Contributing to Bitcoin Core
============================

The Bitcoin Core project operates an open contributor model where anyone is
welcome to contribute towards development in the form of peer review, testing
and patches. This document explains the practical process and guidelines for
contributing.

First, in terms of structure, there is no particular concept of "Bitcoin Core
developers" in the sense of privileged people. Open source often naturally
revolves around a meritocracy where contributors earn trust from the developer
community over time. Nevertheless, some hierarchy is necessary for practical
purposes. As such, there are repository maintainers who are responsible for
merging pull requests, the [release cycle](/doc/release-process.md), and
moderation.

Getting Started
---------------

New contributors are very welcome and needed.

In-depth reviewing and testing are the bottleneck of the project, and are the
most effective way anyone can start to contribute. It will teach you much more
about the code and process than opening pull requests, and may help you uncover
related issues and follow-ups to contribute code for. Please refer to the [peer
review](#peer-review) section below.

Before you start contributing, familiarize yourself with the Bitcoin Core build
system and tests. Refer to the documentation in the repository on how to build
Bitcoin Core and how to run the unit tests, functional tests, and fuzz tests.

You may also participate in the [Bitcoin Core PR Review Club](https://bitcoincore.reviews/).

Communication Channels
----------------------

Most communication about Bitcoin Core development happens on IRC, in the
`#bitcoin-core-dev` channel on Libera Chat. The easiest way to participate on IRC is
with the web client, [web.libera.chat](https://web.libera.chat/#bitcoin-core-dev). Chat
history logs can be found
on [https://www.erisian.com.au/bitcoin-core-dev/](https://www.erisian.com.au/bitcoin-core-dev/)
and [https://gnusha.org/bitcoin-core-dev/](https://gnusha.org/bitcoin-core-dev/).

Discussion about codebase improvements happens in GitHub issues and pull
requests.

The developer
[mailing list](https://groups.google.com/g/bitcoindev)
should be used to discuss complicated or controversial consensus or P2P protocol changes before working on
a patch set.
Archives can be found on [https://gnusha.org/pi/bitcoindev/](https://gnusha.org/pi/bitcoindev/).


Contributor Workflow
--------------------

The codebase is maintained using the "contributor workflow" where everyone
without exception contributes patch proposals using "pull requests" (PRs). This
facilitates social contribution, easy testing and peer review.

Pull request authors must fully and confidently understand their own changes
and must have tested them. Contributors should mention which tests cover their
changes, or include the manual steps they used to confirm the change.
Contributors are expected to be prepared to clearly motivate and explain their
changes. If there is doubt, the pull request may be closed.
Please refer to the [peer review](#peer-review) section below for more details.

To contribute a patch, the workflow is as follows:

  1. Fork repository ([only for the first time](https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/fork-a-repo))
  1. Create topic branch
  1. Commit patches

For GUI-related issues or pull requests, the https://github.com/bitcoin-core/gui repository should be used.
For all other issues and pull requests, the https://github.com/bitcoin/bitcoin node repository should be used.

The master branch for all monotree repositories is identical.

As a rule of thumb, everything that only modifies `src/qt` is a GUI-only pull
request. However:

* For global refactoring or other transversal changes the node repository
  should be used.
* For GUI-related build system changes, the node repository should be used
  because the change needs review by the build systems reviewers.
* Changes in `src/interfaces` need to go to the node repository because they
  might affect other components like the wallet.

For large GUI changes that include build system and interface changes, it is
recommended to first open a pull request against the GUI repository. When there
is agreement to proceed with the changes, a pull request with the build system
and interfaces changes can be submitted to the node repository.

The project coding conventions in the [developer notes](doc/developer-notes.md)
must be followed.

### Committing Patches

In general, [commits should be atomic](https://en.wikipedia.org/wiki/Atomic_commit#Atomic_commit_convention)
and diffs should be easy to read. For this reason, do not mix any formatting
fixes or code moves with actual code changes.

Make sure each individual commit is hygienic: that it builds successfully on its
own without warnings, errors, regressions, or test failures.
This means tests must be updated in the same commit that changes the behavior.

Commit messages should be verbose by default consisting of a short subject line
(50 chars max), a blank line and detailed explanatory text as separate
paragraph(s), unless the title alone is self-explanatory (like "Correct typo
in init.cpp") in which case a single title line is sufficient. Commit messages should be
helpful to people reading your code in the future, so explain the reasoning for
your decisions. Further explanation [here](https://cbea.ms/git-commit/).

If a particular commit references another issue, please add the reference. For
example: `refs #1234` or `fixes #4321`. Using the `fixes` or `closes` keywords
will cause the corresponding issue to be closed when the pull request is merged.

Commit messages should never contain any `@` mentions (usernames prefixed with "@").

Please refer to the [Git manual](https://git-scm.com/doc) for more information
about Git.

  - Push changes to your fork
  - Create pull request

### Creating the Pull Request

The title of the pull request should be prefixed by the component or area that
the pull request affects. Valid areas are:

  - `consensus` for changes to consensus critical code
  - `doc` for changes to the documentation
  - `qt` or `gui` for changes to bitcoin-qt
  - `log` for changes to log messages
  - `mining` for changes to the mining code
  - `net` or `p2p` for changes to the peer-to-peer network code
  - `refactor` for structural changes that do not change behavior
  - `rpc`, `rest` or `zmq` for changes to the RPC, REST or ZMQ APIs
  - `contrib` or `cli` for changes to the scripts and tools
  - `test`, `qa` or `ci` for changes to the unit tests, QA tests or CI code
  - `util` or `lib` for changes to the utils or libraries
  - `wallet` for changes to the wallet code
  - `build` for changes to CMake
  - `guix` for changes to the GUIX reproducible builds

Examples:

    consensus: Add new opcode for BIP-XXXX OP_CHECKAWESOMESIG
    net: Automatically create onion service, listen on Tor
    qt: Add feed bump button
    log: Fix typo in log message

The body of the pull request should contain sufficient description of *what* the
patch does, and even more importantly, *why*, with justification and reasoning.
You should include references to any discussions (for example, other issues or
mailing list discussions).

The description for a new pull request should not contain any `@` mentions. The
PR description will be included in the commit message when the PR is merged and
any users mentioned in the description will be annoyingly notified each time a
fork of Bitcoin Core copies the merge. Instead, make any username mentions in a
subsequent comment to the PR.

### Translation changes

Note that translations should not be submitted as pull requests. Please see
[Translation Process](/doc/translation_process.md)
for more information on helping with translations.

### Work in Progress Changes and Requests for Comments

If a pull request is not to be considered for merging (yet), please
prefix the title with [WIP] or use [Tasks Lists](https://docs.github.com/en/get-started/writing-on-github/getting-started-with-writing-and-formatting-on-github/basic-writing-and-formatting-syntax#task-lists)
in the body of the pull request to indicate tasks are pending.

### Address Feedback

At this stage, one should expect comments and review from other contributors. You
can add more commits to your pull request by committing them locally and pushing
to your fork.

You are expected to reply to any review comments before your pull request is
merged. You may update the code or reject the feedback if you do not agree with
it, but you should express so in a reply. If there is outstanding feedback and
you are not actively working on it, your pull request may be closed.

Please refer to the [peer review](#peer-review) section below for more details.

### Squashing Commits

If your pull request contains fixup commits (commits that change the same line of code repeatedly) or too fine-grained
commits, you may be asked to [squash](https://git-scm.com/docs/git-rebase#_interactive_mode) your commits
before it will be reviewed. The basic squashing workflow is shown below.

    git checkout your_branch_name
    git rebase -i HEAD~n
    # n is normally the number of commits in the pull request.
    # Set commits (except the one in the first line) from 'pick' to 'squash', save and quit.
    # On the next screen, edit/refine commit messages.
    # Save and quit.
    git push -f # (force push to GitHub)

Please update the resulting commit message, if needed. It should read as a
coherent message. In most cases, this means not just listing the interim
commits.

If your change contains a merge commit, the above workflow may not work and you
will need to remove the merge commit first. See the next section for details on
how to rebase.

Please refrain from creating several pull requests for the same change.
Use the pull request that is already open (or was created earlier) to amend
changes. This preserves the discussion and review that happened earlier for
the respective change set.

The length of time required for peer review is unpredictable and will vary from
pull request to pull request.

### Rebasing Changes

When a pull request conflicts with the target branch, you may be asked to rebase it on top of the current target branch.

    git fetch https://github.com/bitcoin/bitcoin  # Fetch the latest upstream commit
    git rebase FETCH_HEAD  # Rebuild commits on top of the new base

This project aims to have a clean git history, where code changes are only made in non-merge commits. This simplifies
auditability because merge commits can be assumed to not contain arbitrary code changes. Merge commits should be signed,
and the resulting git tree hash must be deterministic and reproducible. The script in
[/contrib/verify-commits](/contrib/verify-commits) checks that.

After a rebase, reviewers are encouraged to sign off on the force push. This should be relatively straightforward with
the `git range-diff` tool explained in the [productivity
notes](/doc/productivity.md#diff-the-diffs-with-git-range-diff). To avoid needless review churn, maintainers will
generally merge pull requests that received the most review attention first.

Pull Request Philosophy
-----------------------

Patchsets should always be focused. For example, a pull request could add a
feature, fix a bug, or refactor code; but not a mixture. Please also avoid super
pull requests which attempt to do too much, are overly large, or overly complex
as this makes review difficult.


### Features

When adding a new feature, thought must be given to the long term technical debt
and maintenance that feature may require after inclusion. Before proposing a new
feature that will require maintenance, please consider if you are willing to
maintain it (including bug fixing). If features get orphaned with no maintainer
in the future, they may be removed by the Repository Maintainer.


### Refactoring

Refactoring is a necessary part of any software project's evolution. The
following guidelines cover refactoring pull requests for the project.

There are three categories of refactoring: code-only moves, code style fixes, and
code refactoring. In general, refactoring pull requests should not mix these
three kinds of activities in order to make refactoring pull requests easy to
review and uncontroversial. In all cases, refactoring PRs must not change the
behaviour of code within the pull request (bugs must be preserved as is).

Project maintainers aim for a quick turnaround on refactoring pull requests, so
where possible keep them short, uncomplex and easy to verify.

Pull requests that refactor the code should not be made by new contributors. It
requires a certain level of experience to know where the code belongs to and to
understand the full ramification (including rebase effort of open pull requests).

Trivial pull requests or pull requests that refactor the code with no clear
benefits may be immediately closed by the maintainers to reduce unnecessary
workload on reviewing.


"Decision Making" Process
-------------------------

The following applies to code changes to the Bitcoin Core project (and related
projects such as libsecp256k1), and is not to be confused with overall Bitcoin
Network Protocol consensus changes.

Whether a pull request is merged into Bitcoin Core rests with the project merge
maintainers.

Maintainers will take into consideration if a patch is in line with the general
principles of the project; meets the minimum standards for inclusion; and will
judge the general consensus of contributors.

In general, all pull requests must:

  - Have a clear use case, fix a demonstrable bug or serve the greater good of
    the project (for example refactoring for modularisation);
  - Be well peer-reviewed;
  - Have unit tests, functional tests, and fuzz tests, where appropriate;
  - Follow code style guidelines ([C++](doc/developer-notes.md), [functional tests](test/functional/README.md));
  - Not break the existing test suite;
  - Where bugs are fixed, where possible, there should be unit tests
    demonstrating the bug and also proving the fix. This helps prevent regression.
  - Change relevant comments and documentation when behaviour of code changes.

Patches that change Bitcoin consensus rules are considerably more involved than
normal because they affect the entire ecosystem and so must be preceded by
extensive mailing list discussions and have a numbered BIP. While each case will
be different, one should be prepared to expend more time and effort than for
other kinds of patches because of increased peer review and consensus building
requirements.


### Peer Review

Anyone may participate in peer review which is expressed by comments in the pull
request. Typically reviewers will review the code for obvious errors, as well as
test out the patch set and opine on the technical merits of the patch. Project
maintainers take into account the peer review when determining if there is
consensus to merge a pull request (remember that discussions may have been
spread out over GitHub, mailing list and IRC discussions).

Code review is a burdensome but important part of the development process, and
as such, certain types of pull requests are rejected. In general, if the
**improvements** do not warrant the **review effort** required, the PR has a
high chance of being rejected. It is up to the PR author to convince the
reviewers that the changes warrant the review effort, and if reviewers are
"Concept NACK'ing" the PR, the author may need to present arguments and/or do
research backing their suggested changes.

Moreover, if there is reasonable doubt that the pull request author does not
fully understand the changes they are submitting themselves, or if it becomes
clear that they have not tested the changes on a basic level themselves, the
pull request may be closed immediately.

#### Conceptual Review

A review can be a conceptual review, where the reviewer leaves a comment
 * `Concept (N)ACK`, meaning "I do (not) agree with the general goal of this pull
   request",
 * `Approach (N)ACK`, meaning `Concept ACK`, but "I do (not) agree with the
   approach of this change".

A `NACK` needs to include a rationale why the change is not worthwhile.
NACKs without accompanying reasoning may be disregarded.

#### Code Review

After conceptual agreement on the change, code review can be provided. A review
begins with `ACK BRANCH_COMMIT`, where `BRANCH_COMMIT` is the top of the PR
branch, followed by a description of how the reviewer did the review. The
following language is used within pull request comments:

  - "I have tested the code", involving change-specific manual testing in
    addition to running the unit, functional, or fuzz tests, and in case it is
    not obvious how the manual testing was done, it should be described;
  - "I have not tested the code, but I have reviewed it and it looks
    OK, I agree it can be merged";
  - A "nit" refers to a trivial, often non-blocking issue.

Project maintainers reserve the right to weigh the opinions of peer reviewers
using common sense judgement and may also weigh based on merit. Reviewers that
have demonstrated a deeper commitment and understanding of the project over time
or who have clear domain expertise may naturally have more weight, as one would
expect in all walks of life.

Where a patch set affects consensus-critical code, the bar will be much
higher in terms of discussion and peer review requirements, keeping in mind that
mistakes could be very costly to the wider community. This includes refactoring
of consensus-critical code.

Where a patch set proposes to change the Bitcoin consensus, it must have been
discussed extensively on the mailing list and IRC, be accompanied by a widely
discussed BIP and have a generally widely perceived technical consensus of being
a worthwhile change based on the judgement of the maintainers.

### Finding Reviewers

As most reviewers are themselves developers with their own projects, the review
process can be quite lengthy, and some amount of patience is required. If you find
that you've been waiting for a pull request to be given attention for several
months, there may be a number of reasons for this, some of which you can do something
about:

  - It may be because of a feature freeze due to an upcoming release. During this time,
    only bug fixes are taken into consideration. If your pull request is a new feature,
    it will not be prioritized until after the release. Wait for the release.
  - It may be because the changes you are suggesting do not appeal to people. Rather than
    nits and critique, which require effort and means they care enough to spend time on your
    contribution, thundering silence is a good sign of widespread (mild) dislike of a given change
    (because people don't assume *others* won't actually like the proposal). Don't take
    that personally, though! Instead, take another critical look at what you are suggesting
    and see if it: changes too much, is too broad, doesn't adhere to the
    [developer notes](doc/developer-notes.md), is dangerous or insecure, is messily written, etc.
    Identify and address any of the issues you find. Then ask e.g. on IRC if someone could give
    their opinion on the concept itself.
  - It may be because your code is too complex for all but a few people, and those people
    may not have realized your pull request even exists. A great way to find people who
    are qualified and care about the code you are touching is the
    [Git Blame feature](https://docs.github.com/en/repositories/working-with-files/using-files/viewing-and-understanding-files). Simply
    look up who last modified the code you are changing and see if you can find
    them and give them a nudge. Don't be incessant about the nudging, though.
  - Finally, if all else fails, ask on IRC or elsewhere for someone to give your pull request
    a look. If you think you've been waiting for an unreasonably long time (say,
    more than a month) for no particular reason (a few lines changed, etc.),
    this is totally fine. Try to return the favor when someone else is asking
    for feedback on their code, and the universe balances out.
  - Remember that the best thing you can do while waiting is give review to others!


Backporting
-----------

Security and bug fixes can be backported from `master` to release
branches.
Maintainers will do backports in batches and
use the proper `Needs backport (...)` labels
when needed (the original author does not need to worry about it).

A backport should contain the following metadata in the commit body:

```
Github-Pull: #<PR number>
Rebased-From: <commit hash of the original commit>
```

Have a look at [an example backport PR](
https://github.com/bitcoin/bitcoin/pull/16189).

Also see the [backport.py script](
https://github.com/bitcoin-core/bitcoin-maintainer-tools#backport).

Copyright
---------

By contributing to this repository, you agree to license your work under the
MIT license unless specified otherwise in `contrib/debian/copyright` or at
the top of the file itself. Any work contributed where you are not the original
author must contain its license header with the original author(s) and source.
