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MyFirstContribution.html
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<!DOCTYPE html>
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<title>My First Contribution to the Git Project</title>
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</head>
<body class="article">
<div id="header">
<h1>My First Contribution to the Git Project</h1>
</div>
<div id="content">
<div class="sect1">
<h2 id="summary"><a class="anchor" href="#summary"></a>Summary</h2>
<div class="sectionbody">
<div class="paragraph">
<p>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.</p>
</div>
<div class="sect2">
<h3 id="prerequisites"><a class="anchor" href="#prerequisites"></a>Prerequisites</h3>
<div class="paragraph">
<p>This tutorial assumes you’re already fairly familiar with using Git to manage
source code. The Git workflow steps will largely remain unexplained.</p>
</div>
</div>
<div class="sect2">
<h3 id="related-reading"><a class="anchor" href="#related-reading"></a>Related Reading</h3>
<div class="paragraph">
<p>This tutorial aims to summarize the following documents, but the reader may find
useful additional context:</p>
</div>
<div class="ulist">
<ul>
<li>
<p><code>Documentation/SubmittingPatches</code></p>
</li>
<li>
<p><code>Documentation/howto/new-command.txt</code></p>
</li>
</ul>
</div>
</div>
<div class="sect2">
<h3 id="getting-help"><a class="anchor" href="#getting-help"></a>Getting Help</h3>
<div class="paragraph">
<p>If you get stuck, you can seek help in the following places.</p>
</div>
<div class="sect3">
<h4 id="_gitvger_kernel_org"><a class="anchor" href="#_gitvger_kernel_org"></a><a href="mailto:[email protected]">[email protected]</a></h4>
<div class="paragraph">
<p>This is the main Git project mailing list where code reviews, version
announcements, design discussions, and more take place. Those interested in
contributing are welcome to post questions here. The Git list requires
plain-text-only emails and prefers inline and bottom-posting when replying to
mail; you will be CC’d in all replies to you. Optionally, you can subscribe to
the list by sending an email to <<a href="mailto:[email protected]">[email protected]</a>>
(see <a href="https://subspace.kernel.org/subscribing.html" class="bare">https://subspace.kernel.org/subscribing.html</a> for details).
The <a href="https://lore.kernel.org/git">archive</a> of this mailing list is
available to view in a browser.</p>
</div>
</div>
<div class="sect3">
<h4 id="_git_mentoringgooglegroups_com"><a class="anchor" href="#_git_mentoringgooglegroups_com"></a><a href="https://groups.google.com/forum/#!forum/git-mentoring">[email protected]</a></h4>
<div class="paragraph">
<p>This mailing list is targeted to new contributors and was created as a place to
post questions and receive answers outside of the public eye of the main list.
Veteran contributors who are especially interested in helping mentor newcomers
are present on the list. In order to avoid search indexers, group membership is
required to view messages; anyone can join and no approval is required.</p>
</div>
</div>
<div class="sect3">
<h4 id="_git_devel_on_libera_chat"><a class="anchor" href="#_git_devel_on_libera_chat"></a><a href="https://web.libera.chat/#git-devel">#git-devel</a> on Libera Chat</h4>
<div class="paragraph">
<p>This IRC channel is for conversations between Git contributors. If someone is
currently online and knows the answer to your question, you can receive help
in real time. Otherwise, you can read the
<a href="https://colabti.org/irclogger/irclogger_logs/git-devel">scrollback</a> to see
whether someone answered you. IRC does not allow offline private messaging, so
if you try to private message someone and then log out of IRC, they cannot
respond to you. It’s better to ask your questions in the channel so that you
can be answered if you disconnect and so that others can learn from the
conversation.</p>
</div>
</div>
</div>
</div>
</div>
<div class="sect1">
<h2 id="getting-started"><a class="anchor" href="#getting-started"></a>Getting Started</h2>
<div class="sectionbody">
<div class="sect2">
<h3 id="cloning"><a class="anchor" href="#cloning"></a>Clone the Git Repository</h3>
<div class="paragraph">
<p>Git is mirrored in a number of locations. Clone the repository from one of them;
<a href="https://git-scm.com/downloads" class="bare">https://git-scm.com/downloads</a> suggests one of the best places to clone from is
the mirror on GitHub.</p>
</div>
<div class="listingblock">
<div class="content">
<pre>$ git clone https://github.com/git/git git
$ cd git</pre>
</div>
</div>
</div>
<div class="sect2">
<h3 id="dependencies"><a class="anchor" href="#dependencies"></a>Installing Dependencies</h3>
<div class="paragraph">
<p>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
<code>INSTALL</code>, 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.</p>
</div>
<div class="paragraph">
<p>Make sure that your environment has everything you need by building your brand
new clone of Git from the above step:</p>
</div>
<div class="listingblock">
<div class="content">
<pre>$ make</pre>
</div>
</div>
<div class="admonitionblock note">
<table>
<tr>
<td class="icon">
<div class="title">Note</div>
</td>
<td class="content">
The Git build is parallelizable. <code>-j</code># is not included above but you can
use it as you prefer, here and elsewhere.
</td>
</tr>
</table>
</div>
</div>
<div class="sect2">
<h3 id="identify-problem"><a class="anchor" href="#identify-problem"></a>Identify Problem to Solve</h3>
<div class="paragraph">
<p>In this tutorial, we will add a new command, <code>git</code> <code>psuh</code>, short for “Pony Saying
‘Um, Hello”’ - a feature which has gone unimplemented despite a high frequency
of invocation during users' typical daily workflow.</p>
</div>
<div class="paragraph">
<p>(We’ve seen some other effort in this space with the implementation of popular
commands such as <code>sl</code>.)</p>
</div>
</div>
<div class="sect2">
<h3 id="setup-workspace"><a class="anchor" href="#setup-workspace"></a>Set Up Your Workspace</h3>
<div class="paragraph">
<p>Let’s start by making a development branch to work on our changes. Per
<code>Documentation/SubmittingPatches</code>, since a brand new command is a new feature,
it’s fine to base your work on <code>master</code>. However, in the future for bugfixes,
etc., you should check that document and base it on the appropriate branch.</p>
</div>
<div class="paragraph">
<p>For the purposes of this document, we will base all our work on the <code>master</code>
branch of the upstream project. Create the <code>psuh</code> branch you will use for
development like so:</p>
</div>
<div class="listingblock">
<div class="content">
<pre>$ git checkout -b psuh origin/master</pre>
</div>
</div>
<div class="paragraph">
<p>We’ll make a number of commits here in order to demonstrate how to send a topic
with multiple patches up for review simultaneously.</p>
</div>
</div>
</div>
</div>
<div class="sect1">
<h2 id="code-it-up"><a class="anchor" href="#code-it-up"></a>Code It Up!</h2>
<div class="sectionbody">
<div class="admonitionblock note">
<table>
<tr>
<td class="icon">
<div class="title">Note</div>
</td>
<td class="content">
A reference implementation can be found at
<a href="https://github.com/nasamuffin/git/tree/psuh" class="bare">https://github.com/nasamuffin/git/tree/psuh</a>.
</td>
</tr>
</table>
</div>
<div class="sect2">
<h3 id="add-new-command"><a class="anchor" href="#add-new-command"></a>Adding a New Command</h3>
<div class="paragraph">
<p>Lots of the subcommands are written as builtins, which means they are
implemented in C and compiled into the main <code>git</code> executable. Implementing the
very simple <code>psuh</code> 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.</p>
</div>
<div class="paragraph">
<p>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 <code>builtin/</code>. So it makes sense to implement your
command in <code>builtin/psuh.c</code>. Create that file, and within it, write the entry
point for your command in a function matching the style and signature:</p>
</div>
<div class="listingblock">
<div class="content">
<pre>int cmd_psuh(int argc, const char **argv, const char *prefix)</pre>
</div>
</div>
<div class="paragraph">
<p>We’ll also need to add the declaration of psuh; open up <code>builtin.h</code>, find the
declaration for <code>cmd_pull</code>, and add a new line for <code>psuh</code> immediately before it,
in order to keep the declarations alphabetically sorted:</p>
</div>
<div class="listingblock">
<div class="content">
<pre>int cmd_psuh(int argc, const char **argv, const char *prefix);</pre>
</div>
</div>
<div class="paragraph">
<p>Be sure to #include "builtin.<code>h</code>" in your <code>psuh.c</code>. You’ll also need to
#include "gettext.<code>h</code>" to use functions related to printing output text.</p>
</div>
<div class="paragraph">
<p>Go ahead and add some throwaway printf to the <code>cmd_psuh</code> function. This is a
decent starting point as we can now add build rules and register the command.</p>
</div>
<div class="admonitionblock note">
<table>
<tr>
<td class="icon">
<div class="title">Note</div>
</td>
<td class="content">
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 <code>po/README</code> 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.
</td>
</tr>
</table>
</div>
<div class="listingblock">
<div class="content">
<pre>int cmd_psuh(int argc, const char **argv, const char *prefix)
{
printf(_("Pony saying hello goes here.\n"));
return 0;
}</pre>
</div>
</div>
<div class="paragraph">
<p>Let’s try to build it. Open <code>Makefile</code>, find where <code>builtin/pull.o</code> is added
to <code>BUILTIN_OBJS</code>, and add <code>builtin/psuh.o</code> 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 <code>make</code>. Also add the <code>DEVELOPER=1</code> variable to turn on
some additional warnings:</p>
</div>
<div class="listingblock">
<div class="content">
<pre>$ echo DEVELOPER=1 >config.mak
$ make</pre>
</div>
</div>
<div class="admonitionblock note">
<table>
<tr>
<td class="icon">
<div class="title">Note</div>
</td>
<td class="content">
When you are developing the Git project, it’s preferred that you use the
<code>DEVELOPER</code> 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.
</td>
</tr>
</table>
</div>
<div class="paragraph">
<p>Great, now your new command builds happily on its own. But nobody invokes it.
Let’s change that.</p>
</div>
<div class="paragraph">
<p>The list of commands lives in <code>git.c</code>. We can register a new command by adding
a <code>cmd_struct</code> to the <code>commands</code>[] array. <code>struct</code> <code>cmd_struct</code> 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 <code>push</code>. Find the line where
<code>cmd_push</code> is registered, copy it, and modify it for <code>cmd_psuh</code>, placing the new
line in alphabetical order (immediately before <code>cmd_pull</code>).</p>
</div>
<div class="paragraph">
<p>The options are documented in <code>builtin.h</code> 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 <code>RUN_SETUP</code> as your only option.</p>
</div>
<div class="paragraph">
<p>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
<code>bin-wrappers</code> directory.</p>
</div>
<div class="listingblock">
<div class="content">
<pre>$ ./bin-wrappers/git psuh</pre>
</div>
</div>
<div class="paragraph">
<p>Check it out! You’ve got a command! Nice work! Let’s commit this.</p>
</div>
<div class="paragraph">
<p><code>git</code> <code>status</code> reveals modified <code>Makefile</code>, <code>builtin.h</code>, and <code>git.c</code> as well as
untracked <code>builtin/psuh.c</code> and <code>git-psuh</code>. First, let’s take care of the binary,
which should be ignored. Open .<code>gitignore</code> in your editor, find <code>/git-pull</code>, and
add an entry for your new command in alphabetical order:</p>
</div>
<div class="listingblock">
<div class="content">
<pre>...
/git-prune-packed
/git-psuh
/git-pull
/git-push
/git-quiltimport
/git-range-diff
...</pre>
</div>
</div>
<div class="paragraph">
<p>Checking <code>git</code> <code>status</code> again should show that <code>git-psuh</code> has been removed from
the untracked list and .<code>gitignore</code> has been added to the modified list. Now we
can stage and commit:</p>
</div>
<div class="listingblock">
<div class="content">
<pre>$ git add Makefile builtin.h builtin/psuh.c git.c .gitignore
$ git commit -s</pre>
</div>
</div>
<div class="paragraph">
<p>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 <code>Signed-off-by</code> trailer which was added by <code>-s</code> above.</p>
</div>
<div class="listingblock">
<div class="content">
<pre>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 <[email protected]></pre>
</div>
</div>
<div class="paragraph">
<p>Go ahead and inspect your new commit with <code>git</code> <code>show</code>. "psuh:" indicates you
have modified mainly the <code>psuh</code> command. The subject line gives readers an idea
of what you’ve changed. The sign-off line (<code>-s</code>) indicates that you agree to
the Developer’s Certificate of Origin 1.1 (see the
<code>Documentation/SubmittingPatches</code> [[dco]] header).</p>
</div>
<div class="paragraph">
<p>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.</p>
</div>
</div>
<div class="sect2">
<h3 id="implementation"><a class="anchor" href="#implementation"></a>Implementation</h3>
<div class="paragraph">
<p>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.</p>
</div>
<div class="paragraph">
<p>Modify your <code>cmd_psuh</code> implementation to dump the args you’re passed, keeping
existing <code>printf</code>() calls in place:</p>
</div>
<div class="listingblock">
<div class="content">
<pre> 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 : "");</pre>
</div>
</div>
<div class="paragraph">
<p>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 <code>prefix</code> is empty
for you, try <code>cd</code> <code>Documentation/</code> && <code>..</code><code>/bin-wrappers/git</code> <code>psuh</code>). That’s not so
helpful. So what other context can we get?</p>
</div>
<div class="paragraph">
<p>Add a line to #include "config.<code>h</code>". Then, add the following bits to the
function body:</p>
</div>
<div class="listingblock">
<div class="content">
<pre> const char *cfg_name;
...
git_config(git_default_config, NULL);
if (git_config_get_string_tmp("user.name", &cfg_name) > 0)
printf(_("No name is found in config\n"));
else
printf(_("Your name: %s\n"), cfg_name);</pre>
</div>
</div>
<div class="paragraph">
<p><code>git_config</code>() will grab the configuration from config files known to Git and
apply standard precedence rules. <code>git_config_get_string_tmp</code>() 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 <code>git_config</code>()) in <code>Documentation/technical/api-config.txt</code>.</p>
</div>
<div class="paragraph">
<p>You should see that the name printed matches the one you see when you run:</p>
</div>
<div class="listingblock">
<div class="content">
<pre>$ git config --get user.name</pre>
</div>
</div>
<div class="paragraph">
<p>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.</p>
</div>
<div class="listingblock">
<div class="content">
<pre>$ git add builtin/psuh.c
$ git commit -sm "psuh: show parameters & config opts"</pre>
</div>
</div>
<div class="admonitionblock note">
<table>
<tr>
<td class="icon">
<div class="title">Note</div>
</td>
<td class="content">
Again, the above is for sake of brevity in this tutorial. In a real change
you should not use <code>-m</code> but instead use the editor to write a meaningful
message.
</td>
</tr>
</table>
</div>
<div class="paragraph">
<p>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
<code>git</code> <code>status</code> implementation; the printer is located in <code>wt-status.c</code> and we can
see that the branch is held in a <code>struct</code> <code>wt_status</code>.</p>
</div>
<div class="paragraph">
<p><code>wt_status_print</code>() gets invoked by <code>cmd_status</code>() in <code>builtin/commit.c</code>.
Looking at that implementation we see the status config being populated like so:</p>
</div>
<div class="listingblock">
<div class="content">
<pre>status_init_config(&s, git_status_config);</pre>
</div>
</div>
<div class="paragraph">
<p>But as we drill down, we can find that <code>status_init_config</code>() wraps a call
to <code>git_config</code>(). Let’s modify the code we wrote in the previous commit.</p>
</div>
<div class="paragraph">
<p>Be sure to include the header to allow you to use <code>struct</code> <code>wt_status</code>:</p>
</div>
<div class="listingblock">
<div class="content">
<pre>#include "wt-status.h"</pre>
</div>
</div>
<div class="paragraph">
<p>Then modify your <code>cmd_psuh</code> implementation to declare your <code>struct</code> <code>wt_status</code>,
prepare it, and print its contents:</p>
</div>
<div class="listingblock">
<div class="content">
<pre> struct wt_status status;
...
wt_status_prepare(the_repository, &status);
git_config(git_default_config, &status);
...
printf(_("Your current branch: %s\n"), status.branch);</pre>
</div>
</div>
<div class="paragraph">
<p>Run it again. Check it out - here’s the (verbose) name of your current branch!</p>
</div>
<div class="paragraph">
<p>Let’s commit this as well.</p>
</div>
<div class="listingblock">
<div class="content">
<pre>$ git add builtin/psuh.c
$ git commit -sm "psuh: print the current branch"</pre>
</div>
</div>
<div class="paragraph">
<p>Now let’s see if we can get some info about a specific commit.</p>
</div>
<div class="paragraph">
<p>Luckily, there are some helpers for us here. <code>commit.h</code> has a function called
<code>lookup_commit_reference_by_name</code> to which we can simply provide a hardcoded
string; <code>pretty.h</code> has an extremely handy <code>pp_commit_easy</code>() call which doesn’t
require a full format object to be passed.</p>
</div>
<div class="paragraph">
<p>Add the following includes:</p>
</div>
<div class="listingblock">
<div class="content">
<pre>#include "commit.h"
#include "pretty.h"</pre>
</div>
</div>
<div class="paragraph">
<p>Then, add the following lines within your implementation of <code>cmd_psuh</code>() near
the declarations and the logic, respectively.</p>
</div>
<div class="listingblock">
<div class="content">
<pre> 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);
}</pre>
</div>
</div>