discovery: slowly increase sampling size

Some pathological discovery runs can requires many roundtrip. When this happens
things can get very slow.

To make the algorithm more resilience again such pathological case. We slowly
increase the sample size with each roundtrip (+5%). This will have a negligible
impact on "normal" discovery with few roundtrips, but a large positive impact of
case with many roundtrips. Asking more question per roundtrip helps to reduce
the undecided set faster. Instead of reducing the undecided set a linear speed
(in the worst case), we reduce it as a guaranteed (small) exponential rate. The
data below show this slow ramp up in sample size:

round trip    |    1 |     5 |    10 |    20 |     50 |     100 |       130 |
sample size   |  200 |   254 |   321 |   517 |  2 199 |  25 123 |   108 549 |
covered nodes |  200 | 1 357 | 2 821 | 7 031 | 42 658 | 524 530 | 2 276 755 |

To be a bit more concrete, lets take a very pathological case as an example. We
are doing discovery from a copy of Mozilla-try to a more recent version of
mozilla-unified. Mozilla-unified heads are unknown to the mozilla-try repo and
there are over 1 million "missing" changesets. (the discovery is "local" to
avoid network interference)

Without this change, the discovery:
- last 1858 seconds (31 minutes),
- does 1700 round trip,
- asking about 340 000 nodes.

With this change, the discovery:
- last 218 seconds (3 minutes, 38 seconds a -88% improvement),
- does 94 round trip (-94%),
- asking about 344 211 nodes (+1%).

Of course, this is an extreme case (and 3 minutes is still slow). However this
give a good example of how this sample size increase act as a safety net
catching any bad situations.

We could image a steeper increase than 5%. For example 10% would give the
following number:

round trip    |    1 |     5 |    10 |     20 |     50  |        75 |        100 |
sample size   |  200 |   321 |   514 |  1 326 |  23 060 |   249 812 |  2 706 594 |
covered nodes |  200 | 1 541 | 3 690 | 12 671 | 251 871 | 2 746 254 | 29 770 966 |

In parallel, it is useful to understand these pathological cases and improve
them. However the current change provides a general purpose safety net to smooth
the impact of pathological cases.

To avoid issue with older http server, the increase in sample size only occurs
if the protocol has not limit on command argument size.

mercurial/setdiscovery.py

@@ -256,7 +256,8 b' def findcommonheads(ui, local, remote,'
                     initialsamplesize=100,
                     fullsamplesize=200,
                     abortwhenunrelated=True,
-                    ancestorsof=None):
+                    ancestorsof=None,
+                    samplegrowth=1.05):
     '''Return a tuple (common, anyincoming, remoteheads) used to identify
     missing nodes from or in remote.
     '''
@@ -389,6 +390,8 b' def findcommonheads(ui, local, remote,'
                 ui.debug("taking initial sample\n")
             samplefunc = disco.takefullsample
             targetsize = fullsamplesize
+            if not remote.limitedarguments:
+                fullsamplesize = int(fullsamplesize * samplegrowth)
         else:
             # use even cheaper initial sample
             ui.debug("taking quick initial sample\n")

tests/test-setdiscovery.t

@@ -980,10 +980,10 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: \d+, sample size is: 200 (re)
+  query 4; still undecided: 435, sample size is: 210
   sampling from both directions
   searching: 5 queries
-  query 5; still undecided: 195, sample size is: 195
+  query 5; still undecided: 185, sample size is: 185
   5 total queries in *.????s (glob)
   elapsed time:  * seconds (glob)
   heads summary: