Contributing Changes

How to help us improve Mercurial's code.

This page is intended for (aspiring) developers.

1. Submission checklist

Please double-check your patch meets the following guidelines (explained below) before hitting send:

• patches are sent to the mercurial-devel mailing list (not the BTS)

• patch is in the body of the email in the form produced by hg export for easy review

• first line of commit message is of the form "subsystem: uncapitalized, no trailing period"
• relevant bugs are mentioned in summary in the form "(issueNNNN)"
• all relevant info is in the commit message for posterity (not a "0 of N" message)
• patch does just one thing (if you need a bullet list, split your patch)
• patch contains no other gratuitous changes:
  • whitespace changes
  • code movement
  • typo fixes
  • string changes
• patch leaves Mercurial in a working state (doesn't depend on future patches)
• test suite has been updated and runs cleanly
• code passes check-code and matches coding style (no foo_bar variable names!)

• relevant help text is updated (see mercurial/help/)

• appropriate branch is indicated in email subject (eg hg email --flag stable)

2. Getting started

Start by cloning a copy of Mercurial's main repo:

hg clone http://selenic.com/hg

You'll also want to read the following pages:

• MercurialApi

• CodingStyle

• CompatibilityRules

• TimeBasedReleasePlan

Other pages you may find important:

• LockingDesign

• EncodingStrategy

3. Organizing patches

If you're making a large change, we're probably going to want it broken into a series of smaller patches. This makes for easier review and tests both for us and for you. This can be tricky at first and you might find tools like MQ and record useful in this process.

Each patch in a series should:

• implement one clear step in your process
• leave the code in a working state (so bisect always works!)
• include relevant test changes so they're independently testable
• be sent as a separate email

Do not mix formatting changes, organizational changes, or multiple functional changes in the same patch!

Things to consider:

• put the least controversial pieces first - if you're lucky, they'll get applied right away
• the difficulty of reviewing a patch increases rapidly with size - small patches are more likely to get attention
• the probability of a patch getting rejected is also a function of its size - smaller patches mean fewer review round-trips
• bigger patches have more bugs - smaller patches make it easier to locate regressions

4. Coding style and testing

If you send a patch with an underscore in a variable name, we'll know you haven't read this page!

• See our coding style for what we expect code to look like (yes, we're serious about the underscores)

• Use contrib/check-code.py to check for common style errors (  python contrib/check-code.py --blame `/usr/bin/hg manifest`  )

• Run the test suite to make sure you haven't broken anything

• Add new test cases as needed

• Patches should apply cleanly against the tip of the appropriate branch (default or stable)

• Don't touch the i18n/ directory for code or doc changes (translations is a separate process)

5. Patch descriptions

It's important that you describe your patch. Patch descriptions should be in the following format:

opener: check hardlink count reporting (issue1866)

The Linux CIFS kernel driver (even in 2.6.36) suffers from a hardlink
count blindness bug (lstat() returning 1 in st_nlink when it is expected
to return >1), which causes repository corruption if Mercurial running
...

• lowercase summary line, no trailing period
• start with the most useful identifiable command or subsystem (not necessarily the module name!)
• summarize the fix, not the problem

• add '(issueNNNN)' if it fixes an issue in the BugTracker (and automatically move the issue to testing)

• use '(BC)' to flag backward compatibility changes, use '(API)' to flag major internal API changes.
• a blank line after the summary
• a more complete description of the problem if necessary
• all lines less than 80 characters

Patch descriptions should be aimed at helping the reviewer understand the issue you're addressing. Try to answer the following, where appropriate:

• why we need this patch
• how you've implemented it
• why the choices you've made are the right ones
• why the choices you didn't make are the wrong ones
• what all the corner cases are (consider a table!)
• what shortcomings exist
• what file formats and data structures you've used

• what compatibility issues exist

• what's missing, if anything

• what it looks like, if relevant (include sample output!)

6. Emailing patches

Don't send your patch to the BugTracker - it can't be reviewed there, so it won't go anywhere!

We like to receive changes as patches by email. This allows us to review patches, give feedback, and track which patches need attention.

• Patches go to mercurial-devel@selenic.com - no subscription necessary! (we manually whitelist all legitimate posters)

• Patch emails should have [PATCH] in the subject followed by a summary (not included in the patch)

• We prefer patches in the message body so we can review them (no attachments or URLs!)

• Patches should be in the '# HG changeset patch' form output by 'hg export' - unified diff with author and full patch description

• Sending a patch implies granting permission to use it in our project under an appropriate license

• Apple Mail, Gmail and possibly other mail clients corrupt patches when sending them in the body. Using the patchbomb extension always works; see below for details.

Because this is a community project and our developers are very busy, patches will sometimes fall through the cracks. If you've gotten no response to your patch after a few days, feel free to resend it.

6.1. Mailer issues and patchbomb

As mentioned above, patches should be included in the body of emails to make it easy for developers to review and apply your patches. Some mailers make this difficult by mangling the whitespace in patches or similar. If you have trouble sending clean patches or are sending more than a couple patches, you should probably set up the patchbomb extensions which automates the process.

Add something like the following to your .hgrc:

[extensions]
patchbomb=

[email]
method = smtp
from = Ada Lovelace <adalovelace@gmail.com>
to = mercurial-devel@selenic.com

[smtp]
host = smtp.gmail.com
port = 587
tls = starttls
username = adalovelace@gmail.com

(See the hgrc manual for more SMTP configuration options).

Then run the following to do a dryrun test:

$ hg email --test <change1> <change2> ...

Notable options:

• '--flag' can be used to flag a patch, e.g. "STABLE", "RFC", "v2"

• don't use '--inline', it's a weird MIME thing and not what we want

• make sure everything in your 0 of X summary message is also in your commit messages for posterity

7. Etiquette and advice

• Feel free to resend patches after a few days if you haven't gotten a response
• Try to respond quickly to feedback before we forget what your patch is about
• Tell us everything you want us to know about a patch, every time
• If we ask for fixes, don't send a patch to your patch, send a new fixed patch
• Don't overwhelm us - if you can't get all 10 patches reviewed, try the first 5

• Consult 'hg log' to cc: the most relevant developers for the code you're working on

• Consider giving input on other people's patches

• Discuss your patches on IRC to get faster review and valuable initial feedback

8. See also

• Basic coding style

• Our compatibility rules for new features

• The patchbomb extension automates emailing patches

• The MQ extension is handy for managing patch series

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