hgext/children.py

repair: migrate revlogs during upgrade

Our next step for in-place upgrade is to migrate store data. Revlogs
are the biggest source of data within the store and a store is useless
without them, so we implement their migration first.

Our strategy for migrating revlogs is to walk the store and call
`revlog.clone()` on each revlog. There are some minor complications.

Because revlogs have different storage options (e.g. changelog has
generaldelta and delta chains disabled), we need to obtain the
correct class of revlog so inserted data is encoded properly for its
type.

Various attempts at implementing progress indicators that didn't lead
to frustration from false "it's almost done" indicators were made.

I initially used a single progress bar based on number of revlogs.
However, this quickly churned through all filelogs, got to 99% then
effectively froze at 99.99% when it got to the manifest.

So I converted the progress bar to total revision count. This was a
little bit better. But the manifest was still significantly slower
than filelogs and it took forever to process the last few percent.

I then tried both revision/chunk bytes and raw bytes as the
denominator. This had the opposite effect: because so much data is in
manifests, it would churn through filelogs without showing much
progress. When it got to manifests, it would fill in 90+% of the
progress bar.

I finally gave up having a unified progress bar and instead implemented
3 progress bars: 1 for filelog revisions, 1 for manifest revisions, and
1 for changelog revisions. I added extra messages indicating the total
number of revisions of each so users know there are more progress bars
coming.

I also added extra messages before and after each stage to give extra
details about what is happening. Strictly speaking, this isn't
necessary. But the numbers are impressive. For example, when converting
a non-generaldelta mozilla-central repository, the messages you see are:

   migrating 2475593 total revisions (1833043 in filelogs, 321156 in manifests, 321394 in changelog)
   migrating 1.67 GB in store; 2508 GB tracked data
   migrating 267868 filelogs containing 1833043 revisions (1.09 GB in store; 57.3 GB tracked data)
   finished migrating 1833043 filelog revisions across 267868 filelogs; change in size: -415776 bytes
   migrating 1 manifests containing 321156 revisions (518 MB in store; 2451 GB tracked data)

That "2508 GB" figure really blew me away. I had no clue that the raw
tracked data in mozilla-central was that large. Granted, 2451 GB is in
the manifest and "only" 57.3 GB is in filelogs. But still.

It's worth noting that gratuitous loading of source revlogs in order
to display numbers and progress bars does serve a purpose: it ensures
we can open all source revlogs. We don't want to spend several minutes
copying revlogs only to encounter a permissions error or similar later.

As part of this commit, we also add swapping of the store directory
to the upgrade function. After revlogs are converted, we move the
old store into the backup directory then move the temporary repo's
store into the old store's location. On well-behaved systems, this
should be 2 atomic operations and the window of inconsistency show be
very narrow.

There are still a few improvements to be made to store copying and
upgrading. But this commit gets the bulk of the work out of the way.

# Mercurial extension to provide the 'hg children' command
#
# Copyright 2007 by Intevation GmbH <[e-mail]>
#
# Author(s):
# Thomas Arendsen Hein <[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.

'''command to display child changesets (DEPRECATED)

This extension is deprecated. You should use :hg:`log -r
"children(REV)"` instead.
'''

from __future__ import absolute_import

from mercurial.i18n import _
from mercurial import (
    cmdutil,
    commands,
)

templateopts = commands.templateopts

cmdtable = {}
command = cmdutil.command(cmdtable)
# Note for extension authors: ONLY specify testedwith = 'ships-with-hg-core' for
# extensions which SHIP WITH MERCURIAL. Non-mainline extensions should
# be specifying the version(s) of Mercurial they are tested with, or
# leave the attribute unspecified.
testedwith = 'ships-with-hg-core'

@command('children',
    [('r', 'rev', '',
     _('show children of the specified revision'), _('REV')),
    ] + templateopts,
    _('hg children [-r REV] [FILE]'),
    inferrepo=True)
def children(ui, repo, file_=None, **opts):
    """show the children of the given or working directory revision

    Print the children of the working directory's revisions. If a
    revision is given via -r/--rev, the children of that revision will
    be printed. If a file argument is given, revision in which the
    file was last changed (after the working directory revision or the
    argument to --rev if given) is printed.

    Please use :hg:`log` instead::

        hg children => hg log -r "children()"
        hg children -r REV => hg log -r "children(REV)"

    See :hg:`help log` and :hg:`help revsets.children`.

    """
    rev = opts.get('rev')
    if file_:
        fctx = repo.filectx(file_, changeid=rev)
        childctxs = [fcctx.changectx() for fcctx in fctx.children()]
    else:
        ctx = repo[rev]
        childctxs = ctx.children()

    displayer = cmdutil.show_changeset(ui, repo, opts)
    for cctx in childctxs:
        displayer.show(cctx)
    displayer.close()