Basic information about hooks

1. Hook return values

Hooks can be implemented as either external programs, or internal python calls. The meaning of the return value in both cases is based on the convention for external executables; in other words, a value of 0 means "success". For hooks implemented in python this can be a bit misleading, since it means you return "False" to indicate success and "True" (or throw an exception) to indicate failure.

2. Commit Hook

You can use the hook feature to automatically send mail when a commit occurs. Add the following to .hg/hgrc:

commit = commithook

And put this in a file called commithook in your path:


SUBJECT=$(hg log -r $HG_NODE --template '{desc|firstline}') hg log -vpr $HG_NODE | mail -s "commit: $SUBJECT" commit-list@example.com

And something like this can be used for closing bugs in roundup:


ISSUE=`hg log -vr $HG_NODE | grep -o "\<fix:[[:alnum:]]*" | head -n 1 | sed s/fix://` [ -z $ISSUE ] && exit SUBJECT=`hg log -r $HG_NODE | grep "^summary:" | cut -b 14-` hg log -vr $HG_NODE | mail -s "[$ISSUE] [status=testing] commit: $SUBJECT" roundup@example.com

If you want multiple hooks, just add a suffix to the action, an empty command can be used to unset a site-wide hook:

# do not use the site-wide hook
commit =
commit.email = /my/email/hook
commit.autobuild = /my/build/hook

3. Tips for centralized repositories

(!) If you are using some central repo with the shared ssh method and want to get notified on pushes to there, don't use the commit hook (won't get triggered), but the incoming hook.

(!) If you push a large number of changesets, the script above will flood users with one email per commit. Instead, consider using the changegroup hook, which is activated once for each push/pull/unbundle instead of once for each changeset.

4. The changegroup hook

The changegroup hook is activated once for each push/pull/unbundle, unlike the commit hook, which is run once for each changeset.

To send a mail containing all changesets of a push/pull/unbundle, put this in .hg/hgrc:

changegroup = /path/to/changegrouphook

Then put something like this in /path/to/changegrouphook:


dest=" commit-list@example.com "
subject="New changesets in $repo"

hg log -r $HG_NODE: | mail -s "$subject" $dest

For a nicer display of the changes, use hg log -G like this:


dest=" commit-list@example.com "
subject="New changesets in $repo"

hg log -G -r $HG_NODE: | mail -s "$subject" $dest

An alternative is to use the NotifyExtension, which sends email notifications to a list of subscribed addresses.

5. Commit mail for patch review

One common reason for commit emails is to review changes. When multiple committers push to a central, canonical repository and have changes mailed only when they push to the repository rather than whenever they do commits to their local clones, you can get the same data coming through multiple times, making it hard to review the changes. This happens because of merge commits; hg pull; hg update do work hg commit; hg pull; hg update -m; hg commit will give two commit messages; one for the work and one for the merge. Except when there are conflicts, the text of the commit message for the merge is all diff that is really part of the commit, but which doesn't represent an actual change to the central, canonical repository. So while the commit message is absolutely technically correct, the duplicate information that it provides tends to degrade the reviewing process, and in practice, we (rPath) saw it obscure accidental reversion from a bad merge.

It is possible to remove almost all of the duplicate information with a bit of scripting. The following script does the trick:



while [ $# -gt 0 ] ; do

 . case $1 in
  . --recip)
   . shift RECIP="$RECIP $1" shift ;;
   . shift MODULE=$1 shift ;;


hg update -C SUBJECT=$(hg log -r $HG_NODE --template '{desc|firstline}')

PARENT=$(hg log --debug -r $HG_NODE | sed -n '/^parent:/{s/^.*: *.*://;p}' | head -1 ) FILES=$(hg log -v -r $HG_NODE | sed -n '/^files:/{s/^.*: *//;p}')

if [ -z "$FILES" ]; then

 . exit 0


(hg log -vr $HG_NODE | egrep -v '^(changeset|parent|date):' | sed 's/^description:$//' | cat -s ; hg diff -r $PARENT -r $HG_NODE $FILES) | mail -s "$MODULE: $SUBJECT" $RECIP

This script filters out some of the other information that is really not needed for patch review: the changeset hash exists in the diff; the parents are available by using hg log on the changeset hash; the word "description" isn't really needed in this context, and so is filtered out as noise; the date is in the email header, etc. Some of the brevity introduced in this script was driven by one of its authors being in the habit of reading changemail on his Treo, but it seems to generally help focus the review process on the things that really changed. When we pointed this script at the node with the accidental inappropriate reversion, it went from being hidden in the noise to being painfully obvious.

6. Change temporary directory used on remote when pushing

By default Mercurial uses /tmp (or one of the directories defined by environment variables TMPDIR, TEMP, TMP) to uncompress the bundle received from a remote host. This may be problematic on some servers with a small /tmp directory or with small quotas on that partition. To circumvent that, you can define a hook to set TMPDIR to another location before mercurial sets up its serving thread.

On remote global hgrc (/etc/mercurial/hgrc or MERCURIAL.INI, set the following hook:

pre-serve.tmpdir = python:hgenviron.settmpdir

Somewhere in your $PYTHONPATH, put the following hgenviron.py file :

import os
#see http://docs.python.org/lib/module-tempfile.html
def settmpdir(ui, repo, hooktype, node=None, source=None, **kwargs):
        os.environ["TMPDIR"] = "/home/tmp"

Now mercurial on remote will use /home/tmp as the temporary directory when receiving changesets, for every user (but only for mercurial).

7. Automatically add a "Signed-off-by:" tag line to commit messages

To automate the process of adding the "signed-off-by" tag line like it's used by the Linux Kernel community and many other projects, you could use a commit hook like this one: https://bitbucket.org/snippets/LenzGr/aygXb To enable it, simply download and place the file signoff.py into your repository's .hg directory. Then, add the following to the [hooks] section of .hg/hgrc:

        precommit = python:.hg/signoff.py:sign_commit_message

8. Dealing with demandimport in hooks

With in-process hooks, you can not turn off demandimport because has already been enabled by mercurial.

So appending to the ignore list might not work, ex:

# this might not work
from mercurial import demandimport
       ".ssl_",  # requests uses relative imports
       ".util.ssl_",  # requests uses relative imports

However, there is another way to "by-pass" demandimport: it is to actually use something in the module.

For example, if you have this error with your server-side push hook:

remote:   File "/usr/local/lib/python2.7/site-packages/requests/packages/urllib3/util/ssl_.py", line 229, in create_urllib3_context
remote:     options |= OP_NO_COMPRESSION
remote: TypeError: unsupported operand type(s) for |=: 'int' and '_demandmod'

This is because requests/packages/urllib3/util/ssl_.py has this code

    from ssl import OP_NO_SSLv2, OP_NO_SSLv3, OP_NO_COMPRESSION
except ImportError:
    OP_NO_SSLv2, OP_NO_SSLv3 = 0x1000000, 0x2000000
    OP_NO_COMPRESSION = 0x20000

One way to deal with this "try except ImportError" not working with demandimport is to have these code before the code that actually needs it.

from requests.packages.urllib3.util import ssl_
ssl_.OP_NO_COMPRESSION = 0x20000
# then the code that uses the request package with ssl that used to blow up as above

9. Listing active hooks

In case hooks are causing problems it is essential to determine which hooks are defined/active and in which configuration file they have been defined in.

Using the following command you will get a list of files hooks are read from and the list of active hooks:

hg config hooks --debug

10. See also

Hook (last edited 2017-10-09 11:47:26 by JanStotz)