My First Contribution to the Git Project
========================================
:sectanchors:

[[summary]]
== Summary

This is a tutorial demonstrating the end-to-end workflow of creating a change to
the Git tree, sending it for review, and making changes based on comments.

[[prerequisites]]
=== Prerequisites

This tutorial assumes you're already fairly familiar with using Git to manage
source code.  The Git workflow steps will largely remain unexplained.

[[related-reading]]
=== Related Reading

This tutorial aims to summarize the following documents, but the reader may find
useful additional context:

- `Documentation/SubmittingPatches`
- `Documentation/howto/new-command.txt`

[[getting-started]]
== Getting Started

[[cloning]]
=== Clone the Git Repository

Git is mirrored in a number of locations. Clone the repository from one of them;
https://git-scm.com/downloads suggests one of the best places to clone from is
the mirror on GitHub.

----
$ git clone https://github.com/git/git git
$ cd git
----

[[dependencies]]
=== Installing Dependencies

To build Git from source, you need to have a handful of dependencies installed
on your system. For a hint of what's needed, you can take a look at
`INSTALL`, paying close attention to the section about Git's dependencies on
external programs and libraries. That document mentions a way to "test-drive"
our freshly built Git without installing; that's the method we'll be using in
this tutorial.

Make sure that your environment has everything you need by building your brand
new clone of Git from the above step:

----
$ make
----

NOTE: The Git build is parallelizable. `-j#` is not included above but you can
use it as you prefer, here and elsewhere.

[[identify-problem]]
=== Identify Problem to Solve

////
Use + to indicate fixed-width here; couldn't get ` to work nicely with the
quotes around "Pony Saying 'Um, Hello'".
////
In this tutorial, we will add a new command, +git psuh+, short for ``Pony Saying
`Um, Hello''' - a feature which has gone unimplemented despite a high frequency
of invocation during users' typical daily workflow.

(We've seen some other effort in this space with the implementation of popular
commands such as `sl`.)

[[setup-workspace]]
=== Set Up Your Workspace

Let's start by making a development branch to work on our changes. Per
`Documentation/SubmittingPatches`, since a brand new command is a new feature,
it's fine to base your work on `master`. However, in the future for bugfixes,
etc., you should check that document and base it on the appropriate branch.

For the purposes of this document, we will base all our work on the `master`
branch of the upstream project. Create the `psuh` branch you will use for
development like so:

----
$ git checkout -b psuh origin/master
----

We'll make a number of commits here in order to demonstrate how to send a topic
with multiple patches up for review simultaneously.

[[code-it-up]]
== Code It Up!

NOTE: A reference implementation can be found at
https://github.com/nasamuffin/git/tree/psuh.

[[add-new-command]]
=== Adding a New Command

Lots of the subcommands are written as builtins, which means they are
implemented in C and compiled into the main `git` executable. Implementing the
very simple `psuh` command as a built-in will demonstrate the structure of the
codebase, the internal API, and the process of working together as a contributor
with the reviewers and maintainer to integrate this change into the system.

Built-in subcommands are typically implemented in a function named "cmd_"
followed by the name of the subcommand, in a source file named after the
subcommand and contained within `builtin/`. So it makes sense to implement your
command in `builtin/psuh.c`. Create that file, and within it, write the entry
point for your command in a function matching the style and signature:

----
int cmd_psuh(int argc, const char **argv, const char *prefix)
----

We'll also need to add the declaration of psuh; open up `builtin.h`, find the
declaration for `cmd_pull`, and add a new line for `psuh` immediately before it,
in order to keep the declarations alphabetically sorted:

----
int cmd_psuh(int argc, const char **argv, const char *prefix);
----

Be sure to `#include "builtin.h"` in your `psuh.c`.

Go ahead and add some throwaway printf to that function. This is a decent
starting point as we can now add build rules and register the command.

NOTE: Your throwaway text, as well as much of the text you will be adding over
the course of this tutorial, is user-facing. That means it needs to be
localizable. Take a look at `po/README` under "Marking strings for translation".
Throughout the tutorial, we will mark strings for translation as necessary; you
should also do so when writing your user-facing commands in the future.

----
int cmd_psuh(int argc, const char **argv, const char *prefix)
{
	printf(_("Pony saying hello goes here.\n"));
	return 0;
}
----

Let's try to build it.  Open `Makefile`, find where `builtin/pull.o` is added
to `BUILTIN_OBJS`, and add `builtin/psuh.o` in the same way next to it in
alphabetical order. Once you've done so, move to the top-level directory and
build simply with `make`. Also add the `DEVELOPER=1` variable to turn on
some additional warnings:

----
$ echo DEVELOPER=1 >config.mak
$ make
----

NOTE: When you are developing the Git project, it's preferred that you use the
`DEVELOPER` flag; if there's some reason it doesn't work for you, you can turn
it off, but it's a good idea to mention the problem to the mailing list.

Great, now your new command builds happily on its own. But nobody invokes it.
Let's change that.

The list of commands lives in `git.c`. We can register a new command by adding
a `cmd_struct` to the `commands[]` array. `struct cmd_struct` takes a string
with the command name, a function pointer to the command implementation, and a
setup option flag. For now, let's keep mimicking `push`. Find the line where
`cmd_push` is registered, copy it, and modify it for `cmd_psuh`, placing the new
line in alphabetical order (immediately before `cmd_pull`).

The options are documented in `builtin.h` under "Adding a new built-in." Since
we hope to print some data about the user's current workspace context later,
we need a Git directory, so choose `RUN_SETUP` as your only option.

Go ahead and build again. You should see a clean build, so let's kick the tires
and see if it works. There's a binary you can use to test with in the
`bin-wrappers` directory.

----
$ ./bin-wrappers/git psuh
----

Check it out! You've got a command! Nice work! Let's commit this.

`git status` reveals modified `Makefile`, `builtin.h`, and `git.c` as well as
untracked `builtin/psuh.c` and `git-psuh`. First, let's take care of the binary,
which should be ignored. Open `.gitignore` in your editor, find `/git-pull`, and
add an entry for your new command in alphabetical order:

----
...
/git-prune-packed
/git-psuh
/git-pull
/git-push
/git-quiltimport
/git-range-diff
...
----

Checking `git status` again should show that `git-psuh` has been removed from
the untracked list and `.gitignore` has been added to the modified list. Now we
can stage and commit:

----
$ git add Makefile builtin.h builtin/psuh.c git.c .gitignore
$ git commit -s
----

You will be presented with your editor in order to write a commit message. Start
the commit with a 50-column or less subject line, including the name of the
component you're working on, followed by a blank line (always required) and then
the body of your commit message, which should provide the bulk of the context.
Remember to be explicit and provide the "Why" of your change, especially if it
couldn't easily be understood from your diff. When editing your commit message,
don't remove the Signed-off-by line which was added by `-s` above.

----
psuh: add a built-in by popular demand

Internal metrics indicate this is a command many users expect to be
present. So here's an implementation to help drive customer
satisfaction and engagement: a pony which doubtfully greets the user,
or, a Pony Saying "Um, Hello" (PSUH).

This commit message is intentionally formatted to 72 columns per line,
starts with a single line as "commit message subject" that is written as
if to command the codebase to do something (add this, teach a command
that). The body of the message is designed to add information about the
commit that is not readily deduced from reading the associated diff,
such as answering the question "why?".

Signed-off-by: A U Thor <author@example.com>
----

Go ahead and inspect your new commit with `git show`. "psuh:" indicates you
have modified mainly the `psuh` command. The subject line gives readers an idea
of what you've changed. The sign-off line (`-s`) indicates that you agree to
the Developer's Certificate of Origin 1.1 (see the
`Documentation/SubmittingPatches` +++[[dco]]+++ header).

For the remainder of the tutorial, the subject line only will be listed for the
sake of brevity. However, fully-fleshed example commit messages are available
on the reference implementation linked at the top of this document.

[[implementation]]
=== Implementation

It's probably useful to do at least something besides printing out a string.
Let's start by having a look at everything we get.

Modify your `cmd_psuh` implementation to dump the args you're passed, keeping
existing `printf()` calls in place:

----
	int i;

	...

	printf(Q_("Your args (there is %d):\n",
		  "Your args (there are %d):\n",
		  argc),
	       argc);
	for (i = 0; i < argc; i++)
		printf("%d: %s\n", i, argv[i]);

	printf(_("Your current working directory:\n<top-level>%s%s\n"),
	       prefix ? "/" : "", prefix ? prefix : "");

----

Build and try it. As you may expect, there's pretty much just whatever we give
on the command line, including the name of our command. (If `prefix` is empty
for you, try `cd Documentation/ && ../bin-wrappers/git psuh`). That's not so
helpful. So what other context can we get?

Add a line to `#include "config.h"`. Then, add the following bits to the
function body:

----
	const char *cfg_name;

...

	git_config(git_default_config, NULL);
	if (git_config_get_string_const("user.name", &cfg_name) > 0)
		printf(_("No name is found in config\n"));
	else
		printf(_("Your name: %s\n"), cfg_name);
----

`git_config()` will grab the configuration from config files known to Git and
apply standard precedence rules. `git_config_get_string_const()` will look up
a specific key ("user.name") and give you the value. There are a number of
single-key lookup functions like this one; you can see them all (and more info
about how to use `git_config()`) in `Documentation/technical/api-config.txt`.

You should see that the name printed matches the one you see when you run:

----
$ git config --get user.name
----

Great! Now we know how to check for values in the Git config. Let's commit this
too, so we don't lose our progress.

----
$ git add builtin/psuh.c
$ git commit -sm "psuh: show parameters & config opts"
----

NOTE: Again, the above is for sake of brevity in this tutorial. In a real change
you should not use `-m` but instead use the editor to write a meaningful
message.

Still, it'd be nice to know what the user's working context is like. Let's see
if we can print the name of the user's current branch. We can mimic the
`git status` implementation; the printer is located in `wt-status.c` and we can
see that the branch is held in a `struct wt_status`.

`wt_status_print()` gets invoked by `cmd_status()` in `builtin/commit.c`.
Looking at that implementation we see the status config being populated like so:

----
status_init_config(&s, git_status_config);
----

But as we drill down, we can find that `status_init_config()` wraps a call
to `git_config()`. Let's modify the code we wrote in the previous commit.

Be sure to include the header to allow you to use `struct wt_status`:
----
#include "wt-status.h"
----

Then modify your `cmd_psuh` implementation to declare your `struct wt_status`,
prepare it, and print its contents:

----
	struct wt_status status;

...

	wt_status_prepare(the_repository, &status);
	git_config(git_default_config, &status);

...

	printf(_("Your current branch: %s\n"), status.branch);
----

Run it again. Check it out - here's the (verbose) name of your current branch!

Let's commit this as well.

----
$ git add builtin/psuh.c
$ git commit -sm "psuh: print the current branch"
----

Now let's see if we can get some info about a specific commit.

Luckily, there are some helpers for us here. `commit.h` has a function called
`lookup_commit_reference_by_name` to which we can simply provide a hardcoded
string; `pretty.h` has an extremely handy `pp_commit_easy()` call which doesn't
require a full format object to be passed.

Add the following includes:

----
#include "commit.h"
#include "pretty.h"
----

Then, add the following lines within your implementation of `cmd_psuh()` near
the declarations and the logic, respectively.

----
	struct commit *c = NULL;
	struct strbuf commitline = STRBUF_INIT;

...

	c = lookup_commit_reference_by_name("origin/master");

	if (c != NULL) {
		pp_commit_easy(CMIT_FMT_ONELINE, c, &commitline);
		printf(_("Current commit: %s\n"), commitline.buf);
	}
----

The `struct strbuf` provides some safety belts to your basic `char*`, one of
which is a length member to prevent buffer overruns. It needs to be initialized
nicely with `STRBUF_INIT`. Keep it in mind when you need to pass around `char*`.

`lookup_commit_reference_by_name` resolves the name you pass it, so you can play
with the value there and see what kind of things you can come up with.

`pp_commit_easy` is a convenience wrapper in `pretty.h` that takes a single
format enum shorthand, rather than an entire format struct. It then
pretty-prints the commit according to that shorthand. These are similar to the
formats available with `--pretty=FOO` in many Git commands.

Build it and run, and if you're using the same name in the example, you should
see the subject line of the most recent commit in `origin/master` that you know
about. Neat! Let's commit that as well.

----
$ git add builtin/psuh.c
$ git commit -sm "psuh: display the top of origin/master"
----

[[add-documentation]]
=== Adding Documentation

Awesome! You've got a fantastic new command that you're ready to share with the
community. But hang on just a minute - this isn't very user-friendly. Run the
following:

----
$ ./bin-wrappers/git help psuh
----

Your new command is undocumented! Let's fix that.

Take a look at `Documentation/git-*.txt`. These are the manpages for the
subcommands that Git knows about. You can open these up and take a look to get
acquainted with the format, but then go ahead and make a new file
`Documentation/git-psuh.txt`. Like with most of the documentation in the Git
project, help pages are written with AsciiDoc (see CodingGuidelines, "Writing
Documentation" section). Use the following template to fill out your own
manpage:

// Surprisingly difficult to embed AsciiDoc source within AsciiDoc.
[listing]
....
git-psuh(1)
===========

NAME
----
git-psuh - Delight users' typo with a shy horse


SYNOPSIS
--------
[verse]
'git-psuh [<arg>...]'

DESCRIPTION
-----------
...

OPTIONS[[OPTIONS]]
------------------
...

OUTPUT
------
...

GIT
---
Part of the linkgit:git[1] suite
....

The most important pieces of this to note are the file header, underlined by =,
the NAME section, and the SYNOPSIS, which would normally contain the grammar if
your command took arguments. Try to use well-established manpage headers so your
documentation is consistent with other Git and UNIX manpages; this makes life
easier for your user, who can skip to the section they know contains the
information they need.

Now that you've written your manpage, you'll need to build it explicitly. We
convert your AsciiDoc to troff which is man-readable like so:

----
$ make all doc
$ man Documentation/git-psuh.1
----

or

----
$ make -C Documentation/ git-psuh.1
$ man Documentation/git-psuh.1
----

NOTE: You may need to install the package `asciidoc` to get this to work.

While this isn't as satisfying as running through `git help`, you can at least
check that your help page looks right.

You can also check that the documentation coverage is good (that is, the project
sees that your command has been implemented as well as documented) by running
`make check-docs` from the top-level.

Go ahead and commit your new documentation change.

[[add-usage]]
=== Adding Usage Text

Try and run `./bin-wrappers/git psuh -h`. Your command should crash at the end.
That's because `-h` is a special case which your command should handle by
printing usage.

Take a look at `Documentation/technical/api-parse-options.txt`. This is a handy
tool for pulling out options you need to be able to handle, and it takes a
usage string.

In order to use it, we'll need to prepare a NULL-terminated array of usage
strings and a `builtin_psuh_options` array.

Add a line to `#include "parse-options.h"`.

At global scope, add your array of usage strings:

----
static const char * const psuh_usage[] = {
	N_("git psuh [<arg>...]"),
	NULL,
};
----

Then, within your `cmd_psuh()` implementation, we can declare and populate our
`option` struct. Ours is pretty boring but you can add more to it if you want to
explore `parse_options()` in more detail:

----
	struct option options[] = {
		OPT_END()
	};
----

Finally, before you print your args and prefix, add the call to
`parse-options()`:

----
	argc = parse_options(argc, argv, prefix, options, psuh_usage, 0);
----

This call will modify your `argv` parameter. It will strip the options you
specified in `options` from `argv` and the locations pointed to from `options`
entries will be updated. Be sure to replace your `argc` with the result from
`parse_options()`, or you will be confused if you try to parse `argv` later.

It's worth noting the special argument `--`. As you may be aware, many Unix
commands use `--` to indicate "end of named parameters" - all parameters after
the `--` are interpreted merely as positional arguments. (This can be handy if
you want to pass as a parameter something which would usually be interpreted as
a flag.) `parse_options()` will terminate parsing when it reaches `--` and give
you the rest of the options afterwards, untouched.

Now that you have a usage hint, you can teach Git how to show it in the general
command list shown by `git help git` or `git help -a`, which is generated from
`command-list.txt`. Find the line for 'git-pull' so you can add your 'git-psuh'
line above it in alphabetical order. Now, we can add some attributes about the
command which impacts where it shows up in the aforementioned help commands. The
top of `command-list.txt` shares some information about what each attribute
means; in those help pages, the commands are sorted according to these
attributes. `git psuh` is user-facing, or porcelain - so we will mark it as
"mainporcelain". For "mainporcelain" commands, the comments at the top of
`command-list.txt` indicate we can also optionally add an attribute from another
list; since `git psuh` shows some information about the user's workspace but
doesn't modify anything, let's mark it as "info". Make sure to keep your
attributes in the same style as the rest of `command-list.txt` using spaces to
align and delineate them:

----
git-prune-packed                        plumbingmanipulators
git-psuh                                mainporcelain		info
git-pull                                mainporcelain           remote
git-push                                mainporcelain           remote
----

Build again. Now, when you run with `-h`, you should see your usage printed and
your command terminated before anything else interesting happens. Great!

Go ahead and commit this one, too.

[[testing]]
== Testing

It's important to test your code - even for a little toy command like this one.
Moreover, your patch won't be accepted into the Git tree without tests. Your
tests should:

* Illustrate the current behavior of the feature
* Prove the current behavior matches the expected behavior
* Ensure the externally-visible behavior isn't broken in later changes

So let's write some tests.

Related reading: `t/README`

[[overview-test-structure]]
=== Overview of Testing Structure

The tests in Git live in `t/` and are named with a 4-digit decimal number using
the schema shown in the Naming Tests section of `t/README`.

[[write-new-test]]
=== Writing Your Test

Since this a toy command, let's go ahead and name the test with t9999. However,
as many of the family/subcmd combinations are full, best practice seems to be
to find a command close enough to the one you've added and share its naming
space.

Create a new file `t/t9999-psuh-tutorial.sh`. Begin with the header as so (see
"Writing Tests" and "Source 'test-lib.sh'" in `t/README`):

----
#!/bin/sh

test_description='git-psuh test

This test runs git-psuh and makes sure it does not crash.'

. ./test-lib.sh
----

Tests are framed inside of a `test_expect_success` in order to output TAP
formatted results. Let's make sure that `git psuh` doesn't exit poorly and does
mention the right animal somewhere:

----
test_expect_success 'runs correctly with no args and good output' '
	git psuh >actual &&
	test_i18ngrep Pony actual
'
----

Indicate that you've run everything you wanted by adding the following at the
bottom of your script:

----
test_done
----

Make sure you mark your test script executable:

----
$ chmod +x t/t9999-psuh-tutorial.sh
----

You can get an idea of whether you created your new test script successfully
by running `make -C t test-lint`, which will check for things like test number
uniqueness, executable bit, and so on.

[[local-test]]
=== Running Locally

Let's try and run locally:

----
$ make
$ cd t/ && prove t9999-psuh-tutorial.sh
----

You can run the full test suite and ensure `git-psuh` didn't break anything:

----
$ cd t/
$ prove -j$(nproc) --shuffle t[0-9]*.sh
----

NOTE: You can also do this with `make test` or use any testing harness which can
speak TAP. `prove` can run concurrently. `shuffle` randomizes the order the
tests are run in, which makes them resilient against unwanted inter-test
dependencies. `prove` also makes the output nicer.

Go ahead and commit this change, as well.

[[ready-to-share]]
== Getting Ready to Share

You may have noticed already that the Git project performs its code reviews via
emailed patches, which are then applied by the maintainer when they are ready
and approved by the community. The Git project does not accept patches from
pull requests, and the patches emailed for review need to be formatted a
specific way. At this point the tutorial diverges, in order to demonstrate two
different methods of formatting your patchset and getting it reviewed.

The first method to be covered is GitGitGadget, which is useful for those
already familiar with GitHub's common pull request workflow. This method
requires a GitHub account.

The second method to be covered is `git send-email`, which can give slightly
more fine-grained control over the emails to be sent. This method requires some
setup which can change depending on your system and will not be covered in this
tutorial.

Regardless of which method you choose, your engagement with reviewers will be
the same; the review process will be covered after the sections on GitGitGadget
and `git send-email`.

[[howto-ggg]]
== Sending Patches via GitGitGadget

One option for sending patches is to follow a typical pull request workflow and
send your patches out via GitGitGadget. GitGitGadget is a tool created by
Johannes Schindelin to make life as a Git contributor easier for those used to
the GitHub PR workflow. It allows contributors to open pull requests against its
mirror of the Git project, and does some magic to turn the PR into a set of
emails and send them out for you. It also runs the Git continuous integration
suite for you. It's documented at http://gitgitgadget.github.io.

[[create-fork]]
=== Forking `git/git` on GitHub

Before you can send your patch off to be reviewed using GitGitGadget, you will
need to fork the Git project and upload your changes. First thing - make sure
you have a GitHub account.

Head to the https://github.com/git/git[GitHub mirror] and look for the Fork
button. Place your fork wherever you deem appropriate and create it.

[[upload-to-fork]]
=== Uploading to Your Own Fork

To upload your branch to your own fork, you'll need to add the new fork as a
remote. You can use `git remote -v` to show the remotes you have added already.
From your new fork's page on GitHub, you can press "Clone or download" to get
the URL; then you need to run the following to add, replacing your own URL and
remote name for the examples provided:

----
$ git remote add remotename git@github.com:remotename/git.git
----

or to use the HTTPS URL:

----
$ git remote add remotename https://github.com/remotename/git/.git
----

Run `git remote -v` again and you should see the new remote showing up.
`git fetch remotename` (with the real name of your remote replaced) in order to
get ready to push.

Next, double-check that you've been doing all your development in a new branch
by running `git branch`. If you didn't, now is a good time to move your new
commits to their own branch.

As mentioned briefly at the beginning of this document, we are basing our work
on `master`, so go ahead and update as shown below, or using your preferred
workflow.

----
$ git checkout master
$ git pull -r
$ git rebase master psuh
----

Finally, you're ready to push your new topic branch! (Due to our branch and
command name choices, be careful when you type the command below.)

----
$ git push remotename psuh
----

Now you should be able to go and check out your newly created branch on GitHub.

[[send-pr-ggg]]
=== Sending a PR to GitGitGadget

In order to have your code tested and formatted for review, you need to start by
opening a Pull Request against `gitgitgadget/git`. Head to
https://github.com/gitgitgadget/git and open a PR either with the "New pull
request" button or the convenient "Compare & pull request" button that may
appear with the name of your newly pushed branch.

Review the PR's title and description, as it's used by GitGitGadget as the cover
letter for your change. When you're happy, submit your pull request.

[[run-ci-ggg]]
=== Running CI and Getting Ready to Send

If it's your first time using GitGitGadget (which is likely, as you're using
this tutorial) then someone will need to give you permission to use the tool.
As mentioned in the GitGitGadget documentation, you just need someone who
already uses it to comment on your PR with `/allow <username>`. GitGitGadget
will automatically run your PRs through the CI even without the permission given
but you will not be able to `/submit` your changes until someone allows you to
use the tool.

NOTE: You can typically find someone who can `/allow` you on GitGitGadget by
either examining recent pull requests where someone has been granted `/allow`
(https://github.com/gitgitgadget/git/pulls?utf8=%E2%9C%93&q=is%3Apr+is%3Aopen+%22%2Fallow%22[Search:
is:pr is:open "/allow"]), in which case both the author and the person who
granted the `/allow` can now `/allow` you, or by inquiring on the
https://webchat.freenode.net/#git-devel[#git-devel] IRC channel on Freenode
linking your pull request and asking for someone to `/allow` you.

If the CI fails, you can update your changes with `git rebase -i` and push your
branch again:

----
$ git push -f remotename psuh
----

In fact, you should continue to make changes this way up until the point when
your patch is accepted into `next`.

////
TODO https://github.com/gitgitgadget/gitgitgadget/issues/83
It'd be nice to be able to verify that the patch looks good before sending it
to everyone on Git mailing list.
[[check-work-ggg]]
=== Check Your Work
////

[[send-mail-ggg]]
=== Sending Your Patches

Now that your CI is passing and someone has granted you permission to use
GitGitGadget with the `/allow` command, sending out for review is as simple as
commenting on your PR with `/submit`.

[[responding-ggg]]
=== Updating With Comments

Skip ahead to <<reviewing,Responding to Reviews>> for information on how to
reply to review comments you will receive on the mailing list.

Once you have your branch again in the shape you want following all review
comments, you can submit again:

----
$ git push -f remotename psuh
----

Next, go look at your pull request against GitGitGadget; you should see the CI
has been kicked off again. Now while the CI is running is a good time for you
to modify your description at the top of the pull request thread; it will be
used again as the cover letter. You should use this space to describe what
has changed since your previous version, so that your reviewers have some idea
of what they're looking at. When the CI is done running, you can comment once
more with `/submit` - GitGitGadget will automatically add a v2 mark to your
changes.

[[howto-git-send-email]]
== Sending Patches with `git send-email`

If you don't want to use GitGitGadget, you can also use Git itself to mail your
patches. Some benefits of using Git this way include finer grained control of
subject line (for example, being able to use the tag [RFC PATCH] in the subject)
and being able to send a ``dry run'' mail to yourself to ensure it all looks
good before going out to the list.

[[setup-git-send-email]]
=== Prerequisite: Setting Up `git send-email`

Configuration for `send-email` can vary based on your operating system and email
provider, and so will not be covered in this tutorial, beyond stating that in
many distributions of Linux, `git-send-email` is not packaged alongside the
typical `git` install. You may need to install this additional package; there
are a number of resources online to help you do so. You will also need to
determine the right way to configure it to use your SMTP server; again, as this
configuration can change significantly based on your system and email setup, it
is out of scope for the context of this tutorial.

[[format-patch]]
=== Preparing Initial Patchset

Sending emails with Git is a two-part process; before you can prepare the emails
themselves, you'll need to prepare the patches. Luckily, this is pretty simple:

----
$ git format-patch --cover-letter -o psuh/ master..psuh
----

The `--cover-letter` parameter tells `format-patch` to create a cover letter
template for you. You will need to fill in the template before you're ready
to send - but for now, the template will be next to your other patches.

The `-o psuh/` parameter tells `format-patch` to place the patch files into a
directory. This is useful because `git send-email` can take a directory and
send out all the patches from there.

`master..psuh` tells `format-patch` to generate patches for the difference
between `master` and `psuh`. It will make one patch file per commit. After you
run, you can go have a look at each of the patches with your favorite text
editor and make sure everything looks alright; however, it's not recommended to
make code fixups via the patch file. It's a better idea to make the change the
normal way using `git rebase -i` or by adding a new commit than by modifying a
patch.

NOTE: Optionally, you can also use the `--rfc` flag to prefix your patch subject
with ``[RFC PATCH]'' instead of ``[PATCH]''. RFC stands for ``request for
comments'' and indicates that while your code isn't quite ready for submission,
you'd like to begin the code review process. This can also be used when your
patch is a proposal, but you aren't sure whether the community wants to solve
the problem with that approach or not - to conduct a sort of design review. You
may also see on the list patches marked ``WIP'' - this means they are incomplete
but want reviewers to look at what they have so far. You can add this flag with
`--subject-prefix=WIP`.

Check and make sure that your patches and cover letter template exist in the
directory you specified - you're nearly ready to send out your review!

[[cover-letter]]
=== Preparing Email

In addition to an email per patch, the Git community also expects your patches
to come with a cover letter, typically with a subject line [PATCH 0/x] (where
x is the number of patches you're sending). Since you invoked `format-patch`
with `--cover-letter`, you've already got a template ready. Open it up in your
favorite editor.

You should see a number of headers present already. Check that your `From:`
header is correct. Then modify your `Subject:` to something which succinctly
covers the purpose of your entire topic branch, for example:

----
Subject: [PATCH 0/7] adding the 'psuh' command
----

Make sure you retain the ``[PATCH 0/X]'' part; that's what indicates to the Git
community that this email is the beginning of a review, and many reviewers
filter their email for this type of flag.

You'll need to add some extra parameters when you invoke `git send-email` to add
the cover letter.

Next you'll have to fill out the body of your cover letter. This is an important
component of change submission as it explains to the community from a high level
what you're trying to do, and why, in a way that's more apparent than just
looking at your diff. Be sure to explain anything your diff doesn't make clear
on its own.

Here's an example body for `psuh`:

----
Our internal metrics indicate widespread interest in the command
git-psuh - that is, many users are trying to use it, but finding it is
unavailable, using some unknown workaround instead.

The following handful of patches add the psuh command and implement some
handy features on top of it.

This patchset is part of the MyFirstContribution tutorial and should not
be merged.
----

The template created by `git format-patch --cover-letter` includes a diffstat.
This gives reviewers a summary of what they're in for when reviewing your topic.
The one generated for `psuh` from the sample implementation looks like this:

----
 Documentation/git-psuh.txt | 40 +++++++++++++++++++++
 Makefile                   |  1 +
 builtin.h                  |  1 +
 builtin/psuh.c             | 73 ++++++++++++++++++++++++++++++++++++++
 git.c                      |  1 +
 t/t9999-psuh-tutorial.sh   | 12 +++++++
 6 files changed, 128 insertions(+)
 create mode 100644 Documentation/git-psuh.txt
 create mode 100644 builtin/psuh.c
 create mode 100755 t/t9999-psuh-tutorial.sh
----

Finally, the letter will include the version of Git used to generate the
patches. You can leave that string alone.

[[sending-git-send-email]]
=== Sending Email

At this point you should have a directory `psuh/` which is filled with your
patches and a cover letter. Time to mail it out! You can send it like this:

----
$ git send-email --to=target@example.com psuh/*.patch
----

NOTE: Check `git help send-email` for some other options which you may find
valuable, such as changing the Reply-to address or adding more CC and BCC lines.

NOTE: When you are sending a real patch, it will go to git@vger.kernel.org - but
please don't send your patchset from the tutorial to the real mailing list! For
now, you can send it to yourself, to make sure you understand how it will look.

After you run the command above, you will be presented with an interactive
prompt for each patch that's about to go out. This gives you one last chance to
edit or quit sending something (but again, don't edit code this way). Once you
press `y` or `a` at these prompts your emails will be sent! Congratulations!

Awesome, now the community will drop everything and review your changes. (Just
kidding - be patient!)

[[v2-git-send-email]]
=== Sending v2

Skip ahead to <<reviewing,Responding to Reviews>> for information on how to
handle comments from reviewers. Continue this section when your topic branch is
shaped the way you want it to look for your patchset v2.

When you're ready with the next iteration of your patch, the process is fairly
similar.

First, generate your v2 patches again:

----
$ git format-patch -v2 --cover-letter -o psuh/ master..psuh
----

This will add your v2 patches, all named like `v2-000n-my-commit-subject.patch`,
to the `psuh/` directory. You may notice that they are sitting alongside the v1
patches; that's fine, but be careful when you are ready to send them.

Edit your cover letter again. Now is a good time to mention what's different
between your last version and now, if it's something significant. You do not
need the exact same body in your second cover letter; focus on explaining to
reviewers the changes you've made that may not be as visible.

You will also need to go and find the Message-Id of your previous cover letter.
You can either note it when you send the first series, from the output of `git
send-email`, or you can look it up on the
https://lore.kernel.org/git[mailing list]. Find your cover letter in the
archives, click on it, then click "permalink" or "raw" to reveal the Message-Id
header. It should match:

----
Message-Id: <foo.12345.author@example.com>
----

Your Message-Id is `<foo.12345.author@example.com>`. This example will be used
below as well; make sure to replace it with the correct Message-Id for your
**previous cover letter** - that is, if you're sending v2, use the Message-Id
from v1; if you're sending v3, use the Message-Id from v2.

While you're looking at the email, you should also note who is CC'd, as it's
common practice in the mailing list to keep all CCs on a thread. You can add
these CC lines directly to your cover letter with a line like so in the header
(before the Subject line):

----
CC: author@example.com, Othe R <other@example.com>
----

Now send the emails again, paying close attention to which messages you pass in
to the command:

----
$ git send-email --to=target@example.com
		 --in-reply-to="<foo.12345.author@example.com>"
		 psuh/v2*
----

[[single-patch]]
=== Bonus Chapter: One-Patch Changes

In some cases, your very small change may consist of only one patch. When that
happens, you only need to send one email. Your commit message should already be
meaningful and explain at a high level the purpose (what is happening and why)
of your patch, but if you need to supply even more context, you can do so below
the `---` in your patch. Take the example below, which was generated with `git
format-patch` on a single commit, and then edited to add the content between
the `---` and the diffstat.

----
From 1345bbb3f7ac74abde040c12e737204689a72723 Mon Sep 17 00:00:00 2001
From: A U Thor <author@example.com>
Date: Thu, 18 Apr 2019 15:11:02 -0700
Subject: [PATCH] README: change the grammar

I think it looks better this way. This part of the commit message will
end up in the commit-log.

Signed-off-by: A U Thor <author@example.com>
---
Let's have a wild discussion about grammar on the mailing list. This
part of my email will never end up in the commit log. Here is where I
can add additional context to the mailing list about my intent, outside
of the context of the commit log. This section was added after `git
format-patch` was run, by editing the patch file in a text editor.

 README.md | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/README.md b/README.md
index 88f126184c..38da593a60 100644
--- a/README.md
+++ b/README.md
@@ -3,7 +3,7 @@
 Git - fast, scalable, distributed revision control system
 =========================================================

-Git is a fast, scalable, distributed revision control system with an
+Git is a fast, scalable, and distributed revision control system with an
 unusually rich command set that provides both high-level operations
 and full access to internals.

--
2.21.0.392.gf8f6787159e-goog
----

[[now-what]]
== My Patch Got Emailed - Now What?

[[reviewing]]
=== Responding to Reviews

After a few days, you will hopefully receive a reply to your patchset with some
comments. Woohoo! Now you can get back to work.

It's good manners to reply to each comment, notifying the reviewer that you have
made the change requested, feel the original is better, or that the comment
inspired you to do something a new way which is superior to both the original
and the suggested change. This way reviewers don't need to inspect your v2 to
figure out whether you implemented their comment or not.

If you are going to push back on a comment, be polite and explain why you feel
your original is better; be prepared that the reviewer may still disagree with
you, and the rest of the community may weigh in on one side or the other. As
with all code reviews, it's important to keep an open mind to doing something a
different way than you originally planned; other reviewers have a different
perspective on the project than you do, and may be thinking of a valid side
effect which had not occurred to you. It is always okay to ask for clarification
if you aren't sure why a change was suggested, or what the reviewer is asking
you to do.

Make sure your email client has a plaintext email mode and it is turned on; the
Git list rejects HTML email. Please also follow the mailing list etiquette
outlined in the
https://kernel.googlesource.com/pub/scm/git/git/+/todo/MaintNotes[Maintainer's
Note], which are similar to etiquette rules in most open source communities
surrounding bottom-posting and inline replies.

When you're making changes to your code, it is cleanest - that is, the resulting
commits are easiest to look at - if you use `git rebase -i` (interactive
rebase). Take a look at this
https://www.oreilly.com/library/view/git-pocket-guide/9781449327507/ch10.html[overview]
from O'Reilly. The general idea is to modify each commit which requires changes;
this way, instead of having a patch A with a mistake, a patch B which was fine
and required no upstream reviews in v1, and a patch C which fixes patch A for
v2, you can just ship a v2 with a correct patch A and correct patch B. This is
changing history, but since it's local history which you haven't shared with
anyone, that is okay for now! (Later, it may not make sense to do this; take a
look at the section below this one for some context.)

[[after-approval]]
=== After Review Approval

The Git project has four integration branches: `pu`, `next`, `master`, and
`maint`. Your change will be placed into `pu` fairly early on by the maintainer
while it is still in the review process; from there, when it is ready for wider
testing, it will be merged into `next`. Plenty of early testers use `next` and
may report issues. Eventually, changes in `next` will make it to `master`,
which is typically considered stable. Finally, when a new release is cut,
`maint` is used to base bugfixes onto. As mentioned at the beginning of this
document, you can read `Documents/SubmittingPatches` for some more info about
the use of the various integration branches.

Back to now: your code has been lauded by the upstream reviewers. It is perfect.
It is ready to be accepted. You don't need to do anything else; the maintainer
will merge your topic branch to `next` and life is good.

However, if you discover it isn't so perfect after this point, you may need to
take some special steps depending on where you are in the process.

If the maintainer has announced in the "What's cooking in git.git" email that
your topic is marked for `next` - that is, that they plan to merge it to `next`
but have not yet done so - you should send an email asking the maintainer to
wait a little longer: "I've sent v4 of my series and you marked it for `next`,
but I need to change this and that - please wait for v5 before you merge it."

If the topic has already been merged to `next`, rather than modifying your
patches with `git rebase -i`, you should make further changes incrementally -
that is, with another commit, based on top of the maintainer's topic branch as
detailed in https://github.com/gitster/git. Your work is still in the same topic
but is now incremental, rather than a wholesale rewrite of the topic branch.

The topic branches in the maintainer's GitHub are mirrored in GitGitGadget, so
if you're sending your reviews out that way, you should be sure to open your PR
against the appropriate GitGitGadget/Git branch.

If you're using `git send-email`, you can use it the same way as before, but you
should generate your diffs from `<topic>..<mybranch>` and base your work on
`<topic>` instead of `master`.
