mercurial/setdiscovery.py

discovery: fix embarrassing typo in slice definition

The code introduced in e514799e4e07 ended up having a silly bug. The indexing
selected a single item slice picking only p1. The discovery result was still
correct, but the sampling was hampered, sometime leading to much more round
trips being performed.

Fixing this issue restore the previous sampling behavior.

This fix has a negative performance impact on the pathological case the previous
test has been built.

# parent of this changesets
! wall 5.313884 comb 5.310000 user 5.260000 sys 0.050000 (best of 5)
! wall 6.711860 comb 6.710000 user 6.670000 sys 0.040000 (max of 5)
! wall 5.844016 comb 5.842000 user 5.784000 sys 0.058000 (avg of 5)
! wall 5.778635 comb 5.780000 user 5.740000 sys 0.040000 (median of 5)

# With this changesets.
! wall 6.350879 comb 6.350000 user 6.300000 sys 0.050000 (best of 5)
! wall 6.653647 comb 6.660000 user 6.480000 sys 0.180000 (max of 5)
! wall 6.492762 comb 6.494000 user 6.414000 sys 0.080000 (avg of 5)
! wall 6.547577 comb 6.550000 user 6.490000 sys 0.060000 (median of 5)

Changeset e514799e4e07 raised the question of using the "_uncheckedparentrevs"
instead of the current code. So I ran comparative timing:

# old code: 55919b96c02a (e514799e4e07 parent)
! wall 64.078708 comb 64.080000 user 63.160000 sys 0.920000 (best of 5)
! wall 68.296300 comb 68.290000 user 67.410000 sys 0.880000 (max of 5)
! wall 65.899075 comb 65.894000 user 65.082000 sys 0.812000 (avg of 5)
! wall 66.140286 comb 66.130000 user 65.330000 sys 0.800000 (median of 5)

# buggy code: e514799e4e07
! wall 46.605362 comb 46.610000 user 45.880000 sys 0.730000 (best of 5)
! wall 48.619659 comb 48.620000 user 47.890000 sys 0.730000 (max of 5)
! wall 47.350247 comb 47.350000 user 46.672000 sys 0.678000 (avg of 5)
! wall 46.983224 comb 46.980000 user 46.350000 sys 0.630000 (median of 5)

# fixed code: e514799e4e07 with this fix
! wall 55.858460 comb 55.850000 user 55.090000 sys 0.760000 (best of 5)
! wall 59.048805 comb 59.060000 user 58.110000 sys 0.950000 (max of 5)
! wall 57.192639 comb 57.192000 user 56.350000 sys 0.842000 (avg of 5)
! wall 57.056373 comb 57.060000 user 56.160000 sys 0.900000 (median of 5)

# version using uncheckedparents
! wall 56.471916 comb 56.470000 user 55.630000 sys 0.840000 (best of 5)
! wall 58.228793 comb 58.230000 user 57.600000 sys 0.630000 (max of 5)
! wall 57.377583 comb 57.378000 user 56.674000 sys 0.704000 (avg of 5)
! wall 57.008843 comb 57.010000 user 56.330000 sys 0.680000 (median of 5)

So it looks like the overhead from `_uncheckedparentrevs` is not that impactful.
I'll investigate this shortly. I'm almost done updating our benchmark suite
with more meaningful discovery cases.

# 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 (
    error,
    util,
)

def _updatesample(revs, heads, sample, parentfn, quicksamplesize=0):
    """update an existing sample to match the expected size

    The sample is updated with revs exponentially distant from each head of the
    <revs> 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 <revs> set are
    reached.

    :revs:  set of revs we want to discover (if None, assume the whole dag)
    :heads: set of DAG head revs
    :sample: a sample to update
    :parentfn: a callable to resolve parents for a revision
    :quicksamplesize: optional target size of the sample"""
    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 parentfn(curr):
            if p != nullrev and (not revs or p in revs):
                dist.setdefault(p, d + 1)
                visit.append(p)

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

class partialdiscovery(object):
    """an object representing ongoing discovery

    Feed with data from the remote repository, this object keep track of the
    current set of changeset in various states:

    - common:    revs also known remotely
    - undecided: revs we don't have information on yet
    - missing:   revs missing remotely
    (all tracked revisions are known locally)
    """

    def __init__(self, repo, targetheads):
        self._repo = repo
        self._targetheads = targetheads
        self._common = repo.changelog.incrementalmissingrevs()
        self._undecided = None
        self.missing = set()
        self._childrenmap = None

    def addcommons(self, commons):
        """registrer nodes known as common"""
        self._common.addbases(commons)
        if self._undecided is not None:
            self._common.removeancestorsfrom(self._undecided)

    def addmissings(self, missings):
        """registrer some nodes as missing"""
        newmissing = self._repo.revs('%ld::%ld', missings, self.undecided)
        if newmissing:
            self.missing.update(newmissing)
            self.undecided.difference_update(newmissing)

    def addinfo(self, sample):
        """consume an iterable of (rev, known) tuples"""
        common = set()
        missing = set()
        for rev, known in sample:
            if known:
                common.add(rev)
            else:
                missing.add(rev)
        if common:
            self.addcommons(common)
        if missing:
            self.addmissings(missing)

    def hasinfo(self):
        """return True is we have any clue about the remote state"""
        return self._common.hasbases()

    def iscomplete(self):
        """True if all the necessary data have been gathered"""
        return self._undecided is not None and not self._undecided

    @property
    def undecided(self):
        if self._undecided is not None:
            return self._undecided
        self._undecided = set(self._common.missingancestors(self._targetheads))
        return self._undecided

    def commonheads(self):
        """the heads of the known common set"""
        # heads(common) == heads(common.bases) since common represents
        # common.bases and all its ancestors
        return self._common.basesheads()

    def _parentsgetter(self):
        getrev = self._repo.changelog.index.__getitem__
        def getparents(r):
            return getrev(r)[5:7]
        return getparents

    def _childrengetter(self):

        if self._childrenmap is not None:
            # During discovery, the `undecided` set keep shrinking.
            # Therefore, the map computed for an iteration N will be
            # valid for iteration N+1. Instead of computing the same
            # data over and over we cached it the first time.
            return self._childrenmap.__getitem__

        # _updatesample() essentially does interaction over revisions to look
        # up their children. This lookup is expensive and doing it in a loop is
        # quadratic. We precompute the children for all relevant revisions and
        # make the lookup in _updatesample() a simple dict lookup.
        self._childrenmap = children = {}

        parentrevs = self._parentsgetter()
        revs = self.undecided

        for rev in sorted(revs):
            # Always ensure revision has an entry so we don't need to worry
            # about missing keys.
            children[rev] = []
            for prev in parentrevs(rev):
                if prev == nullrev:
                    continue
                c = children.get(prev)
                if c is not None:
                    c.append(rev)
        return children.__getitem__

    def takequicksample(self, headrevs, size):
        """takes a quick sample of size <size>

        It is meant for initial sampling and focuses on querying heads and close
        ancestors of heads.

        :headrevs: set of head revisions in local DAG to consider
        :size: the maximum size of the sample"""
        revs = self.undecided
        if len(revs) <= size:
            return list(revs)
        sample = set(self._repo.revs('heads(%ld)', revs))

        if len(sample) >= size:
            return _limitsample(sample, size)

        _updatesample(None, headrevs, sample, self._parentsgetter(),
                      quicksamplesize=size)
        return sample

    def takefullsample(self, headrevs, size):
        revs = self.undecided
        if len(revs) <= size:
            return list(revs)
        repo = self._repo
        sample = set(repo.revs('heads(%ld)', revs))
        parentrevs = self._parentsgetter()

        # update from heads
        revsheads = sample.copy()
        _updatesample(revs, revsheads, sample, parentrevs)

        # update from roots
        revsroots = set(repo.revs('roots(%ld)', revs))

        childrenrevs = self._childrengetter()

        _updatesample(revs, revsroots, sample, childrenrevs)
        assert sample
        sample = _limitsample(sample, size)
        if len(sample) < size:
            more = size - len(sample)
            sample.update(random.sample(list(revs - sample), more))
        return sample

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
    clnode = cl.node
    clrev = cl.rev

    if ancestorsof is not None:
        ownheads = [clrev(n) for n in ancestorsof]
    else:
        ownheads = [rev for rev in cl.headrevs() if rev != nullrev]

    # early exit if we know all the specified remote heads already
    ui.debug("query 1; heads\n")
    roundtrips += 1
    sample = _limitsample(ownheads, initialsamplesize)
    # indices between sample and externalized version must match
    sample = list(sample)

    with remote.commandexecutor() as e:
        fheads = e.callcommand('heads', {})
        fknown = e.callcommand('known', {
            'nodes': [clnode(r) for r in sample],
        })

    srvheadhashes, yesno = fheads.result(), fknown.result()

    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"))

    knownsrvheads = []  # revnos of remote heads that are known locally
    for node in srvheadhashes:
        if node == nullid:
            continue

        try:
            knownsrvheads.append(clrev(node))
        # Catches unknown and filtered nodes.
        except error.LookupError:
            continue

    if len(knownsrvheads) == len(srvheadhashes):
        ui.debug("all remote heads known locally\n")
        return srvheadhashes, False, srvheadhashes

    if len(sample) == len(ownheads) and all(yesno):
        ui.note(_("all local heads known remotely\n"))
        ownheadhashes = [clnode(r) for r in ownheads]
        return ownheadhashes, True, srvheadhashes

    # full blown discovery

    disco = partialdiscovery(local, ownheads)
    # treat remote heads (and maybe own heads) as a first implicit sample
    # response
    disco.addcommons(knownsrvheads)
    disco.addinfo(zip(sample, yesno))

    full = False
    progress = ui.makeprogress(_('searching'), unit=_('queries'))
    while not disco.iscomplete():

        if full or disco.hasinfo():
            if full:
                ui.note(_("sampling from both directions\n"))
            else:
                ui.debug("taking initial sample\n")
            samplefunc = disco.takefullsample
            targetsize = fullsamplesize
        else:
            # use even cheaper initial sample
            ui.debug("taking quick initial sample\n")
            samplefunc = disco.takequicksample
            targetsize = initialsamplesize
        sample = samplefunc(ownheads, targetsize)

        roundtrips += 1
        progress.update(roundtrips)
        ui.debug("query %i; still undecided: %i, sample size is: %i\n"
                 % (roundtrips, len(disco.undecided), len(sample)))
        # indices between sample and externalized version must match
        sample = list(sample)

        with remote.commandexecutor() as e:
            yesno = e.callcommand('known', {
                'nodes': [clnode(r) for r in sample],
            }).result()

        full = True

        disco.addinfo(zip(sample, yesno))

    result = disco.commonheads()
    elapsed = util.timer() - start
    progress.complete()
    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(knownsrvheads)
    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])
    result = {clnode(r) for r in result}
    return result, anyincoming, srvheadhashes