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LTTng-tools contributor's guide

Being an open source project, the LTTng-tools project welcomes contributions from anyone. This guide walks you through the process of contributing a patch to LTTng-tools.

Getting the source code

The LTTng-tools project uses Git for version control. The upstream Git repository URL is:

git:https://git.lttng.org/lttng-tools.git

Coding standard

See CodingStyle for guidelines style and design guidelines.

Although the LTTng-tools code base is primarily written in C++, it does contain C, shell, and Python code as well. There is no official coding standard for these languages. However, using a style consistent with the rest of the code written in that language is strongly encouraged.

Creating and sending a patch

LTTng-tools's development flow is primarily based on Gerrit Code Review, although we also accept e-mail based patch series on the lttng-dev mailing list and pull requests on our GitHub mirror. If you're going to create GitHub pull requests, make sure you still follow the guidelines below.

The mailing list is also used to share and comment on RFCs and answer user questions.

A template commit messsage is available below, and as a file that you may configure your local check out to use:

git config commit.template .commit_template

Once your changes have been comitted to your local branch, you may use the git-review plugin to submit them directly to Gerrit using the following command:

git review

Please note that you will need to create an account on Gerrit and add an SSH public key.

For e-mail based patches you may use Git's format-patch command to generate a patch file. The following command line generates a patch from the latest commit:

git format-patch -N1 -s --subject-prefix="PATCH lttng-tools"

The custom PATCH lttng-tools subject prefix is mandatory when submitting patches that apply to the LTTng-tools project.

The patch's subject (the commit message's first line) should:

  • Begin with an uppercase letter.
  • Be written in the present tense.
  • Not exceed 72 characters in length.
  • Not end with a period.

In the case of bug fixes, the patch's subject must be prefixed with Fix: and a suitable sub-system name. For instance, a patch addressing a bug in the session daemon should start with Fix: sessiond:. Patches targeting shared code can either use the namespace of the interface or of the internal library, whichever is more precise.

A non-exhaustive list of common sub-system prefixes follows:

  • relayd (relay daemon).
  • sessiond (session daemon).
  • lttng (LTTng CLI client).
  • ust-consumerd (user space consumer daemon).
  • kernel-consumerd (kernel space consumer daemon).
  • consumerd (common consumer daemon).
  • common (internal libcommon).
  • trace-chunk (internal lttng_trace_chunk_* interface).
  • lttng-ctl (liblttng-ctl library).
  • mi (LTTng client's machine interface).

When possible, the commit title should describe the issue as observed and not the underlying cause. For instance, prefer Fix: sessiond: hang on SIGTERM after session rotation to Fix: sessiond: unchecked status on exit.

The commit message's body must be as detailed as possible and explain the reasons behind the proposed change. Keep in mind that this message will be read in a number of years and must still be clear. Any related bug report(s) should be mentioned at the end of the message using the #123 format, where 123 is the bug number:

  • Use Refs: #123 if the patch is related to bug 123, but does not fix it yet.
  • Use Fixes: #123 to signify that this patch fixes the bug.

In the case of bug fixes, the following structure must be used:

  • Observed issue
  • Cause
  • Solution
  • Optional: Known drawbacks

A short commit message can be used when submitting typo fixes or minor cleanups that don't introduce behaviour changes.

When submitting a patch that affects existing code, implement changes to the existing code as prelude patches in a patch series. Explain why those changes are needed and how they make follow-up changes easier/possible.

Make sure to sign-off your submitted patches (the -s argument to Git's commit and format-patch commands).

Here's a complete example:

Fix: relayd: missing thingy in the doodad folder on error

Observed issue
==============
After a communication error, the relay daemon will not produce
a thingy in the doodad folder. This results in the knickknack
baring the foo.

Steps to reproduce (list of commands or narrative description).

Cause
=====
The thingy_do_the_doodad() callback is only invoked when
the thread responsible for receiving messages and dispatching
them to the correct actors encounters an emoji.

However, an emoji is not guaranteed to be present in the ELF
section header [1].

Solution
========
Flushing the doodad on every reception of a thingo ensures that
the thingy is present in the doodad folder even if a communication
error occurs.

Known drawbacks
===============
Flushing the doodad too often may spam the widget and result in
degradation of the gizmo. This doesn't matter right now since
it happens exactly once per blue moon.

If this becomes a serious issue, we could machine learn the MVP
through the big O terminal.

References
==========
[1] https://www.thedocs.com/elf/proving-my-point-unambiguously.aspx

Fixes: #321
Refs: #456
Refs: #1987

Signed-off-by: Jeanne Mance <[email protected]>

Please note that patches should be as focused as possible. Do not, for instance, fix a bug and correct the indentation of an unrelated block of code as part of the same patch.

The project contains a script, extras/checkpatch.pl, that performs a number of checks on a patch to ensure it is ready for submission. Run this script on your patch and correct any reported errors before posting it to the mailing list:

extras/checkpatch.pl --no-tree 0001-Fix...patch

Once you are confident your patch meets the required guidelines, you may use Git's send-email command to send your patch to the mailing list:

git send-email --suppress-cc=self --to [email protected] *.patch

Make sure you are subscribed to the mailing list to follow and take part in discussions about your changes. You may join the file to an email as an attachment if you can't send the patch directly using git send‑email.

Reviews

Once your patch has been posted to the mailing list or as a GitHub pull request, other contributors may propose modifications. This is completely normal. This collaborative code review is an integral part of the open source development process in general and LTTng-tools makes no exception.

Keep in mind that reviewing patches is a time-consuming process and, as such, may not be done right away. The delays may be affected by the current release cycle phase and the complexity of the proposed changes. If you think your patch might have been forgotten, please mention it on the #lttng IRC channel rather than resubmitting.

Release cycle

The LTTng-tools project follows a release cycle that alternates between development and release candidate (RC) phases. The master branch is feature-frozen during RC phases: only bug fixes are accepted during this period. However, patches adding new functionality may still be submitted and reviewed during the RC. The upcoming features and release dates are posted in a monthly digest on the mailing list.