[RFC] Amend commit messages

Gilles Moris gilles.moris at free.fr
Wed Feb 23 02:41:01 CST 2011


I don't remember any extension in this area, but this is a recurrent question
on the mailing list:
"I made a mistake in my commit message. How can I change it?"

We can redirect them to either mq or histedit, but usually we can only answer
that this is not possible if the commit is already published.
However, this question is legitimate. Whatever the number of reviews you can
do, there always be some typo that can escape, like switched digits in a bug
number or similar issues. The idea to have a message forever wrong is quite

1. Goal
The goal is to overlay another commit description to replace a previous commit
message. The current history (committed SHA1) is not changed and those commit
message amendments appear as additional commits.

2. Design
The idea to wrap the changectx.description() method and replace the original
commit message with something else. The design is somewhat borrowed to tags:
get the new description from another file; if multiple heads exist for this
file, take the latest filelog revision of that file by convention.

Now see how the something else can structured:

- as a single ".hgmessage" file located in the working directory, much like
.hgtags". However this is more complex to handle than tags, as commit message
can be multi-lines. This would require some markup language to delineate the
mapping between the commit identifier and the message content. This would be
specifically a problem during merges of this file, which would require knowledge
of the format and increase the risk of incorrectly merge this file.

- the second solution is to create a ".hgmessage" directory this time, and have
file for each commit amended. The file name is the full hex SHA1 identifier of
the amended commit and the content is just the new commit message. This lowers
the risk of merge, and in case of merge it is quite straightforward to
understand how to merge. However, this risk is not null. The problem here is
that the commit message description history is intermangled with the regular
code history. If the commit amendments are made in 2 different branches that
are not supposed to be merged from a code standpoint, we're stuck with also
2 divergent commit descriptions.

- so I came up with a third solution which is to put the commit messages in
their owned named branch "hgmessage". That way the amendment history is
decorrelated from the code history. The need to amend a commit message should
be sufficiently rare that the overhead of a separate branch is acceptable.

3. Implementation
As an roughly implemented extension inlined below.

4. Open issues
- The name of the extension is message. The name of the command is "amend". I
started with "message", then "editmessage". I like "amend" as it shows I do not
edit the history.
- More generally naming and wording.
- Deleting one of the commit amendment files does not cancel the message
edition as I am looking at the latest filelog revision, not at the manifest.
Currently, you cancel a message amendment by creating a new amendment with the
original content. That should be acceptable.
- Performance: I did not find how to walk a specific directory of the store at
start up, so I am walking the entire store (code taken from the cifiles
extension). This also why I kept a directory even though I have a separate
named branch: to provide further optimization of the start up time. Also there
is no caching. Is some needed ?
- Next steps: what do we do with this? I will wait for your comments to see if
I am heading in the right direction, and probably then post the extension on
Google Code or Bitbuckets. But then, does it deserve to be considered for
inclusion as a bundled extension or a core patch?


import os, re
from mercurial import hg, extensions, context, node, util, commands, cmdutil
from mercurial import match as matchmod
from mercurial.i18n import _

def getmsgfolder(ui):
    return ui.config('editmessage', 'folder', 'hgmessage')

def getmsgbranch(ui):
    return ui.config('editmessage', 'branch', 'hgmessage')

def msgread(filelog, node):
    return filelog.read(node).splitlines()

def amenddesc(orig, ctx):
    """wraps the changectx.description() method"""
    repo = ctx._repo
    nid = node.hex(ctx.node())

    if nid in repo.amendedmsg:
        fl = repo.file(getmsgfolder(repo.ui) + '/' + nid)
        heads = [fl.rev(h) for h in fl.heads()]

        if repo.ui.debugflag:
            lines = []
            for r in range(len(fl)-1, -1, -1):
                amctx = repo[fl.linkrev(r)]
                lines.append(_("amendment version: %d (%s)") %
                             (r + 1, node.hex(amctx.node())))
                if len(heads) > 1 and r in heads:
                    ln = _("WARNING: message amendment conflicts in rev %s") % \
                         ', '.join(str(fl.linkrev(h)) for h in heads if h != r)
                lines.append(_("amended by: %s") % amctx.user())
                lines.append(_("amended on: %s") % util.datestr(amctx.date()))
                lines.extend(msgread(fl, fl.node(r)))
            lines.append(_("original description:"))
        elif repo.ui.verbose:
            lines = msgread(fl, fl.tip())
            amctx = repo[fl.linkrev(len(fl)-1)]
            lines.insert(1, _("(amended by %s on %s)") % (
            if len(heads) > 1:
                             _("WARNING: message amendment conflicts in rev %s") %
                             ", ".join(str(fl.linkrev(h)) for h in heads))
            lines = msgread(fl, fl.tip())

        msg = '\n'.join(lines)
        msg = orig(ctx)
    return msg

def uisetup(ui):
    extensions.wrapfunction(context.changectx, 'description', amenddesc)

def reposetup(ui, repo):
    repo.amendedmsg = set()

    prefix = "data/%s/" % getmsgfolder(ui)
    suffix = ".i"
    plen = len(prefix)
    slen = len(suffix)
    lock = repo.lock()
        # this part can probably be greatly optimized by walking only one folder
        # but using which API
        for fn, efn, sz in repo.store.datafiles():
            if fn[-slen:] == suffix and fn[:plen] == prefix:

def editmsg(ui, repo, n, nidfn):
    """creates the text template of the commit message to be edited"""
    ctx = repo[n]
    # use changelog for original message to bypass the
    # overloaded ctx.description()
    origdesc = repo.changelog.read(n)[4]
    edittext = []

    # user edits the last commit message available
    if node.hex(n) in repo.amendedmsg:
        fl = repo.file(nidfn)
        edittext.extend(msgread(fl, fl.tip()))

    # standard HG comments when editing commit messages.
    edittext.append(_("HG: Enter commit message."
                      "  Lines beginning with 'HG:' are removed."))
    edittext.append(_("HG: Leave message as is to abort edit."))
    edittext.append("HG: --")
    edittext.append(_("HG: user: %s") % ctx.user())
    if ctx.p2():
        edittext.append(_("HG: branch merge"))
    if ctx.branch():
        edittext.append(_("HG: branch '%s'") % ctx.branch())
    edittext.extend([_("HG: files %s") % f for f in ctx.files()])

    # Display all the previous version of the message for reference
    edittext.append("HG:" + '=' * 35 + "8<" + '-' * 35)
    if node.hex(n) in repo.amendedmsg:
        for r in range(len(fl)-1, -1, -1):
            amctx = repo[fl.linkrev(r)]
            edittext.append("HG: amendment version: %d (%s)" %
                            (r + 1, node.hex(amctx.node())))
            edittext.append("HG: amended by: %s" % amctx.user())
            edittext.append("HG: amended on: %s" % util.datestr(amctx.date()))
            edittext.extend("HG: " + l for l in msgread(fl, fl.node(r)))
            edittext.append("HG:" + '=' * 35 + "8<" + '-' * 35)
    edittext.append("HG: original description:")
    origdesc = repo.changelog.read(n)[4]
    edittext.extend(map(lambda l: "HG: " + l, origdesc.splitlines()))
    return '\n'.join(edittext)

def amend(ui, repo, rev, **opts):
    """amend the commit message of the given revision

    Change a previous commit message. The commit is given as the only argument
    of the command. The history is not really edited. Another commit is created
    by this command and will overlay the previous commit message. This command
    can be run multiple times: the log will show the latest message.

    Without -m or -l options, an editor will show up with the commit message
    edition history as a reference.

    If divergent amendment already exists for this commit, the command will
    refuse to work. You will have first to merge the multiple heads on the named
    branch (by default "hgmessage") used to handle the message amendment. The -f
    option enables to override this behavior.

    Only the commit message is amended. The -u and -d options allow only to
    change the user and date of the amendement commit.

    Returns 0 on success, 1 if nothing changed.

    n = repo.lookup(rev)
    msgbranch = getmsgbranch(ui)
    hgmsgdir = getmsgfolder(ui)
    fl = repo.file(hgmsgdir + '/' + node.hex(n))
    if not opts.get('force') and len(fl.heads()) > 1:
        raise util.Abort("multiple heads for %s: merge %s branch or use -f" %
                         (node.short(n), msgbranch))
    savectx = os.getcwd(), repo.dirstate.parents()

    # Create or check out the hgmessage branch
    if msgbranch not in repo.branchtags():
        hg.clean(repo, node.nullid, False)
        hg.clean(repo, msgbranch, False)

    if not os.path.lexists(repo.wjoin(hgmsgdir)):
    nidfn = os.path.join(hgmsgdir, node.hex(n))

    text = cmdutil.logmessage(opts)
    if not text:
        temp = editmsg(ui, repo, n, nidfn)
        text = ui.edit(temp, repo[n].user())
        if text == temp:
            # nothing changed
            text = ""
            text = re.sub("(?m)^HG:.*(\n|$)", "", text)

    if not text:
        ui.status(_("nothing changed\n"))
        return 1

    text = text.strip() + '\n'
    f = open(repo.wjoin(nidfn), 'w')
    wctx = repo[None]
    wctx.status(ignored=True, unknown=True)
    if nidfn in wctx.ignored() or nidfn in wctx.unknown():
    match = matchmod.exact(repo.root, '', [nidfn])
    date = opts.get('date')
    if date:
        date = util.parsedate(date)
    cimsg = "amend commit message of %s" % node.hex(n)
    repo.commit(cimsg, opts.get('user'), date, match)

    # restores the saved context
    hg.clean(repo, savectx[1][0], False)
    return 0

cmdtable = {
         [('f', 'force', None, _('force edit message')),
          ] + commands.commitopts + commands.commitopts2,
        _('hg editmessage [OPTION] REV')),

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