setdiscovery: back out changeset 5cfdf6137af8 (issue5809)

As explained in the bug report, this commit caused a performance
regression. The problem occurs when the local repo has very many
heads. Before 5cfdf6137af8, we used to get the remote's list of heads
and if these heads mostly overlapped with the local repo's heads, we
would mark these common heads as common, which would greatly reduce
the size of the set of undecided nodes.

Note that a similar problem existed before 5cfdf6137af8: If the local
repo had very many heads and the server just had a few (or many heads
from a disjoint set), we would do the same kind of slow discovery as
we would with 5cfdf6137af8 in the case where local and remote repos
share a large set of common nodes.

For now, we just back out 5cfdf6137af8. We should improve the
discovery in the "local has many heads, remote has few heads" case,
but let's do that after backing this out.

Differential Revision: https://phab.mercurial-scm.org/D2643

mercurial/discovery.py

@@ -1,525 +1,525
 # discovery.py - protocol changeset discovery functions
 #
 # Copyright 2010 Matt Mackall <[e-mail]>
 #
 # This software may be used and distributed according to the terms of the
 # GNU General Public License version 2 or any later version.
 
 from __future__ import absolute_import
 
 import functools
 
 from .i18n import _
 from .node import (
     hex,
     nullid,
     short,
 )
 
 from . import (
     bookmarks,
     branchmap,
     error,
     phases,
     scmutil,
     setdiscovery,
     treediscovery,
     util,
 )
 
 def findcommonincoming(repo, remote, heads=None, force=False, ancestorsof=None):
     """Return a tuple (common, anyincoming, heads) used to identify the common
     subset of nodes between repo and remote.
 
     "common" is a list of (at least) the heads of the common subset.
     "anyincoming" is testable as a boolean indicating if any nodes are missing
       locally. If remote does not support getbundle, this actually is a list of
       roots of the nodes that would be incoming, to be supplied to
       changegroupsubset. No code except for pull should be relying on this fact
       any longer.
     "heads" is either the supplied heads, or else the remote's heads.
     "ancestorsof" if not None, restrict the discovery to a subset defined by
       these nodes. Changeset outside of this set won't be considered (and
       won't appears in "common")
 
     If you pass heads and they are all known locally, the response lists just
     these heads in "common" and in "heads".
 
     Please use findcommonoutgoing to compute the set of outgoing nodes to give
     extensions a good hook into outgoing.
     """
 
     if not remote.capable('getbundle'):
         return treediscovery.findcommonincoming(repo, remote, heads, force)
 
     if heads:
         knownnode = repo.changelog.hasnode # no nodemap until it is filtered
         if all(knownnode(h) for h in heads):
             return (heads, False, heads)
 
-    res = setdiscovery.findcommonheads(repo.ui, repo, remote, heads,
+    res = setdiscovery.findcommonheads(repo.ui, repo, remote,
                                        abortwhenunrelated=not force,
                                        ancestorsof=ancestorsof)
     common, anyinc, srvheads = res
     return (list(common), anyinc, heads or list(srvheads))
 
 class outgoing(object):
     '''Represents the set of nodes present in a local repo but not in a
     (possibly) remote one.
 
     Members:
 
       missing is a list of all nodes present in local but not in remote.
       common is a list of all nodes shared between the two repos.
       excluded is the list of missing changeset that shouldn't be sent remotely.
       missingheads is the list of heads of missing.
       commonheads is the list of heads of common.
 
     The sets are computed on demand from the heads, unless provided upfront
     by discovery.'''
 
     def __init__(self, repo, commonheads=None, missingheads=None,
                  missingroots=None):
         # at least one of them must not be set
         assert None in (commonheads, missingroots)
         cl = repo.changelog
         if missingheads is None:
             missingheads = cl.heads()
         if missingroots:
             discbases = []
             for n in missingroots:
                 discbases.extend([p for p in cl.parents(n) if p != nullid])
             # TODO remove call to nodesbetween.
             # TODO populate attributes on outgoing instance instead of setting
             # discbases.
             csets, roots, heads = cl.nodesbetween(missingroots, missingheads)
             included = set(csets)
             missingheads = heads
             commonheads = [n for n in discbases if n not in included]
         elif not commonheads:
             commonheads = [nullid]
         self.commonheads = commonheads
         self.missingheads = missingheads
         self._revlog = cl
         self._common = None
         self._missing = None
         self.excluded = []
 
     def _computecommonmissing(self):
         sets = self._revlog.findcommonmissing(self.commonheads,
                                               self.missingheads)
         self._common, self._missing = sets
 
     @util.propertycache
     def common(self):
         if self._common is None:
             self._computecommonmissing()
         return self._common
 
     @util.propertycache
     def missing(self):
         if self._missing is None:
             self._computecommonmissing()
         return self._missing
 
 def findcommonoutgoing(repo, other, onlyheads=None, force=False,
                        commoninc=None, portable=False):
     '''Return an outgoing instance to identify the nodes present in repo but
     not in other.
 
     If onlyheads is given, only nodes ancestral to nodes in onlyheads
     (inclusive) are included. If you already know the local repo's heads,
     passing them in onlyheads is faster than letting them be recomputed here.
 
     If commoninc is given, it must be the result of a prior call to
     findcommonincoming(repo, other, force) to avoid recomputing it here.
 
     If portable is given, compute more conservative common and missingheads,
     to make bundles created from the instance more portable.'''
     # declare an empty outgoing object to be filled later
     og = outgoing(repo, None, None)
 
     # get common set if not provided
     if commoninc is None:
         commoninc = findcommonincoming(repo, other, force=force,
                                        ancestorsof=onlyheads)
     og.commonheads, _any, _hds = commoninc
 
     # compute outgoing
     mayexclude = (repo._phasecache.phaseroots[phases.secret] or repo.obsstore)
     if not mayexclude:
         og.missingheads = onlyheads or repo.heads()
     elif onlyheads is None:
         # use visible heads as it should be cached
         og.missingheads = repo.filtered("served").heads()
         og.excluded = [ctx.node() for ctx in repo.set('secret() or extinct()')]
     else:
         # compute common, missing and exclude secret stuff
         sets = repo.changelog.findcommonmissing(og.commonheads, onlyheads)
         og._common, allmissing = sets
         og._missing = missing = []
         og.excluded = excluded = []
         for node in allmissing:
             ctx = repo[node]
             if ctx.phase() >= phases.secret or ctx.extinct():
                 excluded.append(node)
             else:
                 missing.append(node)
         if len(missing) == len(allmissing):
             missingheads = onlyheads
         else: # update missing heads
             missingheads = phases.newheads(repo, onlyheads, excluded)
         og.missingheads = missingheads
     if portable:
         # recompute common and missingheads as if -r<rev> had been given for
         # each head of missing, and --base <rev> for each head of the proper
         # ancestors of missing
         og._computecommonmissing()
         cl = repo.changelog
         missingrevs = set(cl.rev(n) for n in og._missing)
         og._common = set(cl.ancestors(missingrevs)) - missingrevs
         commonheads = set(og.commonheads)
         og.missingheads = [h for h in og.missingheads if h not in commonheads]
 
     return og
 
 def _headssummary(pushop):
     """compute a summary of branch and heads status before and after push
 
     return {'branch': ([remoteheads], [newheads],
                        [unsyncedheads], [discardedheads])} mapping
 
     - branch: the branch name,
     - remoteheads: the list of remote heads known locally
                    None if the branch is new,
     - newheads: the new remote heads (known locally) with outgoing pushed,
     - unsyncedheads: the list of remote heads unknown locally,
     - discardedheads: the list of heads made obsolete by the push.
     """
     repo = pushop.repo.unfiltered()
     remote = pushop.remote
     outgoing = pushop.outgoing
     cl = repo.changelog
     headssum = {}
     # A. Create set of branches involved in the push.
     branches = set(repo[n].branch() for n in outgoing.missing)
     remotemap = remote.branchmap()
     newbranches = branches - set(remotemap)
     branches.difference_update(newbranches)
 
     # A. register remote heads
     remotebranches = set()
     for branch, heads in remote.branchmap().iteritems():
         remotebranches.add(branch)
         known = []
         unsynced = []
         knownnode = cl.hasnode # do not use nodemap until it is filtered
         for h in heads:
             if knownnode(h):
                 known.append(h)
             else:
                 unsynced.append(h)
         headssum[branch] = (known, list(known), unsynced)
     # B. add new branch data
     missingctx = list(repo[n] for n in outgoing.missing)
     touchedbranches = set()
     for ctx in missingctx:
         branch = ctx.branch()
         touchedbranches.add(branch)
         if branch not in headssum:
             headssum[branch] = (None, [], [])
 
     # C drop data about untouched branches:
     for branch in remotebranches - touchedbranches:
         del headssum[branch]
 
     # D. Update newmap with outgoing changes.
     # This will possibly add new heads and remove existing ones.
     newmap = branchmap.branchcache((branch, heads[1])
                                  for branch, heads in headssum.iteritems()
                                  if heads[0] is not None)
     newmap.update(repo, (ctx.rev() for ctx in missingctx))
     for branch, newheads in newmap.iteritems():
         headssum[branch][1][:] = newheads
     for branch, items in headssum.iteritems():
         for l in items:
             if l is not None:
                 l.sort()
         headssum[branch] = items + ([],)
 
     # If there are no obsstore, no post processing are needed.
     if repo.obsstore:
         torev = repo.changelog.rev
         futureheads = set(torev(h) for h in outgoing.missingheads)
         futureheads |= set(torev(h) for h in outgoing.commonheads)
         allfuturecommon = repo.changelog.ancestors(futureheads, inclusive=True)
         for branch, heads in sorted(headssum.iteritems()):
             remoteheads, newheads, unsyncedheads, placeholder = heads
             result = _postprocessobsolete(pushop, allfuturecommon, newheads)
             headssum[branch] = (remoteheads, sorted(result[0]), unsyncedheads,
                                 sorted(result[1]))
     return headssum
 
 def _oldheadssummary(repo, remoteheads, outgoing, inc=False):
     """Compute branchmapsummary for repo without branchmap support"""
 
     # 1-4b. old servers: Check for new topological heads.
     # Construct {old,new}map with branch = None (topological branch).
     # (code based on update)
     knownnode = repo.changelog.hasnode # no nodemap until it is filtered
     oldheads = sorted(h for h in remoteheads if knownnode(h))
     # all nodes in outgoing.missing are children of either:
     # - an element of oldheads
     # - another element of outgoing.missing
     # - nullrev
     # This explains why the new head are very simple to compute.
     r = repo.set('heads(%ln + %ln)', oldheads, outgoing.missing)
     newheads = sorted(c.node() for c in r)
     # set some unsynced head to issue the "unsynced changes" warning
     if inc:
         unsynced = [None]
     else:
         unsynced = []
     return {None: (oldheads, newheads, unsynced, [])}
 
 def _nowarnheads(pushop):
     # Compute newly pushed bookmarks. We don't warn about bookmarked heads.
     repo = pushop.repo.unfiltered()
     remote = pushop.remote
     localbookmarks = repo._bookmarks
     remotebookmarks = remote.listkeys('bookmarks')
     bookmarkedheads = set()
 
     # internal config: bookmarks.pushing
     newbookmarks = [localbookmarks.expandname(b)
                     for b in pushop.ui.configlist('bookmarks', 'pushing')]
 
     for bm in localbookmarks:
         rnode = remotebookmarks.get(bm)
         if rnode and rnode in repo:
             lctx, rctx = repo[bm], repo[rnode]
             if bookmarks.validdest(repo, rctx, lctx):
                 bookmarkedheads.add(lctx.node())
         else:
             if bm in newbookmarks and bm not in remotebookmarks:
                 bookmarkedheads.add(repo[bm].node())
 
     return bookmarkedheads
 
 def checkheads(pushop):
     """Check that a push won't add any outgoing head
 
     raise Abort error and display ui message as needed.
     """
 
     repo = pushop.repo.unfiltered()
     remote = pushop.remote
     outgoing = pushop.outgoing
     remoteheads = pushop.remoteheads
     newbranch = pushop.newbranch
     inc = bool(pushop.incoming)
 
     # Check for each named branch if we're creating new remote heads.
     # To be a remote head after push, node must be either:
     # - unknown locally
     # - a local outgoing head descended from update
     # - a remote head that's known locally and not
     #   ancestral to an outgoing head
     if remoteheads == [nullid]:
         # remote is empty, nothing to check.
         return
 
     if remote.capable('branchmap'):
         headssum = _headssummary(pushop)
     else:
         headssum = _oldheadssummary(repo, remoteheads, outgoing, inc)
     pushop.pushbranchmap = headssum
     newbranches = [branch for branch, heads in headssum.iteritems()
                    if heads[0] is None]
     # 1. Check for new branches on the remote.
     if newbranches and not newbranch:  # new branch requires --new-branch
         branchnames = ', '.join(sorted(newbranches))
         raise error.Abort(_("push creates new remote branches: %s!")
                            % branchnames,
                          hint=_("use 'hg push --new-branch' to create"
                                 " new remote branches"))
 
     # 2. Find heads that we need not warn about
     nowarnheads = _nowarnheads(pushop)
 
     # 3. Check for new heads.
     # If there are more heads after the push than before, a suitable
     # error message, depending on unsynced status, is displayed.
     errormsg = None
     for branch, heads in sorted(headssum.iteritems()):
         remoteheads, newheads, unsyncedheads, discardedheads = heads
         # add unsynced data
         if remoteheads is None:
             oldhs = set()
         else:
             oldhs = set(remoteheads)
         oldhs.update(unsyncedheads)
         dhs = None # delta heads, the new heads on branch
         newhs = set(newheads)
         newhs.update(unsyncedheads)
         if unsyncedheads:
             if None in unsyncedheads:
                 # old remote, no heads data
                 heads = None
             else:
                 heads = scmutil.nodesummaries(repo, unsyncedheads)
             if heads is None:
                 repo.ui.status(_("remote has heads that are "
                                  "not known locally\n"))
             elif branch is None:
                 repo.ui.status(_("remote has heads that are "
                                  "not known locally: %s\n") % heads)
             else:
                 repo.ui.status(_("remote has heads on branch '%s' that are "
                                  "not known locally: %s\n") % (branch, heads))
         if remoteheads is None:
             if len(newhs) > 1:
                 dhs = list(newhs)
                 if errormsg is None:
                     errormsg = (_("push creates new branch '%s' "
                                   "with multiple heads") % (branch))
                     hint = _("merge or"
                              " see 'hg help push' for details about"
                              " pushing new heads")
         elif len(newhs) > len(oldhs):
             # remove bookmarked or existing remote heads from the new heads list
             dhs = sorted(newhs - nowarnheads - oldhs)
         if dhs:
             if errormsg is None:
                 if branch not in ('default', None):
                     errormsg = _("push creates new remote head %s "
                                  "on branch '%s'!") % (short(dhs[0]), branch)
                 elif repo[dhs[0]].bookmarks():
                     errormsg = _("push creates new remote head %s "
                                  "with bookmark '%s'!") % (
                                  short(dhs[0]), repo[dhs[0]].bookmarks()[0])
                 else:
                     errormsg = _("push creates new remote head %s!"
                                  ) % short(dhs[0])
                 if unsyncedheads:
                     hint = _("pull and merge or"
                              " see 'hg help push' for details about"
                              " pushing new heads")
                 else:
                     hint = _("merge or"
                              " see 'hg help push' for details about"
                              " pushing new heads")
             if branch is None:
                 repo.ui.note(_("new remote heads:\n"))
             else:
                 repo.ui.note(_("new remote heads on branch '%s':\n") % branch)
             for h in dhs:
                 repo.ui.note((" %s\n") % short(h))
     if errormsg:
         raise error.Abort(errormsg, hint=hint)
 
 def _postprocessobsolete(pushop, futurecommon, candidate_newhs):
     """post process the list of new heads with obsolescence information
 
     Exists as a sub-function to contain the complexity and allow extensions to
     experiment with smarter logic.
 
     Returns (newheads, discarded_heads) tuple
     """
     # known issue
     #
     # * We "silently" skip processing on all changeset unknown locally
     #
     # * if <nh> is public on the remote, it won't be affected by obsolete
     #     marker and a new is created
 
     # define various utilities and containers
     repo = pushop.repo
     unfi = repo.unfiltered()
     tonode = unfi.changelog.node
     torev = unfi.changelog.nodemap.get
     public = phases.public
     getphase = unfi._phasecache.phase
     ispublic = (lambda r: getphase(unfi, r) == public)
     ispushed = (lambda n: torev(n) in futurecommon)
     hasoutmarker = functools.partial(pushingmarkerfor, unfi.obsstore, ispushed)
     successorsmarkers = unfi.obsstore.successors
     newhs = set() # final set of new heads
     discarded = set() # new head of fully replaced branch
 
     localcandidate = set() # candidate heads known locally
     unknownheads = set() # candidate heads unknown locally
     for h in candidate_newhs:
         if h in unfi:
             localcandidate.add(h)
         else:
             if successorsmarkers.get(h) is not None:
                 msg = ('checkheads: remote head unknown locally has'
                        ' local marker: %s\n')
                 repo.ui.debug(msg % hex(h))
             unknownheads.add(h)
 
     # fast path the simple case
     if len(localcandidate) == 1:
         return unknownheads | set(candidate_newhs), set()
 
     # actually process branch replacement
     while localcandidate:
         nh = localcandidate.pop()
         # run this check early to skip the evaluation of the whole branch
         if (torev(nh) in futurecommon or ispublic(torev(nh))):
             newhs.add(nh)
             continue
 
         # Get all revs/nodes on the branch exclusive to this head
         # (already filtered heads are "ignored"))
         branchrevs = unfi.revs('only(%n, (%ln+%ln))',
                                nh, localcandidate, newhs)
         branchnodes = [tonode(r) for r in branchrevs]
 
         # The branch won't be hidden on the remote if
         # * any part of it is public,
         # * any part of it is considered part of the result by previous logic,
         # * if we have no markers to push to obsolete it.
         if (any(ispublic(r) for r in branchrevs)
                 or any(torev(n) in futurecommon for n in branchnodes)
                 or any(not hasoutmarker(n) for n in branchnodes)):
             newhs.add(nh)
         else:
             # note: there is a corner case if there is a merge in the branch.
             # we might end up with -more- heads.  However, these heads are not
             # "added" by the push, but more by the "removal" on the remote so I
             # think is a okay to ignore them,
             discarded.add(nh)
     newhs |= unknownheads
     return newhs, discarded
 
 def pushingmarkerfor(obsstore, ispushed, node):
     """true if some markers are to be pushed for node
 
     We cannot just look in to the pushed obsmarkers from the pushop because
     discovery might have filtered relevant markers. In addition listing all
     markers relevant to all changesets in the pushed set would be too expensive
     (O(len(repo)))
 
     (note: There are cache opportunity in this function. but it would requires
     a two dimensional stack.)
     """
     successorsmarkers = obsstore.successors
     stack = [node]
     seen = set(stack)
     while stack:
         current = stack.pop()
         if ispushed(current):
             return True
         markers = successorsmarkers.get(current, ())
         # markers fields = ('prec', 'succs', 'flag', 'meta', 'date', 'parents')
         for m in markers:
             nexts = m[1] # successors
             if not nexts: # this is a prune marker
                 nexts = m[5] or () # parents
             for n in nexts:
                 if n not in seen:
                     seen.add(n)
                     stack.append(n)
     return False

mercurial/setdiscovery.py

@@ -1,267 +1,263
 # setdiscovery.py - improved discovery of common nodeset for mercurial
 #
 # Copyright 2010 Benoit Boissinot <[e-mail]>
 # and Peter Arrenbrecht <[e-mail]>
 #
 # This software may be used and distributed according to the terms of the
 # GNU General Public License version 2 or any later version.
 """
 Algorithm works in the following way. You have two repository: local and
 remote. They both contains a DAG of changelists.
 
 The goal of the discovery protocol is to find one set of node *common*,
 the set of nodes shared by local and remote.
 
 One of the issue with the original protocol was latency, it could
 potentially require lots of roundtrips to discover that the local repo was a
 subset of remote (which is a very common case, you usually have few changes
 compared to upstream, while upstream probably had lots of development).
 
 The new protocol only requires one interface for the remote repo: `known()`,
 which given a set of changelists tells you if they are present in the DAG.
 
 The algorithm then works as follow:
 
  - We will be using three sets, `common`, `missing`, `unknown`. Originally
  all nodes are in `unknown`.
  - Take a sample from `unknown`, call `remote.known(sample)`
    - For each node that remote knows, move it and all its ancestors to `common`
    - For each node that remote doesn't know, move it and all its descendants
    to `missing`
  - Iterate until `unknown` is empty
 
 There are a couple optimizations, first is instead of starting with a random
 sample of missing, start by sending all heads, in the case where the local
 repo is a subset, you computed the answer in one round trip.
 
 Then you can do something similar to the bisecting strategy used when
 finding faulty changesets. Instead of random samples, you can try picking
 nodes that will maximize the number of nodes that will be
 classified with it (since all ancestors or descendants will be marked as well).
 """
 
 from __future__ import absolute_import
 
 import collections
 import random
 
 from .i18n import _
 from .node import (
     nullid,
     nullrev,
 )
 from . import (
     dagutil,
     error,
     util,
 )
 
 def _updatesample(dag, nodes, sample, quicksamplesize=0):
     """update an existing sample to match the expected size
 
     The sample is updated with nodes exponentially distant from each head of the
     <nodes> set. (H~1, H~2, H~4, H~8, etc).
 
     If a target size is specified, the sampling will stop once this size is
     reached. Otherwise sampling will happen until roots of the <nodes> set are
     reached.
 
     :dag: a dag object from dagutil
     :nodes:  set of nodes we want to discover (if None, assume the whole dag)
     :sample: a sample to update
     :quicksamplesize: optional target size of the sample"""
     # if nodes is empty we scan the entire graph
     if nodes:
         heads = dag.headsetofconnecteds(nodes)
     else:
         heads = dag.heads()
     dist = {}
     visit = collections.deque(heads)
     seen = set()
     factor = 1
     while visit:
         curr = visit.popleft()
         if curr in seen:
             continue
         d = dist.setdefault(curr, 1)
         if d > factor:
             factor *= 2
         if d == factor:
             sample.add(curr)
             if quicksamplesize and (len(sample) >= quicksamplesize):
                 return
         seen.add(curr)
         for p in dag.parents(curr):
             if not nodes or p in nodes:
                 dist.setdefault(p, d + 1)
                 visit.append(p)
 
 def _takequicksample(dag, nodes, size):
     """takes a quick sample of size <size>
 
     It is meant for initial sampling and focuses on querying heads and close
     ancestors of heads.
 
     :dag: a dag object
     :nodes: set of nodes to discover
     :size: the maximum size of the sample"""
     sample = dag.headsetofconnecteds(nodes)
     if size <= len(sample):
         return _limitsample(sample, size)
     _updatesample(dag, None, sample, quicksamplesize=size)
     return sample
 
 def _takefullsample(dag, nodes, size):
     sample = dag.headsetofconnecteds(nodes)
     # update from heads
     _updatesample(dag, nodes, sample)
     # update from roots
     _updatesample(dag.inverse(), nodes, sample)
     assert sample
     sample = _limitsample(sample, size)
     if len(sample) < size:
         more = size - len(sample)
         sample.update(random.sample(list(nodes - sample), more))
     return sample
 
 def _limitsample(sample, desiredlen):
     """return a random subset of sample of at most desiredlen item"""
     if len(sample) > desiredlen:
         sample = set(random.sample(sample, desiredlen))
     return sample
 
-def findcommonheads(ui, local, remote, heads=None,
+def findcommonheads(ui, local, remote,
                     initialsamplesize=100,
                     fullsamplesize=200,
                     abortwhenunrelated=True,
                     ancestorsof=None):
     '''Return a tuple (common, anyincoming, remoteheads) used to identify
     missing nodes from or in remote.
     '''
     start = util.timer()
 
     roundtrips = 0
     cl = local.changelog
     localsubset = None
     if ancestorsof is not None:
         rev = local.changelog.rev
         localsubset = [rev(n) for n in ancestorsof]
     dag = dagutil.revlogdag(cl, localsubset=localsubset)
 
     # early exit if we know all the specified remote heads already
     ui.debug("query 1; heads\n")
     roundtrips += 1
     ownheads = dag.heads()
     sample = _limitsample(ownheads, initialsamplesize)
     # indices between sample and externalized version must match
     sample = list(sample)
-    if heads:
-        srvheadhashes = heads
-        yesno = remote.known(dag.externalizeall(sample))
-    else:
-        batch = remote.iterbatch()
-        batch.heads()
-        batch.known(dag.externalizeall(sample))
-        batch.submit()
-        srvheadhashes, yesno = batch.results()
+    batch = remote.iterbatch()
+    batch.heads()
+    batch.known(dag.externalizeall(sample))
+    batch.submit()
+    srvheadhashes, yesno = batch.results()
 
     if cl.tip() == nullid:
         if srvheadhashes != [nullid]:
             return [nullid], True, srvheadhashes
         return [nullid], False, []
 
     # start actual discovery (we note this before the next "if" for
     # compatibility reasons)
     ui.status(_("searching for changes\n"))
 
     srvheads = dag.internalizeall(srvheadhashes, filterunknown=True)
     if len(srvheads) == len(srvheadhashes):
         ui.debug("all remote heads known locally\n")
         return (srvheadhashes, False, srvheadhashes,)
 
     if sample and len(ownheads) <= initialsamplesize and all(yesno):
         ui.note(_("all local heads known remotely\n"))
         ownheadhashes = dag.externalizeall(ownheads)
         return (ownheadhashes, True, srvheadhashes,)
 
     # full blown discovery
 
     # own nodes I know we both know
     # treat remote heads (and maybe own heads) as a first implicit sample
     # response
     common = cl.incrementalmissingrevs(srvheads)
     commoninsample = set(n for i, n in enumerate(sample) if yesno[i])
     common.addbases(commoninsample)
     # own nodes where I don't know if remote knows them
     undecided = set(common.missingancestors(ownheads))
     # own nodes I know remote lacks
     missing = set()
 
     full = False
     while undecided:
 
         if sample:
             missinginsample = [n for i, n in enumerate(sample) if not yesno[i]]
             missing.update(dag.descendantset(missinginsample, missing))
 
             undecided.difference_update(missing)
 
         if not undecided:
             break
 
         if full or common.hasbases():
             if full:
                 ui.note(_("sampling from both directions\n"))
             else:
                 ui.debug("taking initial sample\n")
             samplefunc = _takefullsample
             targetsize = fullsamplesize
         else:
             # use even cheaper initial sample
             ui.debug("taking quick initial sample\n")
             samplefunc = _takequicksample
             targetsize = initialsamplesize
         if len(undecided) < targetsize:
             sample = list(undecided)
         else:
             sample = samplefunc(dag, undecided, targetsize)
             sample = _limitsample(sample, targetsize)
 
         roundtrips += 1
         ui.progress(_('searching'), roundtrips, unit=_('queries'))
         ui.debug("query %i; still undecided: %i, sample size is: %i\n"
                  % (roundtrips, len(undecided), len(sample)))
         # indices between sample and externalized version must match
         sample = list(sample)
         yesno = remote.known(dag.externalizeall(sample))
         full = True
 
         if sample:
             commoninsample = set(n for i, n in enumerate(sample) if yesno[i])
             common.addbases(commoninsample)
             common.removeancestorsfrom(undecided)
 
     # heads(common) == heads(common.bases) since common represents common.bases
     # and all its ancestors
     result = dag.headsetofconnecteds(common.bases)
     # common.bases can include nullrev, but our contract requires us to not
     # return any heads in that case, so discard that
     result.discard(nullrev)
     elapsed = util.timer() - start
     ui.progress(_('searching'), None)
     ui.debug("%d total queries in %.4fs\n" % (roundtrips, elapsed))
     msg = ('found %d common and %d unknown server heads,'
            ' %d roundtrips in %.4fs\n')
     missing = set(result) - set(srvheads)
     ui.log('discovery', msg, len(result), len(missing), roundtrips,
            elapsed)
 
     if not result and srvheadhashes != [nullid]:
         if abortwhenunrelated:
             raise error.Abort(_("repository is unrelated"))
         else:
             ui.warn(_("warning: repository is unrelated\n"))
         return ({nullid}, True, srvheadhashes,)
 
     anyincoming = (srvheadhashes != [nullid])
     return dag.externalizeall(result), anyincoming, srvheadhashes