Notes on using git (#7496)

* general updates to CONTRIBUTING.md
* notes on updating your PR
* Notes on squash-merging or otherwise
* document git branching model
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# Contributing code to Matrix
# Contributing code to Synapse
Everyone is welcome to contribute code to Matrix
(https://github.com/matrix-org), provided that they are willing to license
their contributions under the same license as the project itself. We follow a
simple 'inbound=outbound' model for contributions: the act of submitting an
'inbound' contribution means that the contributor agrees to license the code
under the same terms as the project's overall 'outbound' license - in our
case, this is almost always Apache Software License v2 (see [LICENSE](LICENSE)).
Everyone is welcome to contribute code to [matrix.org
projects](https://github.com/matrix-org), provided that they are willing to
license their contributions under the same license as the project itself. We
follow a simple 'inbound=outbound' model for contributions: the act of
submitting an 'inbound' contribution means that the contributor agrees to
license the code under the same terms as the project's overall 'outbound'
license - in our case, this is almost always Apache Software License v2 (see
[LICENSE](LICENSE)).
## How to contribute
The preferred and easiest way to contribute changes to Matrix is to fork the
relevant project on github, and then [create a pull request](
https://help.github.com/articles/using-pull-requests/) to ask us to pull
your changes into our repo.
The preferred and easiest way to contribute changes is to fork the relevant
project on github, and then [create a pull request](
https://help.github.com/articles/using-pull-requests/) to ask us to pull your
changes into our repo.
**The single biggest thing you need to know is: please base your changes on
the develop branch - *not* master.**
@ -28,35 +29,31 @@ use github's pull request workflow to review the contribution, and either ask
you to make any refinements needed or merge it and make them ourselves. The
changes will then land on master when we next do a release.
We use [Buildkite](https://buildkite.com/matrix-dot-org/synapse) for continuous
integration. If your change breaks the build, this will be shown in GitHub, so
please keep an eye on the pull request for feedback.
Some other things you will need to know when contributing to Synapse:
To run unit tests in a local development environment, you can use:
* Please follow the [code style requirements](#code-style).
- ``tox -e py35`` (requires tox to be installed by ``pip install tox``)
for SQLite-backed Synapse on Python 3.5.
- ``tox -e py36`` for SQLite-backed Synapse on Python 3.6.
- ``tox -e py36-postgres`` for PostgreSQL-backed Synapse on Python 3.6
(requires a running local PostgreSQL with access to create databases).
- ``./test_postgresql.sh`` for PostgreSQL-backed Synapse on Python 3.5
(requires Docker). Entirely self-contained, recommended if you don't want to
set up PostgreSQL yourself.
* Please include a [changelog entry](#changelog) with each PR.
Docker images are available for running the integration tests (SyTest) locally,
see the [documentation in the SyTest repo](
https://github.com/matrix-org/sytest/blob/develop/docker/README.md) for more
information.
* Please [sign off](#sign-off) your contribution.
* Please keep an eye on the pull request for feedback from the [continuous
integration system](#continuous-integration-and-testing) and try to fix any
errors that come up.
* If you need to [update your PR](#updating-your-pull-request), just add new
commits to your branch rather than rebasing.
## Code style
All Matrix projects have a well-defined code-style - and sometimes we've even
got as far as documenting it... For instance, synapse's code style doc lives
[here](docs/code_style.md).
Synapse's code style is documented [here](docs/code_style.md). Please follow
it, including the conventions for the [sample configuration
file](docs/code_style.md#configuration-file-format).
To facilitate meeting these criteria you can run `scripts-dev/lint.sh`
locally. Since this runs the tools listed in the above document, you'll need
python 3.6 and to install each tool:
Many of the conventions are enforced by scripts which are run as part of the
[continuous integration system](#continuous-integration-and-testing). To help
check if you have followed the code style, you can run `scripts-dev/lint.sh`
locally. You'll need python 3.6 or later, and to install a number of tools:
```
# Install the dependencies
@ -67,9 +64,11 @@ pip install -U black flake8 flake8-comprehensions isort
```
**Note that the script does not just test/check, but also reformats code, so you
may wish to ensure any new code is committed first**. By default this script
checks all files and can take some time; if you alter only certain files, you
might wish to specify paths as arguments to reduce the run-time:
may wish to ensure any new code is committed first**.
By default, this script checks all files and can take some time; if you alter
only certain files, you might wish to specify paths as arguments to reduce the
run-time:
```
./scripts-dev/lint.sh path/to/file1.py path/to/file2.py path/to/folder
@ -82,7 +81,6 @@ Please ensure your changes match the cosmetic style of the existing project,
and **never** mix cosmetic and functional changes in the same commit, as it
makes it horribly hard to review otherwise.
## Changelog
All changes, even minor ones, need a corresponding changelog / newsfragment
@ -98,24 +96,55 @@ in the format of `PRnumber.type`. The type can be one of the following:
* `removal` (also used for deprecations)
* `misc` (for internal-only changes)
The content of the file is your changelog entry, which should be a short
description of your change in the same style as the rest of our [changelog](
https://github.com/matrix-org/synapse/blob/master/CHANGES.md). The file can
contain Markdown formatting, and should end with a full stop (.) or an
exclamation mark (!) for consistency.
This file will become part of our [changelog](
https://github.com/matrix-org/synapse/blob/master/CHANGES.md) at the next
release, so the content of the file should be a short description of your
change in the same style as the rest of the changelog. The file can contain Markdown
formatting, and should end with a full stop (.) or an exclamation mark (!) for
consistency.
Adding credits to the changelog is encouraged, we value your
contributions and would like to have you shouted out in the release notes!
For example, a fix in PR #1234 would have its changelog entry in
`changelog.d/1234.bugfix`, and contain content like "The security levels of
Florbs are now validated when received over federation. Contributed by Jane
Matrix.".
`changelog.d/1234.bugfix`, and contain content like:
## Debian changelog
> The security levels of Florbs are now validated when received
> via the `/federation/florb` endpoint. Contributed by Jane Matrix.
If there are multiple pull requests involved in a single bugfix/feature/etc,
then the content for each `changelog.d` file should be the same. Towncrier will
merge the matching files together into a single changelog entry when we come to
release.
### How do I know what to call the changelog file before I create the PR?
Obviously, you don't know if you should call your newsfile
`1234.bugfix` or `5678.bugfix` until you create the PR, which leads to a
chicken-and-egg problem.
There are two options for solving this:
1. Open the PR without a changelog file, see what number you got, and *then*
add the changelog file to your branch (see [Updating your pull
request](#updating-your-pull-request)), or:
1. Look at the [list of all
issues/PRs](https://github.com/matrix-org/synapse/issues?q=), add one to the
highest number you see, and quickly open the PR before somebody else claims
your number.
[This
script](https://github.com/richvdh/scripts/blob/master/next_github_number.sh)
might be helpful if you find yourself doing this a lot.
Sorry, we know it's a bit fiddly, but it's *really* helpful for us when we come
to put together a release!
### Debian changelog
Changes which affect the debian packaging files (in `debian`) are an
exception.
exception to the rule that all changes require a `changelog.d` file.
In this case, you will need to add an entry to the debian changelog for the
next release. For this, run the following command:
@ -200,19 +229,45 @@ Git allows you to add this signoff automatically when using the `-s`
flag to `git commit`, which uses the name and email set in your
`user.name` and `user.email` git configs.
## Merge Strategy
## Continuous integration and testing
We use the commit history of develop/master extensively to identify
when regressions were introduced and what changes have been made.
[Buildkite](https://buildkite.com/matrix-dot-org/synapse) will automatically
run a series of checks and tests against any PR which is opened against the
project; if your change breaks the build, this will be shown in GitHub, with
links to the build results. If your build fails, please try to fix the errors
and update your branch.
We aim to have a clean merge history, which means we normally squash-merge
changes into develop. For small changes this means there is no need to rebase
to clean up your PR before merging. Larger changes with an organised set of
commits may be merged as-is, if the history is judged to be useful.
To run unit tests in a local development environment, you can use:
This use of squash-merging will mean PRs built on each other will be hard to
merge. We suggest avoiding these where possible, and if required, ensuring
each PR has a tidy set of commits to ease merging.
- ``tox -e py35`` (requires tox to be installed by ``pip install tox``)
for SQLite-backed Synapse on Python 3.5.
- ``tox -e py36`` for SQLite-backed Synapse on Python 3.6.
- ``tox -e py36-postgres`` for PostgreSQL-backed Synapse on Python 3.6
(requires a running local PostgreSQL with access to create databases).
- ``./test_postgresql.sh`` for PostgreSQL-backed Synapse on Python 3.5
(requires Docker). Entirely self-contained, recommended if you don't want to
set up PostgreSQL yourself.
Docker images are available for running the integration tests (SyTest) locally,
see the [documentation in the SyTest repo](
https://github.com/matrix-org/sytest/blob/develop/docker/README.md) for more
information.
## Updating your pull request
If you decide to make changes to your pull request - perhaps to address issues
raised in a review, or to fix problems highlighted by [continuous
integration](#continuous-integration-and-testing) - just add new commits to your
branch, and push to GitHub. The pull request will automatically be updated.
Please **avoid** rebasing your branch, especially once the PR has been
reviewed: doing so makes it very difficult for a reviewer to see what has
changed since a previous review.
## Notes for maintainers on merging PRs etc
There are some notes for those with commit access to the project on how we
manage git [here](docs/dev/git.md).
## Conclusion

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Some notes on how we use git
============================
On keeping the commit history clean
-----------------------------------
In an ideal world, our git commit history would be a linear progression of
commits each of which contains a single change building on what came
before. Here, by way of an arbitrary example, is the top of `git log --graph
b2dba0607`:
<img src="git/clean.png" alt="clean git graph" width="500px">
Note how the commit comment explains clearly what is changing and why. Also
note the *absence* of merge commits, as well as the absence of commits called
things like (to pick a few culprits):
[“pep8”](https://github.com/matrix-org/synapse/commit/84691da6c), [“fix broken
test”](https://github.com/matrix-org/synapse/commit/474810d9d),
[“oops”](https://github.com/matrix-org/synapse/commit/c9d72e457),
[“typo”](https://github.com/matrix-org/synapse/commit/836358823), or [“Who's
the president?”](https://github.com/matrix-org/synapse/commit/707374d5d).
There are a number of reasons why keeping a clean commit history is a good
thing:
* From time to time, after a change lands, it turns out to be necessary to
revert it, or to backport it to a release branch. Those operations are
*much* easier when the change is contained in a single commit.
* Similarly, it's much easier to answer questions like “is the fix for
`/publicRooms` on the release branch?” if that change consists of a single
commit.
* Likewise: “what has changed on this branch in the last week?” is much
clearer without merges and “pep8” commits everywhere.
* Sometimes we need to figure out where a bug got introduced, or some
behaviour changed. One way of doing that is with `git bisect`: pick an
arbitrary commit between the known good point and the known bad point, and
see how the code behaves. However, that strategy fails if the commit you
chose is the middle of someone's epic branch in which they broke the world
before putting it back together again.
One counterargument is that it is sometimes useful to see how a PR evolved as
it went through review cycles. This is true, but that information is always
available via the GitHub UI (or via the little-known [refs/pull
namespace](https://help.github.com/en/github/collaborating-with-issues-and-pull-requests/checking-out-pull-requests-locally)).
Of course, in reality, things are more complicated than that. We have release
branches as well as `develop` and `master`, and we deliberately merge changes
between them. Bugs often slip through and have to be fixed later. That's all
fine: this not a cast-iron rule which must be obeyed, but an ideal to aim
towards.
Merges, squashes, rebases: wtf?
-------------------------------
Ok, so that's what we'd like to achieve. How do we achieve it?
The TL;DR is: when you come to merge a pull request, you *probably* want to
“squash and merge”:
![squash and merge](git/squash.png).
(This applies whether you are merging your own PR, or that of another
contributor.)
“Squash and merge”<sup id="a1">[1](#f1)</sup> takes all of the changes in the
PR, and bundles them into a single commit. GitHub gives you the opportunity to
edit the commit message before you confirm, and normally you should do so,
because the default will be useless (again: `* woops typo` is not a useful
thing to keep in the historical record).
The main problem with this approach comes when you have a series of pull
requests which build on top of one another: as soon as you squash-merge the
first PR, you'll end up with a stack of conflicts to resolve in all of the
others. In general, it's best to avoid this situation in the first place by
trying not to have multiple related PRs in flight at the same time. Still,
sometimes that's not possible and doing a regular merge is the lesser evil.
Another occasion in which a regular merge makes more sense is a PR where you've
deliberately created a series of commits each of which makes sense in its own
right. For example: [a PR which gradually propagates a refactoring operation
through the codebase](https://github.com/matrix-org/synapse/pull/6837), or [a
PR which is the culmination of several other
PRs](https://github.com/matrix-org/synapse/pull/5987). In this case the ability
to figure out when a particular change/bug was introduced could be very useful.
Ultimately: **this is not a hard-and-fast-rule**. If in doubt, ask yourself “do
each of the commits I am about to merge make sense in their own right”, but
remember that we're just doing our best to balance “keeping the commit history
clean” with other factors.
Git branching model
-------------------
A [lot](https://nvie.com/posts/a-successful-git-branching-model/)
[of](http://scottchacon.com/2011/08/31/github-flow.html)
[words](https://www.endoflineblog.com/gitflow-considered-harmful) have been
written in the past about git branching models (no really, [a
lot](https://martinfowler.com/articles/branching-patterns.html)). I tend to
think the whole thing is overblown. Fundamentally, it's not that
complicated. Here's how we do it.
Let's start with a picture:
![branching model](git/branches.jpg)
It looks complicated, but it's really not. There's one basic rule: *anyone* is
free to merge from *any* more-stable branch to *any* less-stable branch at
*any* time<sup id="a2">[2](#f2)</sup>. (The principle behind this is that if a
change is good enough for the more-stable branch, then it's also good enough go
put in a less-stable branch.)
Meanwhile, merging (or squashing, as per the above) from a less-stable to a
more-stable branch is a deliberate action in which you want to publish a change
or a set of changes to (some subset of) the world: for example, this happens
when a PR is landed, or as part of our release process.
So, what counts as a more- or less-stable branch? A little reflection will show
that our active branches are ordered thus, from more-stable to less-stable:
* `master` (tracks our last release).
* `release-vX.Y.Z` (the branch where we prepare the next release)<sup
id="a3">[3](#f3)</sup>.
* PR branches which are targeting the release.
* `develop` (our "mainline" branch containing our bleeding-edge).
* regular PR branches.
The corollary is: if you have a bugfix that needs to land in both
`release-vX.Y.Z` *and* `develop`, then you should base your PR on
`release-vX.Y.Z`, get it merged there, and then merge from `release-vX.Y.Z` to
`develop`. (If a fix lands in `develop` and we later need it in a
release-branch, we can of course cherry-pick it, but landing it in the release
branch first helps reduce the chance of annoying conflicts.)
---
<b id="f1">[1]</b>: “Squash and merge” is GitHub's term for this
operation. Given that there is no merge involved, I'm not convinced it's the
most intuitive name. [^](#a1)
<b id="f2">[2]</b>: Well, anyone with commit access.[^](#a2)
<b id="f3">[3]</b>: Very, very occasionally (I think this has happened once in
the history of Synapse), we've had two releases in flight at once. Obviously,
`release-v1.2.3` is more-stable than `release-v1.3.0`. [^](#a3)

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