mercurial/setdiscovery.py

@@ -314,6 +314,8 b' def findcommonheads('
     else:
         ownheads = [rev for rev in cl.headrevs() if rev != nullrev]
 
+    initial_head_exchange = ui.configbool(b'devel', b'discovery.exchange-heads')
+
     # We also ask remote about all the local heads. That set can be arbitrarily
     # large, so we used to limit it size to `initialsamplesize`. We no longer
     # do as it proved counter productive. The skipped heads could lead to a
@@ -365,6 +367,7 b' def findcommonheads('
     #     graph (with many heads) attached to, but very independant to a the
     #     "simple" graph on the server. This is a fairly usual case and have
     #     not been met in the wild so far.
+    if initial_head_exchange:
     if remote.limitedarguments:
         sample = _limitsample(ownheads, initialsamplesize)
         # indices between sample and externalized version must match
@@ -392,6 +395,11 b' def findcommonheads('
         if srvheadhashes != [nullid]:
             return [nullid], True, srvheadhashes
         return [nullid], False, []
+    else:
+        # we still need the remote head for the function return
+        with remote.commandexecutor() as e:
+            fheads = e.callcommand(b'heads', {})
+        srvheadhashes = fheads.result()
 
     # start actual discovery (we note this before the next "if" for
     # compatibility reasons)
@@ -408,6 +416,7 b' def findcommonheads('
         except error.LookupError:
             continue
 
+    if initial_head_exchange:
     # early exit if we know all the specified remote heads already
     if len(knownsrvheads) == len(srvheadhashes):
         ui.debug(b"all remote heads known locally\n")
@@ -429,12 +438,13 b' def findcommonheads('
     disco = partialdiscovery(
         local, ownheads, hard_limit_sample, randomize=randomize
     )
+    if initial_head_exchange:
     # treat remote heads (and maybe own heads) as a first implicit sample
     # response
     disco.addcommons(knownsrvheads)
     disco.addinfo(zip(sample, yesno))
 
-    full = False
+    full = not initial_head_exchange
     progress = ui.makeprogress(_(b'searching'), unit=_(b'queries'))
     while not disco.iscomplete():
 

tests/test-setdiscovery.t

@@ -1412,23 +1412,22 b' One with >200 heads. We now switch to se'
       missing:                1040
   common heads: 3ee37d65064a
 
-  $ hg -R a debugdiscovery b --debug --config devel.discovery.randomize=false --config devel.discovery.grow-sample.rate=1.01
+  $ hg -R a debugdiscovery b --debug --config devel.discovery.exchange-heads=false --config devel.discovery.randomize=false --config devel.discovery.grow-sample.rate=1.01
   comparing with b
-  query 1; heads
   searching for changes
-  taking quick initial sample
-  query 2; still undecided: 1080, sample size is: 100
   sampling from both directions
-  query 3; still undecided: 980, sample size is: 200
+  query 1; still undecided: 1340, sample size is: 200
+  sampling from both directions
+  query 2; still undecided: 795, sample size is: 202
   sampling from both directions
-  query 4; still undecided: 497, sample size is: 202
+  query 3; still undecided: 525, sample size is: 204
   sampling from both directions
-  query 5; still undecided: 294, sample size is: 204
+  query 4; still undecided: 252, sample size is: 206
   sampling from both directions
-  query 6; still undecided: 90, sample size is: 90
-  6 total queries in *s (glob)
+  query 5; still undecided: 44, sample size is: 44
+  5 total queries in *s (glob)
   elapsed time: * seconds (glob)
-  round-trips:                   6
+  round-trips:                   5
   heads summary:
     total common heads:          1
       also local heads:          0