mercurial/merge.py

merge: increase safety of parallel updating/removing on icasefs

"merge.applyupdates()" sorts "actions" in removal first order, and
"workeractions" derived from it should be also sorted.

If each actions in "workeractions" are executed in serial, this
sorting ensures that merging/updating process is collision free,
because updating the file in target context is always executed after
removing the existing file which causes case-folding collision against
the former.

In the other hand, if each actions are executed in parallel, updating
on a worker process may be executed before removing on another worker
process, because "worker.partition()" partitions list of actions
regardless of type of each actions.

This patch divides "workeractions" into removing and updating, and
executes the former first.

This patch still scans "actions"/"workeractions" some times for ease
of patch review, even though large list may cost much in this way.
(total cost should be as same as before)

This also changes some tests, because dividing "workeractions" affects
progress indication.

# merge.py - directory-level update/merge handling for Mercurial
#
# Copyright 2006, 2007 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 node import nullid, nullrev, hex, bin
from i18n import _
from mercurial import obsolete
import error, util, filemerge, copies, subrepo, worker, dicthelpers
import errno, os, shutil

class mergestate(object):
    '''track 3-way merge state of individual files'''
    def __init__(self, repo):
        self._repo = repo
        self._dirty = False
        self._read()
    def reset(self, node=None):
        self._state = {}
        if node:
            self._local = node
        shutil.rmtree(self._repo.join("merge"), True)
        self._dirty = False
    def _read(self):
        self._state = {}
        try:
            f = self._repo.opener("merge/state")
            for i, l in enumerate(f):
                if i == 0:
                    self._local = bin(l[:-1])
                else:
                    bits = l[:-1].split("\0")
                    self._state[bits[0]] = bits[1:]
            f.close()
        except IOError, err:
            if err.errno != errno.ENOENT:
                raise
        self._dirty = False
    def commit(self):
        if self._dirty:
            f = self._repo.opener("merge/state", "w")
            f.write(hex(self._local) + "\n")
            for d, v in self._state.iteritems():
                f.write("\0".join([d] + v) + "\n")
            f.close()
            self._dirty = False
    def add(self, fcl, fco, fca, fd):
        hash = util.sha1(fcl.path()).hexdigest()
        self._repo.opener.write("merge/" + hash, fcl.data())
        self._state[fd] = ['u', hash, fcl.path(), fca.path(),
                           hex(fca.filenode()), fco.path(), fcl.flags()]
        self._dirty = True
    def __contains__(self, dfile):
        return dfile in self._state
    def __getitem__(self, dfile):
        return self._state[dfile][0]
    def __iter__(self):
        l = self._state.keys()
        l.sort()
        for f in l:
            yield f
    def mark(self, dfile, state):
        self._state[dfile][0] = state
        self._dirty = True
    def resolve(self, dfile, wctx, octx):
        if self[dfile] == 'r':
            return 0
        state, hash, lfile, afile, anode, ofile, flags = self._state[dfile]
        fcd = wctx[dfile]
        fco = octx[ofile]
        fca = self._repo.filectx(afile, fileid=anode)
        # "premerge" x flags
        flo = fco.flags()
        fla = fca.flags()
        if 'x' in flags + flo + fla and 'l' not in flags + flo + fla:
            if fca.node() == nullid:
                self._repo.ui.warn(_('warning: cannot merge flags for %s\n') %
                                   afile)
            elif flags == fla:
                flags = flo
        # restore local
        f = self._repo.opener("merge/" + hash)
        self._repo.wwrite(dfile, f.read(), flags)
        f.close()
        r = filemerge.filemerge(self._repo, self._local, lfile, fcd, fco, fca)
        if r is None:
            # no real conflict
            del self._state[dfile]
        elif not r:
            self.mark(dfile, 'r')
        return r

def _checkunknownfile(repo, wctx, mctx, f):
    return (not repo.dirstate._ignore(f)
        and os.path.isfile(repo.wjoin(f))
        and repo.dirstate.normalize(f) not in repo.dirstate
        and mctx[f].cmp(wctx[f]))

def _checkunknown(repo, wctx, mctx):
    "check for collisions between unknown files and files in mctx"

    error = False
    for f in mctx:
        if f not in wctx and _checkunknownfile(repo, wctx, mctx, f):
            error = True
            wctx._repo.ui.warn(_("%s: untracked file differs\n") % f)
    if error:
        raise util.Abort(_("untracked files in working directory differ "
                           "from files in requested revision"))

def _remains(f, m, ma, workingctx=False):
    """check whether specified file remains after merge.

    It is assumed that specified file is not contained in the manifest
    of the other context.
    """
    if f in ma:
        n = m[f]
        if n != ma[f]:
            return True # because it is changed locally
            # even though it doesn't remain, if "remote deleted" is
            # chosen in manifestmerge()
        elif workingctx and n[20:] == "a":
            return True # because it is added locally (linear merge specific)
        else:
            return False # because it is removed remotely
    else:
        return True # because it is added locally

def _checkcollision(mctx, extractxs):
    "check for case folding collisions in the destination context"
    folded = {}
    for fn in mctx:
        fold = util.normcase(fn)
        if fold in folded:
            raise util.Abort(_("case-folding collision between %s and %s")
                             % (fn, folded[fold]))
        folded[fold] = fn

    if extractxs:
        wctx, actx = extractxs
        # class to delay looking up copy mapping
        class pathcopies(object):
            @util.propertycache
            def map(self):
                # {dst@mctx: src@wctx} copy mapping
                return copies.pathcopies(wctx, mctx)
        pc = pathcopies()

        for fn in wctx:
            fold = util.normcase(fn)
            mfn = folded.get(fold, None)
            if (mfn and mfn != fn and pc.map.get(mfn) != fn and
                _remains(fn, wctx.manifest(), actx.manifest(), True) and
                _remains(mfn, mctx.manifest(), actx.manifest())):
                raise util.Abort(_("case-folding collision between %s and %s")
                                 % (mfn, fn))

def _forgetremoved(wctx, mctx, branchmerge):
    """
    Forget removed files

    If we're jumping between revisions (as opposed to merging), and if
    neither the working directory nor the target rev has the file,
    then we need to remove it from the dirstate, to prevent the
    dirstate from listing the file when it is no longer in the
    manifest.

    If we're merging, and the other revision has removed a file
    that is not present in the working directory, we need to mark it
    as removed.
    """

    actions = []
    state = branchmerge and 'r' or 'f'
    for f in wctx.deleted():
        if f not in mctx:
            actions.append((f, state, None, "forget deleted"))

    if not branchmerge:
        for f in wctx.removed():
            if f not in mctx:
                actions.append((f, "f", None, "forget removed"))

    return actions

def manifestmerge(repo, wctx, p2, pa, branchmerge, force, partial,
                  acceptremote=False):
    """
    Merge p1 and p2 with ancestor pa and generate merge action list

    branchmerge and force are as passed in to update
    partial = function to filter file lists
    acceptremote = accept the incoming changes without prompting
    """

    overwrite = force and not branchmerge
    actions, copy, movewithdir = [], {}, {}

    followcopies = False
    if overwrite:
        pa = wctx
    elif pa == p2: # backwards
        pa = wctx.p1()
    elif not branchmerge and not wctx.dirty(missing=True):
        pass
    elif pa and repo.ui.configbool("merge", "followcopies", True):
        followcopies = True

    # manifests fetched in order are going to be faster, so prime the caches
    [x.manifest() for x in
     sorted(wctx.parents() + [p2, pa], key=lambda x: x.rev())]

    if followcopies:
        ret = copies.mergecopies(repo, wctx, p2, pa)
        copy, movewithdir, diverge, renamedelete = ret
        for of, fl in diverge.iteritems():
            actions.append((of, "dr", (fl,), "divergent renames"))
        for of, fl in renamedelete.iteritems():
            actions.append((of, "rd", (fl,), "rename and delete"))

    repo.ui.note(_("resolving manifests\n"))
    repo.ui.debug(" branchmerge: %s, force: %s, partial: %s\n"
                  % (bool(branchmerge), bool(force), bool(partial)))
    repo.ui.debug(" ancestor: %s, local: %s, remote: %s\n" % (pa, wctx, p2))

    m1, m2, ma = wctx.manifest(), p2.manifest(), pa.manifest()
    copied = set(copy.values())
    copied.update(movewithdir.values())

    if '.hgsubstate' in m1:
        # check whether sub state is modified
        for s in sorted(wctx.substate):
            if wctx.sub(s).dirty():
                m1['.hgsubstate'] += "+"
                break

    aborts, prompts = [], []
    # Compare manifests
    fdiff = dicthelpers.diff(m1, m2)
    flagsdiff = m1.flagsdiff(m2)
    diff12 = dicthelpers.join(fdiff, flagsdiff)

    for f, (n12, fl12) in diff12.iteritems():
        if n12:
            n1, n2 = n12
        else: # file contents didn't change, but flags did
            n1 = n2 = m1.get(f, None)
            if n1 is None:
                # Since n1 == n2, the file isn't present in m2 either. This
                # means that the file was removed or deleted locally and
                # removed remotely, but that residual entries remain in flags.
                # This can happen in manifests generated by workingctx.
                continue
        if fl12:
            fl1, fl2 = fl12
        else: # flags didn't change, file contents did
            fl1 = fl2 = m1.flags(f)

        if partial and not partial(f):
            continue
        if n1 and n2:
            fla = ma.flags(f)
            nol = 'l' not in fl1 + fl2 + fla
            a = ma.get(f, nullid)
            if n2 == a and fl2 == fla:
                pass # remote unchanged - keep local
            elif n1 == a and fl1 == fla: # local unchanged - use remote
                if n1 == n2: # optimization: keep local content
                    actions.append((f, "e", (fl2,), "update permissions"))
                else:
                    actions.append((f, "g", (fl2,), "remote is newer"))
            elif nol and n2 == a: # remote only changed 'x'
                actions.append((f, "e", (fl2,), "update permissions"))
            elif nol and n1 == a: # local only changed 'x'
                actions.append((f, "g", (fl1,), "remote is newer"))
            else: # both changed something
                actions.append((f, "m", (f, f, False), "versions differ"))
        elif f in copied: # files we'll deal with on m2 side
            pass
        elif n1 and f in movewithdir: # directory rename
            f2 = movewithdir[f]
            actions.append((f, "d", (None, f2, fl1),
                            "remote renamed directory to " + f2))
        elif n1 and f in copy:
            f2 = copy[f]
            actions.append((f, "m", (f2, f, False),
                            "local copied/moved to " + f2))
        elif n1 and f in ma: # clean, a different, no remote
            if n1 != ma[f]:
                prompts.append((f, "cd")) # prompt changed/deleted
            elif n1[20:] == "a": # added, no remote
                actions.append((f, "f", None, "remote deleted"))
            else:
                actions.append((f, "r", None, "other deleted"))
        elif n2 and f in movewithdir:
            f2 = movewithdir[f]
            actions.append((None, "d", (f, f2, fl2),
                            "local renamed directory to " + f2))
        elif n2 and f in copy:
            f2 = copy[f]
            if f2 in m2:
                actions.append((f2, "m", (f, f, False),
                                "remote copied to " + f))
            else:
                actions.append((f2, "m", (f, f, True),
                                "remote moved to " + f))
        elif n2 and f not in ma:
            # local unknown, remote created: the logic is described by the
            # following table:
            #
            # force  branchmerge  different  |  action
            #   n         *           n      |    get
            #   n         *           y      |   abort
            #   y         n           *      |    get
            #   y         y           n      |    get
            #   y         y           y      |   merge
            #
            # Checking whether the files are different is expensive, so we
            # don't do that when we can avoid it.
            if force and not branchmerge:
                actions.append((f, "g", (fl2,), "remote created"))
            else:
                different = _checkunknownfile(repo, wctx, p2, f)
                if force and branchmerge and different:
                    actions.append((f, "m", (f, f, False),
                                    "remote differs from untracked local"))
                elif not force and different:
                    aborts.append((f, "ud"))
                else:
                    actions.append((f, "g", (fl2,), "remote created"))
        elif n2 and n2 != ma[f]:
            prompts.append((f, "dc")) # prompt deleted/changed

    for f, m in sorted(aborts):
        if m == "ud":
            repo.ui.warn(_("%s: untracked file differs\n") % f)
        else: assert False, m
    if aborts:
        raise util.Abort(_("untracked files in working directory differ "
                           "from files in requested revision"))

    for f, m in sorted(prompts):
        if m == "cd":
            if acceptremote:
                actions.append((f, "r", None, "remote delete"))
            elif repo.ui.promptchoice(
                _("local changed %s which remote deleted\n"
                  "use (c)hanged version or (d)elete?") % f,
                (_("&Changed"), _("&Delete")), 0):
                actions.append((f, "r", None, "prompt delete"))
            else:
                actions.append((f, "a", None, "prompt keep"))
        elif m == "dc":
            if acceptremote:
                actions.append((f, "g", (m2.flags(f),), "remote recreating"))
            elif repo.ui.promptchoice(
                _("remote changed %s which local deleted\n"
                  "use (c)hanged version or leave (d)eleted?") % f,
                (_("&Changed"), _("&Deleted")), 0) == 0:
                actions.append((f, "g", (m2.flags(f),), "prompt recreating"))
        else: assert False, m
    return actions

def actionkey(a):
    return a[1] == "r" and -1 or 0, a

def getremove(repo, mctx, overwrite, args):
    """apply usually-non-interactive updates to the working directory

    mctx is the context to be merged into the working copy

    yields tuples for progress updates
    """
    verbose = repo.ui.verbose
    unlink = util.unlinkpath
    wjoin = repo.wjoin
    fctx = mctx.filectx
    wwrite = repo.wwrite
    audit = repo.wopener.audit
    i = 0
    for arg in args:
        f = arg[0]
        if arg[1] == 'r':
            if verbose:
                repo.ui.note(_("removing %s\n") % f)
            audit(f)
            try:
                unlink(wjoin(f), ignoremissing=True)
            except OSError, inst:
                repo.ui.warn(_("update failed to remove %s: %s!\n") %
                             (f, inst.strerror))
        else:
            if verbose:
                repo.ui.note(_("getting %s\n") % f)
            wwrite(f, fctx(f).data(), arg[2][0])
        if i == 100:
            yield i, f
            i = 0
        i += 1
    if i > 0:
        yield i, f

def applyupdates(repo, actions, wctx, mctx, actx, overwrite):
    """apply the merge action list to the working directory

    wctx is the working copy context
    mctx is the context to be merged into the working copy
    actx is the context of the common ancestor

    Return a tuple of counts (updated, merged, removed, unresolved) that
    describes how many files were affected by the update.
    """

    updated, merged, removed, unresolved = 0, 0, 0, 0
    ms = mergestate(repo)
    ms.reset(wctx.p1().node())
    moves = []
    actions.sort(key=actionkey)

    # prescan for merges
    for a in actions:
        f, m, args, msg = a
        repo.ui.debug(" %s: %s -> %s\n" % (f, msg, m))
        if m == "m": # merge
            f2, fd, move = args
            if fd == '.hgsubstate': # merged internally
                continue
            repo.ui.debug("  preserving %s for resolve of %s\n" % (f, fd))
            fcl = wctx[f]
            fco = mctx[f2]
            if mctx == actx: # backwards, use working dir parent as ancestor
                if fcl.parents():
                    fca = fcl.p1()
                else:
                    fca = repo.filectx(f, fileid=nullrev)
            else:
                fca = fcl.ancestor(fco, actx)
            if not fca:
                fca = repo.filectx(f, fileid=nullrev)
            ms.add(fcl, fco, fca, fd)
            if f != fd and move:
                moves.append(f)

    audit = repo.wopener.audit

    # remove renamed files after safely stored
    for f in moves:
        if os.path.lexists(repo.wjoin(f)):
            repo.ui.debug("removing %s\n" % f)
            audit(f)
            util.unlinkpath(repo.wjoin(f))

    numupdates = len(actions)
    workeractions = [a for a in actions if a[1] in 'gr']
    updateactions = [a for a in workeractions if a[1] == 'g']
    updated = len(updateactions)
    removeactions = [a for a in workeractions if a[1] == 'r']
    removed = len(removeactions)
    actions = [a for a in actions if a[1] not in 'gr']

    hgsub = [a[1] for a in workeractions if a[0] == '.hgsubstate']
    if hgsub and hgsub[0] == 'r':
        subrepo.submerge(repo, wctx, mctx, wctx, overwrite)

    z = 0
    prog = worker.worker(repo.ui, 0.001, getremove, (repo, mctx, overwrite),
                         removeactions)
    for i, item in prog:
        z += i
        repo.ui.progress(_('updating'), z, item=item, total=numupdates,
                         unit=_('files'))
    prog = worker.worker(repo.ui, 0.001, getremove, (repo, mctx, overwrite),
                         updateactions)
    for i, item in prog:
        z += i
        repo.ui.progress(_('updating'), z, item=item, total=numupdates,
                         unit=_('files'))

    if hgsub and hgsub[0] == 'g':
        subrepo.submerge(repo, wctx, mctx, wctx, overwrite)

    _updating = _('updating')
    _files = _('files')
    progress = repo.ui.progress

    for i, a in enumerate(actions):
        f, m, args, msg = a
        progress(_updating, z + i + 1, item=f, total=numupdates, unit=_files)
        if m == "m": # merge
            f2, fd, move = args
            if fd == '.hgsubstate': # subrepo states need updating
                subrepo.submerge(repo, wctx, mctx, wctx.ancestor(mctx),
                                 overwrite)
                continue
            audit(fd)
            r = ms.resolve(fd, wctx, mctx)
            if r is not None and r > 0:
                unresolved += 1
            else:
                if r is None:
                    updated += 1
                else:
                    merged += 1
        elif m == "d": # directory rename
            f2, fd, flags = args
            if f:
                repo.ui.note(_("moving %s to %s\n") % (f, fd))
                audit(f)
                repo.wwrite(fd, wctx.filectx(f).data(), flags)
                util.unlinkpath(repo.wjoin(f))
            if f2:
                repo.ui.note(_("getting %s to %s\n") % (f2, fd))
                repo.wwrite(fd, mctx.filectx(f2).data(), flags)
            updated += 1
        elif m == "dr": # divergent renames
            fl, = args
            repo.ui.warn(_("note: possible conflict - %s was renamed "
                           "multiple times to:\n") % f)
            for nf in fl:
                repo.ui.warn(" %s\n" % nf)
        elif m == "rd": # rename and delete
            fl, = args
            repo.ui.warn(_("note: possible conflict - %s was deleted "
                           "and renamed to:\n") % f)
            for nf in fl:
                repo.ui.warn(" %s\n" % nf)
        elif m == "e": # exec
            flags, = args
            audit(f)
            util.setflags(repo.wjoin(f), 'l' in flags, 'x' in flags)
            updated += 1
    ms.commit()
    progress(_updating, None, total=numupdates, unit=_files)

    return updated, merged, removed, unresolved

def calculateupdates(repo, tctx, mctx, ancestor, branchmerge, force, partial,
                     acceptremote=False):
    "Calculate the actions needed to merge mctx into tctx"
    actions = []
    folding = not util.checkcase(repo.path)
    if folding:
        # collision check is not needed for clean update
        if (not branchmerge and
            (force or not tctx.dirty(missing=True, branch=False))):
            _checkcollision(mctx, None)
        else:
            _checkcollision(mctx, (tctx, ancestor))
    actions += manifestmerge(repo, tctx, mctx,
                             ancestor,
                             branchmerge, force,
                             partial, acceptremote)
    if tctx.rev() is None:
        actions += _forgetremoved(tctx, mctx, branchmerge)
    return actions

def recordupdates(repo, actions, branchmerge):
    "record merge actions to the dirstate"

    for a in actions:
        f, m, args, msg = a
        if m == "r": # remove
            if branchmerge:
                repo.dirstate.remove(f)
            else:
                repo.dirstate.drop(f)
        elif m == "a": # re-add
            if not branchmerge:
                repo.dirstate.add(f)
        elif m == "f": # forget
            repo.dirstate.drop(f)
        elif m == "e": # exec change
            repo.dirstate.normallookup(f)
        elif m == "g": # get
            if branchmerge:
                repo.dirstate.otherparent(f)
            else:
                repo.dirstate.normal(f)
        elif m == "m": # merge
            f2, fd, move = args
            if branchmerge:
                # We've done a branch merge, mark this file as merged
                # so that we properly record the merger later
                repo.dirstate.merge(fd)
                if f != f2: # copy/rename
                    if move:
                        repo.dirstate.remove(f)
                    if f != fd:
                        repo.dirstate.copy(f, fd)
                    else:
                        repo.dirstate.copy(f2, fd)
            else:
                # We've update-merged a locally modified file, so
                # we set the dirstate to emulate a normal checkout
                # of that file some time in the past. Thus our
                # merge will appear as a normal local file
                # modification.
                if f2 == fd: # file not locally copied/moved
                    repo.dirstate.normallookup(fd)
                if move:
                    repo.dirstate.drop(f)
        elif m == "d": # directory rename
            f2, fd, flag = args
            if not f2 and f not in repo.dirstate:
                # untracked file moved
                continue
            if branchmerge:
                repo.dirstate.add(fd)
                if f:
                    repo.dirstate.remove(f)
                    repo.dirstate.copy(f, fd)
                if f2:
                    repo.dirstate.copy(f2, fd)
            else:
                repo.dirstate.normal(fd)
                if f:
                    repo.dirstate.drop(f)

def update(repo, node, branchmerge, force, partial, ancestor=None,
           mergeancestor=False):
    """
    Perform a merge between the working directory and the given node

    node = the node to update to, or None if unspecified
    branchmerge = whether to merge between branches
    force = whether to force branch merging or file overwriting
    partial = a function to filter file lists (dirstate not updated)
    mergeancestor = whether it is merging with an ancestor. If true,
      we should accept the incoming changes for any prompts that occur.
      If false, merging with an ancestor (fast-forward) is only allowed
      between different named branches. This flag is used by rebase extension
      as a temporary fix and should be avoided in general.

    The table below shows all the behaviors of the update command
    given the -c and -C or no options, whether the working directory
    is dirty, whether a revision is specified, and the relationship of
    the parent rev to the target rev (linear, on the same named
    branch, or on another named branch).

    This logic is tested by test-update-branches.t.

    -c  -C  dirty  rev  |  linear   same  cross
     n   n    n     n   |    ok     (1)     x
     n   n    n     y   |    ok     ok     ok
     n   n    y     *   |   merge   (2)    (2)
     n   y    *     *   |    ---  discard  ---
     y   n    y     *   |    ---    (3)    ---
     y   n    n     *   |    ---    ok     ---
     y   y    *     *   |    ---    (4)    ---

    x = can't happen
    * = don't-care
    1 = abort: crosses branches (use 'hg merge' or 'hg update -c')
    2 = abort: crosses branches (use 'hg merge' to merge or
                 use 'hg update -C' to discard changes)
    3 = abort: uncommitted local changes
    4 = incompatible options (checked in commands.py)

    Return the same tuple as applyupdates().
    """

    onode = node
    wlock = repo.wlock()
    try:
        wc = repo[None]
        if node is None:
            # tip of current branch
            try:
                node = repo.branchtip(wc.branch())
            except error.RepoLookupError:
                if wc.branch() == "default": # no default branch!
                    node = repo.lookup("tip") # update to tip
                else:
                    raise util.Abort(_("branch %s not found") % wc.branch())
        overwrite = force and not branchmerge
        pl = wc.parents()
        p1, p2 = pl[0], repo[node]
        if ancestor:
            pa = repo[ancestor]
        else:
            pa = p1.ancestor(p2)

        fp1, fp2, xp1, xp2 = p1.node(), p2.node(), str(p1), str(p2)

        ### check phase
        if not overwrite and len(pl) > 1:
            raise util.Abort(_("outstanding uncommitted merges"))
        if branchmerge:
            if pa == p2:
                raise util.Abort(_("merging with a working directory ancestor"
                                   " has no effect"))
            elif pa == p1:
                if not mergeancestor and p1.branch() == p2.branch():
                    raise util.Abort(_("nothing to merge"),
                                     hint=_("use 'hg update' "
                                            "or check 'hg heads'"))
            if not force and (wc.files() or wc.deleted()):
                raise util.Abort(_("outstanding uncommitted changes"),
                                 hint=_("use 'hg status' to list changes"))
            for s in sorted(wc.substate):
                if wc.sub(s).dirty():
                    raise util.Abort(_("outstanding uncommitted changes in "
                                       "subrepository '%s'") % s)

        elif not overwrite:
            if pa not in (p1, p2):  # nolinear
                dirty = wc.dirty(missing=True)
                if dirty or onode is None:
                    # Branching is a bit strange to ensure we do the minimal
                    # amount of call to obsolete.background.
                    foreground = obsolete.foreground(repo, [p1.node()])
                    # note: the <node> variable contains a random identifier
                    if repo[node].node() in foreground:
                        pa = p1  # allow updating to successors
                    elif dirty:
                        msg = _("crosses branches (merge branches or use"
                                " --clean to discard changes)")
                        raise util.Abort(msg)
                    else:  # node is none
                        msg = _("crosses branches (merge branches or update"
                                " --check to force update)")
                        raise util.Abort(msg)
                else:
                    # Allow jumping branches if clean and specific rev given
                    pa = p1

        ### calculate phase
        actions = calculateupdates(repo, wc, p2, pa,
                                   branchmerge, force, partial, mergeancestor)

        ### apply phase
        if not branchmerge: # just jump to the new rev
            fp1, fp2, xp1, xp2 = fp2, nullid, xp2, ''
        if not partial:
            repo.hook('preupdate', throw=True, parent1=xp1, parent2=xp2)

        stats = applyupdates(repo, actions, wc, p2, pa, overwrite)

        if not partial:
            repo.setparents(fp1, fp2)
            recordupdates(repo, actions, branchmerge)
            if not branchmerge:
                repo.dirstate.setbranch(p2.branch())
    finally:
        wlock.release()

    if not partial:
        repo.hook('update', parent1=xp1, parent2=xp2, error=stats[3])
    return stats