annotate mercurial/metadata.py @ 45059:79f6f9fa18c1

documentation: add some internals documentation about bid merge

This is an important algorithm that was only documented on the wiki so far.

Some update to the algorithm (and associated doc) is to expected in the future
since the bid merge algorithm is bug-ridden when it comes to file deletion comes
to play.

Differential Revision: https://phab.mercurial-scm.org/D8711

# metadata.py -- code related to various metadata computation and access.
#
# Copyright 2019 Google, Inc <[e-mail]>
# Copyright 2020 Pierre-Yves David <[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, print_function

import multiprocessing

from . import (
    error,
    node,
    pycompat,
    util,
)

from .revlogutils import (
    flagutil as sidedataflag,
    sidedata as sidedatamod,
)


def computechangesetfilesadded(ctx):
    """return the list of files added in a changeset
    """
    added = []
    for f in ctx.files():
        if not any(f in p for p in ctx.parents()):
            added.append(f)
    return added


def get_removal_filter(ctx, x=None):
    """return a function to detect files "wrongly" detected as `removed`

    When a file is removed relative to p1 in a merge, this
    function determines whether the absence is due to a
    deletion from a parent, or whether the merge commit
    itself deletes the file. We decide this by doing a
    simplified three way merge of the manifest entry for
    the file. There are two ways we decide the merge
    itself didn't delete a file:
    - neither parent (nor the merge) contain the file
    - exactly one parent contains the file, and that
      parent has the same filelog entry as the merge
      ancestor (or all of them if there two). In other
      words, that parent left the file unchanged while the
      other one deleted it.
    One way to think about this is that deleting a file is
    similar to emptying it, so the list of changed files
    should be similar either way. The computation
    described above is not done directly in _filecommit
    when creating the list of changed files, however
    it does something very similar by comparing filelog
    nodes.
    """

    if x is not None:
        p1, p2, m1, m2 = x
    else:
        p1 = ctx.p1()
        p2 = ctx.p2()
        m1 = p1.manifest()
        m2 = p2.manifest()

    @util.cachefunc
    def mas():
        p1n = p1.node()
        p2n = p2.node()
        cahs = ctx.repo().changelog.commonancestorsheads(p1n, p2n)
        if not cahs:
            cahs = [node.nullrev]
        return [ctx.repo()[r].manifest() for r in cahs]

    def deletionfromparent(f):
        if f in m1:
            return f not in m2 and all(
                f in ma and ma.find(f) == m1.find(f) for ma in mas()
            )
        elif f in m2:
            return all(f in ma and ma.find(f) == m2.find(f) for ma in mas())
        else:
            return True

    return deletionfromparent


def computechangesetfilesremoved(ctx):
    """return the list of files removed in a changeset
    """
    removed = []
    for f in ctx.files():
        if f not in ctx:
            removed.append(f)
    if removed:
        rf = get_removal_filter(ctx)
        removed = [r for r in removed if not rf(r)]
    return removed


def computechangesetcopies(ctx):
    """return the copies data for a changeset

    The copies data are returned as a pair of dictionnary (p1copies, p2copies).

    Each dictionnary are in the form: `{newname: oldname}`
    """
    p1copies = {}
    p2copies = {}
    p1 = ctx.p1()
    p2 = ctx.p2()
    narrowmatch = ctx._repo.narrowmatch()
    for dst in ctx.files():
        if not narrowmatch(dst) or dst not in ctx:
            continue
        copied = ctx[dst].renamed()
        if not copied:
            continue
        src, srcnode = copied
        if src in p1 and p1[src].filenode() == srcnode:
            p1copies[dst] = src
        elif src in p2 and p2[src].filenode() == srcnode:
            p2copies[dst] = src
    return p1copies, p2copies


def encodecopies(files, copies):
    items = []
    for i, dst in enumerate(files):
        if dst in copies:
            items.append(b'%d\0%s' % (i, copies[dst]))
    if len(items) != len(copies):
        raise error.ProgrammingError(
            b'some copy targets missing from file list'
        )
    return b"\n".join(items)


def decodecopies(files, data):
    try:
        copies = {}
        if not data:
            return copies
        for l in data.split(b'\n'):
            strindex, src = l.split(b'\0')
            i = int(strindex)
            dst = files[i]
            copies[dst] = src
        return copies
    except (ValueError, IndexError):
        # Perhaps someone had chosen the same key name (e.g. "p1copies") and
        # used different syntax for the value.
        return None


def encodefileindices(files, subset):
    subset = set(subset)
    indices = []
    for i, f in enumerate(files):
        if f in subset:
            indices.append(b'%d' % i)
    return b'\n'.join(indices)


def decodefileindices(files, data):
    try:
        subset = []
        if not data:
            return subset
        for strindex in data.split(b'\n'):
            i = int(strindex)
            if i < 0 or i >= len(files):
                return None
            subset.append(files[i])
        return subset
    except (ValueError, IndexError):
        # Perhaps someone had chosen the same key name (e.g. "added") and
        # used different syntax for the value.
        return None


def _getsidedata(srcrepo, rev):
    ctx = srcrepo[rev]
    filescopies = computechangesetcopies(ctx)
    filesadded = computechangesetfilesadded(ctx)
    filesremoved = computechangesetfilesremoved(ctx)
    sidedata = {}
    if any([filescopies, filesadded, filesremoved]):
        sortedfiles = sorted(ctx.files())
        p1copies, p2copies = filescopies
        p1copies = encodecopies(sortedfiles, p1copies)
        p2copies = encodecopies(sortedfiles, p2copies)
        filesadded = encodefileindices(sortedfiles, filesadded)
        filesremoved = encodefileindices(sortedfiles, filesremoved)
        if p1copies:
            sidedata[sidedatamod.SD_P1COPIES] = p1copies
        if p2copies:
            sidedata[sidedatamod.SD_P2COPIES] = p2copies
        if filesadded:
            sidedata[sidedatamod.SD_FILESADDED] = filesadded
        if filesremoved:
            sidedata[sidedatamod.SD_FILESREMOVED] = filesremoved
    return sidedata


def getsidedataadder(srcrepo, destrepo):
    use_w = srcrepo.ui.configbool(b'experimental', b'worker.repository-upgrade')
    if pycompat.iswindows or not use_w:
        return _get_simple_sidedata_adder(srcrepo, destrepo)
    else:
        return _get_worker_sidedata_adder(srcrepo, destrepo)


def _sidedata_worker(srcrepo, revs_queue, sidedata_queue, tokens):
    """The function used by worker precomputing sidedata

    It read an input queue containing revision numbers
    It write in an output queue containing (rev, <sidedata-map>)

    The `None` input value is used as a stop signal.

    The `tokens` semaphore is user to avoid having too many unprocessed
    entries. The workers needs to acquire one token before fetching a task.
    They will be released by the consumer of the produced data.
    """
    tokens.acquire()
    rev = revs_queue.get()
    while rev is not None:
        data = _getsidedata(srcrepo, rev)
        sidedata_queue.put((rev, data))
        tokens.acquire()
        rev = revs_queue.get()
    # processing of `None` is completed, release the token.
    tokens.release()


BUFF_PER_WORKER = 50


def _get_worker_sidedata_adder(srcrepo, destrepo):
    """The parallel version of the sidedata computation

    This code spawn a pool of worker that precompute a buffer of sidedata
    before we actually need them"""
    # avoid circular import copies -> scmutil -> worker -> copies
    from . import worker

    nbworkers = worker._numworkers(srcrepo.ui)

    tokens = multiprocessing.BoundedSemaphore(nbworkers * BUFF_PER_WORKER)
    revsq = multiprocessing.Queue()
    sidedataq = multiprocessing.Queue()

    assert srcrepo.filtername is None
    # queue all tasks beforehand, revision numbers are small and it make
    # synchronisation simpler
    #
    # Since the computation for each node can be quite expensive, the overhead
    # of using a single queue is not revelant. In practice, most computation
    # are fast but some are very expensive and dominate all the other smaller
    # cost.
    for r in srcrepo.changelog.revs():
        revsq.put(r)
    # queue the "no more tasks" markers
    for i in range(nbworkers):
        revsq.put(None)

    allworkers = []
    for i in range(nbworkers):
        args = (srcrepo, revsq, sidedataq, tokens)
        w = multiprocessing.Process(target=_sidedata_worker, args=args)
        allworkers.append(w)
        w.start()

    # dictionnary to store results for revision higher than we one we are
    # looking for. For example, if we need the sidedatamap for 42, and 43 is
    # received, when shelve 43 for later use.
    staging = {}

    def sidedata_companion(revlog, rev):
        sidedata = {}
        if util.safehasattr(revlog, b'filteredrevs'):  # this is a changelog
            # Is the data previously shelved ?
            sidedata = staging.pop(rev, None)
            if sidedata is None:
                # look at the queued result until we find the one we are lookig
                # for (shelve the other ones)
                r, sidedata = sidedataq.get()
                while r != rev:
                    staging[r] = sidedata
                    r, sidedata = sidedataq.get()
            tokens.release()
        return False, (), sidedata

    return sidedata_companion


def _get_simple_sidedata_adder(srcrepo, destrepo):
    """The simple version of the sidedata computation

    It just compute it in the same thread on request"""

    def sidedatacompanion(revlog, rev):
        sidedata = {}
        if util.safehasattr(revlog, 'filteredrevs'):  # this is a changelog
            sidedata = _getsidedata(srcrepo, rev)
        return False, (), sidedata

    return sidedatacompanion


def getsidedataremover(srcrepo, destrepo):
    def sidedatacompanion(revlog, rev):
        f = ()
        if util.safehasattr(revlog, 'filteredrevs'):  # this is a changelog
            if revlog.flags(rev) & sidedataflag.REVIDX_SIDEDATA:
                f = (
                    sidedatamod.SD_P1COPIES,
                    sidedatamod.SD_P2COPIES,
                    sidedatamod.SD_FILESADDED,
                    sidedatamod.SD_FILESREMOVED,
                )
        return False, f, {}

    return sidedatacompanion