bitcoin/CONTRIBUTING.md

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.

Firstly in terms of structure, there is no particular concept of "Core
developers" in the sense of privileged people. Open source often naturally
revolves around meritocracy where longer term contributors gain more trust from
the developer community. However, some hierarchy is necessary for practical
purposes. As such there are repository "maintainers" who are responsible for
merging pull requests as well as a "lead maintainer" who is responsible for the
release cycle, overall merging, moderation and appointment of maintainers.

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

New contributors are very welcome and needed.

Reviewing and testing is the most effective way you can contribute as a new
contributor, and it also will teach you much more about the code and process
than opening PRs. Please refer to the section [peer review](#peer-review) later
in this document.

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 and functional tests.

There are many open issues of varying difficulty waiting to be fixed.
If you're looking for somewhere to start contributing, check out the
[good first issue](https://github.com/bitcoin/bitcoin/issues?q=is%3Aopen+is%3Aissue+label%3A%22good+first+issue%22)
list or changes that are
[up for grabs](https://github.com/bitcoin/bitcoin/issues?utf8=%E2%9C%93&q=label%3A%22Up+for+grabs%22).
Some of them might no longer be applicable. So if you are interested, but
unsure, you might want to leave a comment on the issue first.

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

### Good First Issue Label

The purpose of the `good first issue` label is to highlight which issues are
suitable for a new contributor without a deep understanding of the codebase.

However, good first issues can be solved by anyone. If they remain unsolved
for a longer time, a frequent contributor might address them.

You do not need to request permission to start working on an issue. However,
you are encouraged to leave a comment if you are planning to work on it. This
will help other contributors monitor which issues are actively being addressed
and is also an effective way to request assistance if and when you need it.

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

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

Discussion about code base improvements happens in GitHub issues and on pull
requests.

The developer
[mailing list](https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev)
should be used to discuss complicated or controversial consensus or P2P protocol changes before working on
a patch set.


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

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

To contribute a patch, the workflow is as follows:

  1. Fork repository ([only for the first time](https://help.github.com/en/articles/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.

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 "Corrected 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://chris.beams.io/posts/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.

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 as:

  - `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
  - `script` 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 the GNU Autotools or reproducible builds

Examples:

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

The body of the pull request should contain enough description about what the
patch does together with any justification/reasoning. You should include
references to any discussions (for example other tickets or mailing list
discussions).

### Translation changes

Note that translations should not be submitted as pull requests. Please see
[Translation Process](https://github.com/bitcoin/bitcoin/blob/master/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://help.github.com/articles/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 until you have satisfied all feedback.

Note: 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.

### 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 merged. 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 that you should not just list the
interim commits.

If you have problems with squashing (or other workflows with `git`), you can
alternatively enable "Allow edits from maintainers" in the right GitHub
sidebar and ask for help in the pull request.

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.
The `git rebase` command will take care of rebuilding your 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 and ultimately the project lead.

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 and functional 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).

#### Conceptual Review

A review can be a conceptual review, where the reviewer leaves a comment
 * `Concept (N)ACK`, meaning "I do (not) agree in 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. It is
starting with `ACK BRANCH_COMMIT`, where `BRANCH_COMMIT` is the top of the
topic branch. The review is 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 and functional
    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";
  - Nit refers to trivial, often non-blocking issues.

Project maintainers reserve the right to weigh the opinions of peer reviewers
using common sense judgement and also may weight based on meritocracy: Those
that have demonstrated a deeper commitment and understanding towards the project
(over time) or 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 set 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 the release is over. Wait for 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://help.github.com/articles/tracing-changes-in-a-file/). Simply
    find the person touching the code you are touching before you 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 an unreasonably long amount of time (month+) for
    no particular reason (few lines changed, etc), this is totally fine. Try to return the favor
    when someone else is asking for feedback on their code, and universe balances out.


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

Security and bug fixes can be backported from `master` to release
branches.
If the backport is non-trivial, it may be appropriate to open an
additional PR, to backport the change, only after the original PR
has been merged.
Otherwise, backports will be done in batches and
the maintainers will use the proper `Needs backport (...)` labels
when needed (the original author does not need to worry).

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).

Release Policy
--------------

The project leader is the release manager for each Bitcoin Core release.

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.