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<title>My First Object Walk</title> |
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</head> |
|
<body class="article"> |
|
<div id="header"> |
|
<h1>My First Object Walk</h1> |
|
</div> |
|
<div id="content"> |
|
<div class="sect1"> |
|
<h2 id="_whats_an_object_walk">What’s an Object Walk?</h2> |
|
<div class="sectionbody"> |
|
<div class="paragraph"> |
|
<p>The object walk is a key concept in Git - this is the process that underpins |
|
operations like object transfer and fsck. Beginning from a given commit, the |
|
list of objects is found by walking parent relationships between commits (commit |
|
X based on commit W) and containment relationships between objects (tree Y is |
|
contained within commit X, and blob Z is located within tree Y, giving our |
|
working tree for commit X something like <code>y/z.txt</code>).</p> |
|
</div> |
|
<div class="paragraph"> |
|
<p>A related concept is the revision walk, which is focused on commit objects and |
|
their parent relationships and does not delve into other object types. The |
|
revision walk is used for operations like <code>git log</code>.</p> |
|
</div> |
|
<div class="sect2"> |
|
<h3 id="_related_reading">Related Reading</h3> |
|
<div class="ulist"> |
|
<ul> |
|
<li> |
|
<p><code>Documentation/user-manual.txt</code> under "Hacking Git" contains some coverage of |
|
the revision walker in its various incarnations.</p> |
|
</li> |
|
<li> |
|
<p><code>revision.h</code></p> |
|
</li> |
|
<li> |
|
<p><a href="https://eagain.net/articles/git-for-computer-scientists/">Git for Computer Scientists</a> |
|
gives a good overview of the types of objects in Git and what your object |
|
walk is really describing.</p> |
|
</li> |
|
</ul> |
|
</div> |
|
</div> |
|
</div> |
|
</div> |
|
<div class="sect1"> |
|
<h2 id="_setting_up">Setting Up</h2> |
|
<div class="sectionbody"> |
|
<div class="paragraph"> |
|
<p>Create a new branch from <code>master</code>.</p> |
|
</div> |
|
<div class="listingblock"> |
|
<div class="content"> |
|
<pre>git checkout -b revwalk origin/master</pre> |
|
</div> |
|
</div> |
|
<div class="paragraph"> |
|
<p>We’ll put our fiddling into a new command. For fun, let’s name it <code>git walken</code>. |
|
Open up a new file <code>builtin/walken.c</code> and set up the command handler:</p> |
|
</div> |
|
<div class="listingblock"> |
|
<div class="content"> |
|
<pre>/* |
|
* "git walken" |
|
* |
|
* Part of the "My First Object Walk" tutorial. |
|
*/ |
|
|
|
#include "builtin.h" |
|
|
|
int cmd_walken(int argc, const char **argv, const char *prefix) |
|
{ |
|
trace_printf(_("cmd_walken incoming...\n")); |
|
return 0; |
|
}</pre> |
|
</div> |
|
</div> |
|
<div class="admonitionblock note"> |
|
<table> |
|
<tr> |
|
<td class="icon"> |
|
<div class="title">Note</div> |
|
</td> |
|
<td class="content"> |
|
<code>trace_printf()</code> differs from <code>printf()</code> in that it can be turned on or |
|
off at runtime. For the purposes of this tutorial, we will write <code>walken</code> as |
|
though it is intended for use as a "plumbing" command: that is, a command which |
|
is used primarily in scripts, rather than interactively by humans (a "porcelain" |
|
command). So we will send our debug output to <code>trace_printf()</code> instead. When |
|
running, enable trace output by setting the environment variable <code>GIT_TRACE</code>. |
|
</td> |
|
</tr> |
|
</table> |
|
</div> |
|
<div class="paragraph"> |
|
<p>Add usage text and <code>-h</code> handling, like all subcommands should consistently do |
|
(our test suite will notice and complain if you fail to do so). |
|
We’ll need to include the <code>parse-options.h</code> header.</p> |
|
</div> |
|
<div class="listingblock"> |
|
<div class="content"> |
|
<pre>#include "parse-options.h" |
|
|
|
... |
|
|
|
int cmd_walken(int argc, const char **argv, const char *prefix) |
|
{ |
|
const char * const walken_usage[] = { |
|
N_("git walken"), |
|
NULL, |
|
}; |
|
struct option options[] = { |
|
OPT_END() |
|
}; |
|
|
|
argc = parse_options(argc, argv, prefix, options, walken_usage, 0); |
|
|
|
... |
|
}</pre> |
|
</div> |
|
</div> |
|
<div class="paragraph"> |
|
<p>Also add the relevant line in <code>builtin.h</code> near <code>cmd_whatchanged()</code>:</p> |
|
</div> |
|
<div class="listingblock"> |
|
<div class="content"> |
|
<pre>int cmd_walken(int argc, const char **argv, const char *prefix);</pre> |
|
</div> |
|
</div> |
|
<div class="paragraph"> |
|
<p>Include the command in <code>git.c</code> in <code>commands[]</code> near the entry for <code>whatchanged</code>, |
|
maintaining alphabetical ordering:</p> |
|
</div> |
|
<div class="listingblock"> |
|
<div class="content"> |
|
<pre>{ "walken", cmd_walken, RUN_SETUP },</pre> |
|
</div> |
|
</div> |
|
<div class="paragraph"> |
|
<p>Add it to the <code>Makefile</code> near the line for <code>builtin/worktree.o</code>:</p> |
|
</div> |
|
<div class="listingblock"> |
|
<div class="content"> |
|
<pre>BUILTIN_OBJS += builtin/walken.o</pre> |
|
</div> |
|
</div> |
|
<div class="paragraph"> |
|
<p>Build and test out your command, without forgetting to ensure the <code>DEVELOPER</code> |
|
flag is set, and with <code>GIT_TRACE</code> enabled so the debug output can be seen:</p> |
|
</div> |
|
<div class="listingblock"> |
|
<div class="content"> |
|
<pre>$ echo DEVELOPER=1 >>config.mak |
|
$ make |
|
$ GIT_TRACE=1 ./bin-wrappers/git walken</pre> |
|
</div> |
|
</div> |
|
<div class="admonitionblock note"> |
|
<table> |
|
<tr> |
|
<td class="icon"> |
|
<div class="title">Note</div> |
|
</td> |
|
<td class="content"> |
|
For a more exhaustive overview of the new command process, take a look at |
|
<code>Documentation/MyFirstContribution.txt</code>. |
|
</td> |
|
</tr> |
|
</table> |
|
</div> |
|
<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/revwalk" class="bare">https://github.com/nasamuffin/git/tree/revwalk</a>. |
|
</td> |
|
</tr> |
|
</table> |
|
</div> |
|
<div class="sect2"> |
|
<h3 id="_struct_rev_cmdline_info"><code>struct rev_cmdline_info</code></h3> |
|
<div class="paragraph"> |
|
<p>The definition of <code>struct rev_cmdline_info</code> can be found in <code>revision.h</code>.</p> |
|
</div> |
|
<div class="paragraph"> |
|
<p>This struct is contained within the <code>rev_info</code> struct and is used to reflect |
|
parameters provided by the user over the CLI.</p> |
|
</div> |
|
<div class="paragraph"> |
|
<p><code>nr</code> represents the number of <code>rev_cmdline_entry</code> present in the array.</p> |
|
</div> |
|
<div class="paragraph"> |
|
<p><code>alloc</code> is used by the <code>ALLOC_GROW</code> macro. Check <code>cache.h</code> - this variable is |
|
used to track the allocated size of the list.</p> |
|
</div> |
|
<div class="paragraph"> |
|
<p>Per entry, we find:</p> |
|
</div> |
|
<div class="paragraph"> |
|
<p><code>item</code> is the object provided upon which to base the object walk. Items in Git |
|
can be blobs, trees, commits, or tags. (See <code>Documentation/gittutorial-2.txt</code>.)</p> |
|
</div> |
|
<div class="paragraph"> |
|
<p><code>name</code> is the object ID (OID) of the object - a hex string you may be familiar |
|
with from using Git to organize your source in the past. Check the tutorial |
|
mentioned above towards the top for a discussion of where the OID can come |
|
from.</p> |
|
</div> |
|
<div class="paragraph"> |
|
<p><code>whence</code> indicates some information about what to do with the parents of the |
|
specified object. We’ll explore this flag more later on; take a look at |
|
<code>Documentation/revisions.txt</code> to get an idea of what could set the <code>whence</code> |
|
value.</p> |
|
</div> |
|
<div class="paragraph"> |
|
<p><code>flags</code> are used to hint the beginning of the revision walk and are the first |
|
block under the <code>#include`s in `revision.h</code>. The most likely ones to be set in |
|
the <code>rev_cmdline_info</code> are <code>UNINTERESTING</code> and <code>BOTTOM</code>, but these same flags |
|
can be used during the walk, as well.</p> |
|
</div> |
|
</div> |
|
<div class="sect2"> |
|
<h3 id="_struct_rev_info"><code>struct rev_info</code></h3> |
|
<div class="paragraph"> |
|
<p>This one is quite a bit longer, and many fields are only used during the walk |
|
by <code>revision.c</code> - not configuration options. Most of the configurable flags in |
|
<code>struct rev_info</code> have a mirror in <code>Documentation/rev-list-options.txt</code>. It’s a |
|
good idea to take some time and read through that document.</p> |
|
</div> |
|
</div> |
|
</div> |
|
</div> |
|
<div class="sect1"> |
|
<h2 id="_basic_commit_walk">Basic Commit Walk</h2> |
|
<div class="sectionbody"> |
|
<div class="paragraph"> |
|
<p>First, let’s see if we can replicate the output of <code>git log --oneline</code>. We’ll |
|
refer back to the implementation frequently to discover norms when performing |
|
an object walk of our own.</p> |
|
</div> |
|
<div class="paragraph"> |
|
<p>To do so, we’ll first find all the commits, in order, which preceded the current |
|
commit. We’ll extract the name and subject of the commit from each.</p> |
|
</div> |
|
<div class="paragraph"> |
|
<p>Ideally, we will also be able to find out which ones are currently at the tip of |
|
various branches.</p> |
|
</div> |
|
<div class="sect2"> |
|
<h3 id="_setting_up_2">Setting Up</h3> |
|
<div class="paragraph"> |
|
<p>Preparing for your object walk has some distinct stages.</p> |
|
</div> |
|
<div class="olist arabic"> |
|
<ol class="arabic"> |
|
<li> |
|
<p>Perform default setup for this mode, and others which may be invoked.</p> |
|
</li> |
|
<li> |
|
<p>Check configuration files for relevant settings.</p> |
|
</li> |
|
<li> |
|
<p>Set up the <code>rev_info</code> struct.</p> |
|
</li> |
|
<li> |
|
<p>Tweak the initialized <code>rev_info</code> to suit the current walk.</p> |
|
</li> |
|
<li> |
|
<p>Prepare the <code>rev_info</code> for the walk.</p> |
|
</li> |
|
<li> |
|
<p>Iterate over the objects, processing each one.</p> |
|
</li> |
|
</ol> |
|
</div> |
|
<div class="sect3"> |
|
<h4 id="_default_setups">Default Setups</h4> |
|
<div class="paragraph"> |
|
<p>Before examining configuration files which may modify command behavior, set up |
|
default state for switches or options your command may have. If your command |
|
utilizes other Git components, ask them to set up their default states as well. |
|
For instance, <code>git log</code> takes advantage of <code>grep</code> and <code>diff</code> functionality, so |
|
its <code>init_log_defaults()</code> sets its own state (<code>decoration_style</code>) and asks |
|
<code>grep</code> and <code>diff</code> to initialize themselves by calling each of their |
|
initialization functions.</p> |
|
</div> |
|
</div> |
|
<div class="sect3"> |
|
<h4 id="_configuring_from_gitconfig">Configuring From <code>.gitconfig</code></h4> |
|
<div class="paragraph"> |
|
<p>Next, we should have a look at any relevant configuration settings (i.e., |
|
settings readable and settable from <code>git config</code>). This is done by providing a |
|
callback to <code>git_config()</code>; within that callback, you can also invoke methods |
|
from other components you may need that need to intercept these options. Your |
|
callback will be invoked once per each configuration value which Git knows about |
|
(global, local, worktree, etc.).</p> |
|
</div> |
|
<div class="paragraph"> |
|
<p>Similarly to the default values, we don’t have anything to do here yet |
|
ourselves; however, we should call <code>git_default_config()</code> if we aren’t calling |
|
any other existing config callbacks.</p> |
|
</div> |
|
<div class="paragraph"> |
|
<p>Add a new function to <code>builtin/walken.c</code>. |
|
We’ll also need to include the <code>config.h</code> header:</p> |
|
</div> |
|
<div class="listingblock"> |
|
<div class="content"> |
|
<pre>#include "config.h" |
|
|
|
... |
|
|
|
static int git_walken_config(const char *var, const char *value, void *cb) |
|
{ |
|
/* |
|
* For now, we don't have any custom configuration, so fall back to |
|
* the default config. |
|
*/ |
|
return git_default_config(var, value, cb); |
|
}</pre> |
|
</div> |
|
</div> |
|
<div class="paragraph"> |
|
<p>Make sure to invoke <code>git_config()</code> with it in your <code>cmd_walken()</code>:</p> |
|
</div> |
|
<div class="listingblock"> |
|
<div class="content"> |
|
<pre>int cmd_walken(int argc, const char **argv, const char *prefix) |
|
{ |
|
... |
|
|
|
git_config(git_walken_config, NULL); |
|
|
|
... |
|
}</pre> |
|
</div> |
|
</div> |
|
</div> |
|
<div class="sect3"> |
|
<h4 id="_setting_up_rev_info">Setting Up <code>rev_info</code></h4> |
|
<div class="paragraph"> |
|
<p>Now that we’ve gathered external configuration and options, it’s time to |
|
initialize the <code>rev_info</code> object which we will use to perform the walk. This is |
|
typically done by calling <code>repo_init_revisions()</code> with the repository you intend |
|
to target, as well as the <code>prefix</code> argument of <code>cmd_walken</code> and your <code>rev_info</code> |
|
struct.</p> |
|
</div> |
|
<div class="paragraph"> |
|
<p>Add the <code>struct rev_info</code> and the <code>repo_init_revisions()</code> call. |
|
We’ll also need to include the <code>revision.h</code> header:</p> |
|
</div> |
|
<div class="listingblock"> |
|
<div class="content"> |
|
<pre>#include "revision.h" |
|
|
|
... |
|
|
|
int cmd_walken(int argc, const char **argv, const char *prefix) |
|
{ |
|
/* This can go wherever you like in your declarations.*/ |
|
struct rev_info rev; |
|
... |
|
|
|
/* This should go after the git_config() call. */ |
|
repo_init_revisions(the_repository, &rev, prefix); |
|
|
|
... |
|
}</pre> |
|
</div> |
|
</div> |
|
</div> |
|
<div class="sect3"> |
|
<h4 id="_tweaking_rev_info_for_the_walk">Tweaking <code>rev_info</code> For the Walk</h4> |
|
<div class="paragraph"> |
|
<p>We’re getting close, but we’re still not quite ready to go. Now that <code>rev</code> is |
|
initialized, we can modify it to fit our needs. This is usually done within a |
|
helper for clarity, so let’s add one:</p> |
|
</div> |
|
<div class="listingblock"> |
|
<div class="content"> |
|
<pre>static void final_rev_info_setup(struct rev_info *rev) |
|
{ |
|
/* |
|
* We want to mimic the appearance of `git log --oneline`, so let's |
|
* force oneline format. |
|
*/ |
|
get_commit_format("oneline", rev); |
|
|
|
/* Start our object walk at HEAD. */ |
|
add_head_to_pending(rev); |
|
}</pre> |
|
</div> |
|
</div> |
|
<div class="admonitionblock note"> |
|
<table> |
|
<tr> |
|
<td class="icon"> |
|
<div class="title">Note</div> |
|
</td> |
|
<td class="content"> |
|
<div class="paragraph"> |
|
<p>Instead of using the shorthand <code>add_head_to_pending()</code>, you could do |
|
something like this:</p> |
|
</div> |
|
<div class="listingblock"> |
|
<div class="content"> |
|
<pre> struct setup_revision_opt opt; |
|
|
|
memset(&opt, 0, sizeof(opt)); |
|
opt.def = "HEAD"; |
|
opt.revarg_opt = REVARG_COMMITTISH; |
|
setup_revisions(argc, argv, rev, &opt);</pre> |
|
</div> |
|
</div> |
|
<div class="paragraph"> |
|
<p>Using a <code>setup_revision_opt</code> gives you finer control over your walk’s starting |
|
point.</p> |
|
</div> |
|
</td> |
|
</tr> |
|
</table> |
|
</div> |
|
<div class="paragraph"> |
|
<p>Then let’s invoke <code>final_rev_info_setup()</code> after the call to |
|
<code>repo_init_revisions()</code>:</p> |
|
</div> |
|
<div class="listingblock"> |
|
<div class="content"> |
|
<pre>int cmd_walken(int argc, const char **argv, const char *prefix) |
|
{ |
|
... |
|
|
|
final_rev_info_setup(&rev); |
|
|
|
... |
|
}</pre> |
|
</div> |
|
</div> |
|
<div class="paragraph"> |
|
<p>Later, we may wish to add more arguments to <code>final_rev_info_setup()</code>. But for |
|
now, this is all we need.</p> |
|
</div> |
|
</div> |
|
<div class="sect3"> |
|
<h4 id="_preparing_rev_info_for_the_walk">Preparing <code>rev_info</code> For the Walk</h4> |
|
<div class="paragraph"> |
|
<p>Now that <code>rev</code> is all initialized and configured, we’ve got one more setup step |
|
before we get rolling. We can do this in a helper, which will both prepare the |
|
<code>rev_info</code> for the walk, and perform the walk itself. Let’s start the helper |
|
with the call to <code>prepare_revision_walk()</code>, which can return an error without |
|
dying on its own:</p> |
|
</div> |
|
<div class="listingblock"> |
|
<div class="content"> |
|
<pre>static void walken_commit_walk(struct rev_info *rev) |
|
{ |
|
if (prepare_revision_walk(rev)) |
|
die(_("revision walk setup failed")); |
|
}</pre> |
|
</div> |
|
</div> |
|
<div class="admonitionblock note"> |
|
<table> |
|
<tr> |
|
<td class="icon"> |
|
<div class="title">Note</div> |
|
</td> |
|
<td class="content"> |
|
<code>die()</code> prints to <code>stderr</code> and exits the program. Since it will print to |
|
<code>stderr</code> it’s likely to be seen by a human, so we will localize it. |
|
</td> |
|
</tr> |
|
</table> |
|
</div> |
|
</div> |
|
<div class="sect3"> |
|
<h4 id="_performing_the_walk">Performing the Walk!</h4> |
|
<div class="paragraph"> |
|
<p>Finally! We are ready to begin the walk itself. Now we can see that <code>rev_info</code> |
|
can also be used as an iterator; we move to the next item in the walk by using |
|
<code>get_revision()</code> repeatedly. Add the listed variable declarations at the top and |
|
the walk loop below the <code>prepare_revision_walk()</code> call within your |
|
<code>walken_commit_walk()</code>:</p> |
|
</div> |
|
<div class="listingblock"> |
|
<div class="content"> |
|
<pre>static void walken_commit_walk(struct rev_info *rev) |
|
{ |
|
struct commit *commit; |
|
struct strbuf prettybuf = STRBUF_INIT; |
|
|
|
... |
|
|
|
while ((commit = get_revision(rev))) { |
|
strbuf_reset(&prettybuf); |
|
pp_commit_easy(CMIT_FMT_ONELINE, commit, &prettybuf); |
|
puts(prettybuf.buf); |
|
} |
|
strbuf_release(&prettybuf); |
|
}</pre> |
|
</div> |
|
</div> |
|
<div class="admonitionblock note"> |
|
<table> |
|
<tr> |
|
<td class="icon"> |
|
<div class="title">Note</div> |
|
</td> |
|
<td class="content"> |
|
<code>puts()</code> prints a <code>char*</code> to <code>stdout</code>. Since this is the part of the |
|
command we expect to be machine-parsed, we’re sending it directly to stdout. |
|
</td> |
|
</tr> |
|
</table> |
|
</div> |
|
<div class="paragraph"> |
|
<p>Give it a shot.</p> |
|
</div> |
|
<div class="listingblock"> |
|
<div class="content"> |
|
<pre>$ make |
|
$ ./bin-wrappers/git walken</pre> |
|
</div> |
|
</div> |
|
<div class="paragraph"> |
|
<p>You should see all of the subject lines of all the commits in |
|
your tree’s history, in order, ending with the initial commit, "Initial revision |
|
of "git", the information manager from hell". Congratulations! You’ve written |
|
your first revision walk. You can play with printing some additional fields |
|
from each commit if you’re curious; have a look at the functions available in |
|
<code>commit.h</code>.</p> |
|
</div> |
|
</div> |
|
</div> |
|
<div class="sect2"> |
|
<h3 id="_adding_a_filter">Adding a Filter</h3> |
|
<div class="paragraph"> |
|
<p>Next, let’s try to filter the commits we see based on their author. This is |
|
equivalent to running <code>git log --author=<pattern></code>. We can add a filter by |
|
modifying <code>rev_info.grep_filter</code>, which is a <code>struct grep_opt</code>.</p> |
|
</div> |
|
<div class="paragraph"> |
|
<p>First some setup. Add <code>grep_config()</code> to <code>git_walken_config()</code>:</p> |
|
</div> |
|
<div class="listingblock"> |
|
<div class="content"> |
|
<pre>static int git_walken_config(const char *var, const char *value, void *cb) |
|
{ |
|
grep_config(var, value, cb); |
|
return git_default_config(var, value, cb); |
|
}</pre> |
|
</div> |
|
</div> |
|
<div class="paragraph"> |
|
<p>Next, we can modify the <code>grep_filter</code>. This is done with convenience functions |
|
found in <code>grep.h</code>. For fun, we’re filtering to only commits from folks using a |
|
<code>gmail.com</code> email address - a not-very-precise guess at who may be working on |
|
Git as a hobby. Since we’re checking the author, which is a specific line in the |
|
header, we’ll use the <code>append_header_grep_pattern()</code> helper. We can use |
|
the <code>enum grep_header_field</code> to indicate which part of the commit header we want |
|
to search.</p> |
|
</div> |
|
<div class="paragraph"> |
|
<p>In <code>final_rev_info_setup()</code>, add your filter line:</p> |
|
</div> |
|
<div class="listingblock"> |
|
<div class="content"> |
|
<pre>static void final_rev_info_setup(int argc, const char **argv, |
|
const char *prefix, struct rev_info *rev) |
|
{ |
|
... |
|
|
|
append_header_grep_pattern(&rev->grep_filter, GREP_HEADER_AUTHOR, |
|
"gmail"); |
|
compile_grep_patterns(&rev->grep_filter); |
|
|
|
... |
|
}</pre> |
|
</div> |
|
</div> |
|
<div class="paragraph"> |
|
<p><code>append_header_grep_pattern()</code> adds your new "gmail" pattern to <code>rev_info</code>, but |
|
it won’t work unless we compile it with <code>compile_grep_patterns()</code>.</p> |
|
</div> |
|
<div class="admonitionblock note"> |
|
<table> |
|
<tr> |
|
<td class="icon"> |
|
<div class="title">Note</div> |
|
</td> |
|
<td class="content"> |
|
If you are using <code>setup_revisions()</code> (for example, if you are passing a |
|
<code>setup_revision_opt</code> instead of using <code>add_head_to_pending()</code>), you don’t need |
|
to call <code>compile_grep_patterns()</code> because <code>setup_revisions()</code> calls it for you. |
|
</td> |
|
</tr> |
|
</table> |
|
</div> |
|
<div class="admonitionblock note"> |
|
<table> |
|
<tr> |
|
<td class="icon"> |
|
<div class="title">Note</div> |
|
</td> |
|
<td class="content"> |
|
We could add the same filter via the <code>append_grep_pattern()</code> helper if we |
|
wanted to, but <code>append_header_grep_pattern()</code> adds the <code>enum grep_context</code> and |
|
<code>enum grep_pat_token</code> for us. |
|
</td> |
|
</tr> |
|
</table> |
|
</div> |
|
</div> |
|
<div class="sect2"> |
|
<h3 id="_changing_the_order">Changing the Order</h3> |
|
<div class="paragraph"> |
|
<p>There are a few ways that we can change the order of the commits during a |
|
revision walk. Firstly, we can use the <code>enum rev_sort_order</code> to choose from some |
|
typical orderings.</p> |
|
</div> |
|
<div class="paragraph"> |
|
<p><code>topo_order</code> is the same as <code>git log --topo-order</code>: we avoid showing a parent |
|
before all of its children have been shown, and we avoid mixing commits which |
|
are in different lines of history. (<code>git help log</code>'s section on <code>--topo-order</code> |
|
has a very nice diagram to illustrate this.)</p> |
|
</div> |
|
<div class="paragraph"> |
|
<p>Let’s see what happens when we run with <code>REV_SORT_BY_COMMIT_DATE</code> as opposed to |
|
<code>REV_SORT_BY_AUTHOR_DATE</code>. Add the following:</p> |
|
</div> |
|
<div class="listingblock"> |
|
<div class="content"> |
|
<pre>static void final_rev_info_setup(int argc, const char **argv, |
|
const char *prefix, struct rev_info *rev) |
|
{ |
|
... |
|
|
|
rev->topo_order = 1; |
|
rev->sort_order = REV_SORT_BY_COMMIT_DATE; |
|
|
|
... |
|
}</pre> |
|
</div> |
|
</div> |
|
<div class="paragraph"> |
|
<p>Let’s output this into a file so we can easily diff it with the walk sorted by |
|
author date.</p> |
|
</div> |
|
<div class="listingblock"> |
|
<div class="content"> |
|
<pre>$ make |
|
$ ./bin-wrappers/git walken > commit-date.txt</pre> |
|
</div> |
|
</div> |
|
<div class="paragraph"> |
|
<p>Then, let’s sort by author date and run it again.</p> |
|
</div> |
|
<div class="listingblock"> |
|
<div class="content"> |
|
<pre>static void final_rev_info_setup(int argc, const char **argv, |
|
const char *prefix, struct rev_info *rev) |
|
{ |
|
... |
|
|
|
rev->topo_order = 1; |
|
rev->sort_order = REV_SORT_BY_AUTHOR_DATE; |
|
|
|
... |
|
}</pre> |
|
</div> |
|
</div> |
|
<div class="listingblock"> |
|
<div class="content"> |
|
<pre>$ make |
|
$ ./bin-wrappers/git walken > author-date.txt</pre> |
|
</div> |
|
</div> |
|
<div class="paragraph"> |
|
<p>Finally, compare the two. This is a little less helpful without object names or |
|
dates, but hopefully we get the idea.</p> |
|
</div> |
|
<div class="listingblock"> |
|
<div class="content"> |
|
<pre>$ diff -u commit-date.txt author-date.txt</pre> |
|
</div> |
|
</div> |
|
<div class="paragraph"> |
|
<p>This display indicates that commits can be reordered after they’re written, for |
|
example with <code>git rebase</code>.</p> |
|
</div> |
|
<div class="paragraph"> |
|
<p>Let’s try one more reordering of commits. <code>rev_info</code> exposes a <code>reverse</code> flag. |
|
Set that flag somewhere inside of <code>final_rev_info_setup()</code>:</p> |
|
</div> |
|
<div class="listingblock"> |
|
<div class="content"> |
|
<pre>static void final_rev_info_setup(int argc, const char **argv, const char *prefix, |
|
struct rev_info *rev) |
|
{ |
|
... |
|
|
|
rev->reverse = 1; |
|
|
|
... |
|
}</pre> |
|
</div> |
|
</div> |
|
<div class="paragraph"> |
|
<p>Run your walk again and note the difference in order. (If you remove the grep |
|
pattern, you should see the last commit this call gives you as your current |
|
HEAD.)</p> |
|
</div> |
|
</div> |
|
</div> |
|
</div> |
|
<div class="sect1"> |
|
<h2 id="_basic_object_walk">Basic Object Walk</h2> |
|
<div class="sectionbody"> |
|
<div class="paragraph"> |
|
<p>So far we’ve been walking only commits. But Git has more types of objects than |
|
that! Let’s see if we can walk <em>all</em> objects, and find out some information |
|
about each one.</p> |
|
</div> |
|
<div class="paragraph"> |
|
<p>We can base our work on an example. <code>git pack-objects</code> prepares all kinds of |
|
objects for packing into a bitmap or packfile. The work we are interested in |
|
resides in <code>builtins/pack-objects.c:get_object_list()</code>; examination of that |
|
function shows that the all-object walk is being performed by |
|
<code>traverse_commit_list()</code> or <code>traverse_commit_list_filtered()</code>. Those two |
|
functions reside in <code>list-objects.c</code>; examining the source shows that, despite |
|
the name, these functions traverse all kinds of objects. Let’s have a look at |
|
the arguments to <code>traverse_commit_list()</code>.</p> |
|
</div> |
|
<div class="ulist"> |
|
<ul> |
|
<li> |
|
<p><code>struct rev_info *revs</code>: This is the <code>rev_info</code> used for the walk. If |
|
its <code>filter</code> member is not <code>NULL</code>, then <code>filter</code> contains information for |
|
how to filter the object list.</p> |
|
</li> |
|
<li> |
|
<p><code>show_commit_fn show_commit</code>: A callback which will be used to handle each |
|
individual commit object.</p> |
|
</li> |
|
<li> |
|
<p><code>show_object_fn show_object</code>: A callback which will be used to handle each |
|
non-commit object (so each blob, tree, or tag).</p> |
|
</li> |
|
<li> |
|
<p><code>void *show_data</code>: A context buffer which is passed in turn to <code>show_commit</code> |
|
and <code>show_object</code>.</p> |
|
</li> |
|
</ul> |
|
</div> |
|
<div class="paragraph"> |
|
<p>In addition, <code>traverse_commit_list_filtered()</code> has an additional parameter:</p> |
|
</div> |
|
<div class="ulist"> |
|
<ul> |
|
<li> |
|
<p><code>struct oidset *omitted</code>: A linked-list of object IDs which the provided |
|
filter caused to be omitted.</p> |
|
</li> |
|
</ul> |
|
</div> |
|
<div class="paragraph"> |
|
<p>It looks like these methods use callbacks we provide instead of needing us |
|
to call it repeatedly ourselves. Cool! Let’s add the callbacks first.</p> |
|
</div> |
|
<div class="paragraph"> |
|
<p>For the sake of this tutorial, we’ll simply keep track of how many of each kind |
|
of object we find. At file scope in <code>builtin/walken.c</code> add the following |
|
tracking variables:</p> |
|
</div> |
|
<div class="listingblock"> |
|
<div class="content"> |
|
<pre>static int commit_count; |
|
static int tag_count; |
|
static int blob_count; |
|
static int tree_count;</pre> |
|
</div> |
|
</div> |
|
<div class="paragraph"> |
|
<p>Commits are handled by a different callback than other objects; let’s do that |
|
one first:</p> |
|
</div> |
|
<div class="listingblock"> |
|
<div class="content"> |
|
<pre>static void walken_show_commit(struct commit *cmt, void *buf) |
|
{ |
|
commit_count++; |
|
}</pre> |
|
</div> |
|
</div> |
|
<div class="paragraph"> |
|
<p>The <code>cmt</code> argument is fairly self-explanatory. But it’s worth mentioning that |
|
the <code>buf</code> argument is actually the context buffer that we can provide to the |
|
traversal calls - <code>show_data</code>, which we mentioned a moment ago.</p> |
|
</div> |
|
<div class="paragraph"> |
|
<p>Since we have the <code>struct commit</code> object, we can look at all the same parts that |
|
we looked at in our earlier commit-only walk. For the sake of this tutorial, |
|
though, we’ll just increment the commit counter and move on.</p> |
|
</div> |
|
<div class="paragraph"> |
|
<p>The callback for non-commits is a little different, as we’ll need to check |
|
which kind of object we’re dealing with:</p> |
|
</div> |
|
<div class="listingblock"> |
|
<div class="content"> |
|
<pre>static void walken_show_object(struct object *obj, const char *str, void *buf) |
|
{ |
|
switch (obj->type) { |
|
case OBJ_TREE: |
|
tree_count++; |
|
break; |
|
case OBJ_BLOB: |
|
blob_count++; |
|
break; |
|
case OBJ_TAG: |
|
tag_count++; |
|
break; |
|
case OBJ_COMMIT: |
|
BUG("unexpected commit object in walken_show_object\n"); |
|
default: |
|
BUG("unexpected object type %s in walken_show_object\n", |
|
type_name(obj->type)); |
|
} |
|
}</pre> |
|
</div> |
|
</div> |
|
<div class="paragraph"> |
|
<p>Again, <code>obj</code> is fairly self-explanatory, and we can guess that <code>buf</code> is the same |
|
context pointer that <code>walken_show_commit()</code> receives: the <code>show_data</code> argument |
|
to <code>traverse_commit_list()</code> and <code>traverse_commit_list_filtered()</code>. Finally, |
|
<code>str</code> contains the name of the object, which ends up being something like |
|
<code>foo.txt</code> (blob), <code>bar/baz</code> (tree), or <code>v1.2.3</code> (tag).</p> |
|
</div> |
|
<div class="paragraph"> |
|
<p>To help assure us that we aren’t double-counting commits, we’ll include some |
|
complaining if a commit object is routed through our non-commit callback; we’ll |
|
also complain if we see an invalid object type. Since those two cases should be |
|
unreachable, and would only change in the event of a semantic change to the Git |
|
codebase, we complain by using <code>BUG()</code> - which is a signal to a developer that |
|
the change they made caused unintended consequences, and the rest of the |
|
codebase needs to be updated to understand that change. <code>BUG()</code> is not intended |
|
to be seen by the public, so it is not localized.</p> |
|
</div> |
|
<div class="paragraph"> |
|
<p>Our main object walk implementation is substantially different from our commit |
|
walk implementation, so let’s make a new function to perform the object walk. We |
|
can perform setup which is applicable to all objects here, too, to keep separate |
|
from setup which is applicable to commit-only walks.</p> |
|
</div> |
|
<div class="paragraph"> |
|
<p>We’ll start by enabling all types of objects in the <code>struct rev_info</code>. We’ll |
|
also turn on <code>tree_blobs_in_commit_order</code>, which means that we will walk a |
|
commit’s tree and everything it points to immediately after we find each commit, |
|
as opposed to waiting for the end and walking through all trees after the commit |
|
history has been discovered. With the appropriate settings configured, we are |
|
ready to call <code>prepare_revision_walk()</code>.</p> |
|
</div> |
|
<div class="listingblock"> |
|
<div class="content"> |
|
<pre>static void walken_object_walk(struct rev_info *rev) |
|
{ |
|
rev->tree_objects = 1; |
|
rev->blob_objects = 1; |
|
rev->tag_objects = 1; |
|
rev->tree_blobs_in_commit_order = 1; |
|
|
|
if (prepare_revision_walk(rev)) |
|
die(_("revision walk setup failed")); |
|
|
|
commit_count = 0; |
|
tag_count = 0; |
|
blob_count = 0; |
|
tree_count = 0;</pre> |
|
</div> |
|
</div> |
|
<div class="paragraph"> |
|
<p>Let’s start by calling just the unfiltered walk and reporting our counts. |
|
Complete your implementation of <code>walken_object_walk()</code>. |
|
We’ll also need to include the <code>list-objects.h</code> header.</p> |
|
</div> |
|
<div class="listingblock"> |
|
<div class="content"> |
|
<pre>#include "list-objects.h" |
|
|
|
... |
|
|
|
traverse_commit_list(rev, walken_show_commit, walken_show_object, NULL); |
|
|
|
printf("commits %d\nblobs %d\ntags %d\ntrees %d\n", commit_count, |
|
blob_count, tag_count, tree_count); |
|
}</pre> |
|
</div> |
|
</div> |
|
<div class="admonitionblock note"> |
|
<table> |
|
<tr> |
|
<td class="icon"> |
|
<div class="title">Note</div> |
|
</td> |
|
<td class="content"> |
|
This output is intended to be machine-parsed. Therefore, we are not |
|
sending it to <code>trace_printf()</code>, and we are not localizing it - we need scripts |
|
to be able to count on the formatting to be exactly the way it is shown here. |
|
If we were intending this output to be read by humans, we would need to localize |
|
it with <code>_()</code>. |
|
</td> |
|
</tr> |
|
</table> |
|
</div> |
|
<div class="paragraph"> |
|
<p>Finally, we’ll ask <code>cmd_walken()</code> to use the object walk instead. Discussing |
|
command line options is out of scope for this tutorial, so we’ll just hardcode |
|
a branch we can change at compile time. Where you call <code>final_rev_info_setup()</code> |
|
and <code>walken_commit_walk()</code>, instead branch like so:</p> |
|
</div> |
|
<div class="listingblock"> |
|
<div class="content"> |
|
<pre> if (1) { |
|
add_head_to_pending(&rev); |
|
walken_object_walk(&rev); |
|
} else { |
|
final_rev_info_setup(argc, argv, prefix, &rev); |
|
walken_commit_walk(&rev); |
|
}</pre> |
|
</div> |
|
</div> |
|
<div class="admonitionblock note"> |
|
<table> |
|
<tr> |
|
<td class="icon"> |
|
<div class="title">Note</div> |
|
</td> |
|
<td class="content"> |
|
For simplicity, we’ve avoided all the filters and sorts we applied in |
|
<code>final_rev_info_setup()</code> and simply added <code>HEAD</code> to our pending queue. If you |
|
want, you can certainly use the filters we added before by moving |
|
<code>final_rev_info_setup()</code> out of the conditional and removing the call to |
|
<code>add_head_to_pending()</code>. |
|
</td> |
|
</tr> |
|
</table> |
|
</div> |
|
<div class="paragraph"> |
|
<p>Now we can try to run our command! It should take noticeably longer than the |
|
commit walk, but an examination of the output will give you an idea why. Your |
|
output should look similar to this example, but with different counts:</p> |
|
</div> |
|
<div class="listingblock"> |
|
<div class="content"> |
|
<pre>Object walk completed. Found 55733 commits, 100274 blobs, 0 tags, and 104210 trees.</pre> |
|
</div> |
|
</div> |
|
<div class="paragraph"> |
|
<p>This makes sense. We have more trees than commits because the Git project has |
|
lots of subdirectories which can change, plus at least one tree per commit. We |
|
have no tags because we started on a commit (<code>HEAD</code>) and while tags can point to |
|
commits, commits can’t point to tags.</p> |
|
</div> |
|
<div class="admonitionblock note"> |
|
<table> |
|
<tr> |
|
<td class="icon"> |
|
<div class="title">Note</div> |
|
</td> |
|
<td class="content"> |
|
You will have different counts when you run this yourself! The number of |
|
objects grows along with the Git project. |
|
</td> |
|
</tr> |
|
</table> |
|
</div> |
|
<div class="sect2"> |
|
<h3 id="_adding_a_filter_2">Adding a Filter</h3> |
|
<div class="paragraph"> |
|
<p>There are a handful of filters that we can apply to the object walk laid out in |
|
<code>Documentation/rev-list-options.txt</code>. These filters are typically useful for |
|
operations such as creating packfiles or performing a partial clone. They are |
|
defined in <code>list-objects-filter-options.h</code>. For the purposes of this tutorial we |
|
will use the "tree:1" filter, which causes the walk to omit all trees and blobs |
|
which are not directly referenced by commits reachable from the commit in |
|
<code>pending</code> when the walk begins. (<code>pending</code> is the list of objects which need to |
|
be traversed during a walk; you can imagine a breadth-first tree traversal to |
|
help understand. In our case, that means we omit trees and blobs not directly |
|
referenced by <code>HEAD</code> or <code>HEAD</code>'s history, because we begin the walk with only |
|
<code>HEAD</code> in the <code>pending</code> list.)</p> |
|
</div> |
|
<div class="paragraph"> |
|
<p>For now, we are not going to track the omitted objects, so we’ll replace those |
|
parameters with <code>NULL</code>. For the sake of simplicity, we’ll add a simple |
|
build-time branch to use our filter or not. Preface the line calling |
|
<code>traverse_commit_list()</code> with the following, which will remind us which kind of |
|
walk we’ve just performed:</p> |
|
</div> |
|
<div class="listingblock"> |
|
<div class="content"> |
|
<pre> if (0) { |
|
/* Unfiltered: */ |
|
trace_printf(_("Unfiltered object walk.\n")); |
|
} else { |
|
trace_printf( |
|
_("Filtered object walk with filterspec 'tree:1'.\n")); |
|
CALLOC_ARRAY(rev->filter, 1); |
|
parse_list_objects_filter(rev->filter, "tree:1"); |
|
} |
|
traverse_commit_list(rev, walken_show_commit, |
|
walken_show_object, NULL);</pre> |
|
</div> |
|
</div> |
|
<div class="paragraph"> |
|
<p>The <code>rev->filter</code> member is usually built directly from a command |
|
line argument, so the module provides an easy way to build one from a string. |
|
Even though we aren’t taking user input right now, we can still build one with |
|
a hardcoded string using <code>parse_list_objects_filter()</code>.</p> |
|
</div> |
|
<div class="paragraph"> |
|
<p>With the filter spec "tree:1", we are expecting to see <em>only</em> the root tree for |
|
each commit; therefore, the tree object count should be less than or equal to |
|
the number of commits. (For an example of why that’s true: <code>git commit --revert</code> |
|
points to the same tree object as its grandparent.)</p> |
|
</div> |
|
</div> |
|
<div class="sect2"> |
|
<h3 id="_counting_omitted_objects">Counting Omitted Objects</h3> |
|
<div class="paragraph"> |
|
<p>We also have the capability to enumerate all objects which were omitted by a |
|
filter, like with <code>git log --filter=<spec> --filter-print-omitted</code>. Asking |
|
<code>traverse_commit_list_filtered()</code> to populate the <code>omitted</code> list means that our |
|
object walk does not perform any better than an unfiltered object walk; all |
|
reachable objects are walked in order to populate the list.</p> |
|
</div> |
|
<div class="paragraph"> |
|
<p>First, add the <code>struct oidset</code> and related items we will use to iterate it:</p> |
|
</div> |
|
<div class="listingblock"> |
|
<div class="content"> |
|
<pre>static void walken_object_walk( |
|
... |
|
|
|
struct oidset omitted; |
|
struct oidset_iter oit; |
|
struct object_id *oid = NULL; |
|
int omitted_count = 0; |
|
oidset_init(&omitted, 0); |
|
|
|
...</pre> |
|
</div> |
|
</div> |
|
<div class="paragraph"> |
|
<p>Modify the call to <code>traverse_commit_list_filtered()</code> to include your <code>omitted</code> |
|
object:</p> |
|
</div> |
|
<div class="listingblock"> |
|
<div class="content"> |
|
<pre> ... |
|
|
|
traverse_commit_list_filtered(rev, |
|
walken_show_commit, walken_show_object, NULL, &omitted); |
|
|
|
...</pre> |
|
</div> |
|
</div> |
|
<div class="paragraph"> |
|
<p>Then, after your traversal, the <code>oidset</code> traversal is pretty straightforward. |
|
Count all the objects within and modify the print statement:</p> |
|
</div> |
|
<div class="listingblock"> |
|
<div class="content"> |
|
<pre> /* Count the omitted objects. */ |
|
oidset_iter_init(&omitted, &oit); |
|
|
|
while ((oid = oidset_iter_next(&oit))) |
|
omitted_count++; |
|
|
|
printf("commits %d\nblobs %d\ntags %d\ntrees %d\nomitted %d\n", |
|
commit_count, blob_count, tag_count, tree_count, omitted_count);</pre> |
|
</div> |
|
</div> |
|
<div class="paragraph"> |
|
<p>By running your walk with and without the filter, you should find that the total |
|
object count in each case is identical. You can also time each invocation of |
|
the <code>walken</code> subcommand, with and without <code>omitted</code> being passed in, to confirm |
|
to yourself the runtime impact of tracking all omitted objects.</p> |
|
</div> |
|
</div> |
|
<div class="sect2"> |
|
<h3 id="_changing_the_order_2">Changing the Order</h3> |
|
<div class="paragraph"> |
|
<p>Finally, let’s demonstrate that you can also reorder walks of all objects, not |
|
just walks of commits. First, we’ll make our handlers chattier - modify |
|
<code>walken_show_commit()</code> and <code>walken_show_object()</code> to print the object as they |
|
go:</p> |
|
</div> |
|
<div class="listingblock"> |
|
<div class="content"> |
|
<pre>static void walken_show_commit(struct commit *cmt, void *buf) |
|
{ |
|
trace_printf("commit: %s\n", oid_to_hex(&cmt->object.oid)); |
|
commit_count++; |
|
} |
|
|
|
static void walken_show_object(struct object *obj, const char *str, void *buf) |
|
{ |
|
trace_printf("%s: %s\n", type_name(obj->type), oid_to_hex(&obj->oid)); |
|
|
|
... |
|
}</pre> |
|
</div> |
|
</div> |
|
<div class="admonitionblock note"> |
|
<table> |
|
<tr> |
|
<td class="icon"> |
|
<div class="title">Note</div> |
|
</td> |
|
<td class="content"> |
|
Since we will be examining this output directly as humans, we’ll use |
|
<code>trace_printf()</code> here. Additionally, since this change introduces a significant |
|
number of printed lines, using <code>trace_printf()</code> will allow us to easily silence |
|
those lines without having to recompile. |
|
</td> |
|
</tr> |
|
</table> |
|
</div> |
|
<div class="paragraph"> |
|
<p>(Leave the counter increment logic in place.)</p> |
|
</div> |
|
<div class="paragraph"> |
|
<p>With only that change, run again (but save yourself some scrollback):</p> |
|
</div> |
|
<div class="listingblock"> |
|
<div class="content"> |
|
<pre>$ GIT_TRACE=1 ./bin-wrappers/git walken | head -n 10</pre> |
|
</div> |
|
</div> |
|
<div class="paragraph"> |
|
<p>Take a look at the top commit with <code>git show</code> and the object ID you printed; it |
|
should be the same as the output of <code>git show HEAD</code>.</p> |
|
</div> |
|
<div class="paragraph"> |
|
<p>Next, let’s change a setting on our <code>struct rev_info</code> within |
|
<code>walken_object_walk()</code>. Find where you’re changing the other settings on <code>rev</code>, |
|
such as <code>rev->tree_objects</code> and <code>rev->tree_blobs_in_commit_order</code>, and add the |
|
<code>reverse</code> setting at the bottom:</p> |
|
</div> |
|
<div class="listingblock"> |
|
<div class="content"> |
|
<pre> ... |
|
|
|
rev->tree_objects = 1; |
|
rev->blob_objects = 1; |
|
rev->tag_objects = 1; |
|
rev->tree_blobs_in_commit_order = 1; |
|
rev->reverse = 1; |
|
|
|
...</pre> |
|
</div> |
|
</div> |
|
<div class="paragraph"> |
|
<p>Now, run again, but this time, let’s grab the last handful of objects instead |
|
of the first handful:</p> |
|
</div> |
|
<div class="listingblock"> |
|
<div class="content"> |
|
<pre>$ make |
|
$ GIT_TRACE=1 ./bin-wrappers git walken | tail -n 10</pre> |
|
</div> |
|
</div> |
|
<div class="paragraph"> |
|
<p>The last commit object given should have the same OID as the one we saw at the |
|
top before, and running <code>git show <oid></code> with that OID should give you again |
|
the same results as <code>git show HEAD</code>. Furthermore, if you run and examine the |
|
first ten lines again (with <code>head</code> instead of <code>tail</code> like we did before applying |
|
the <code>reverse</code> setting), you should see that now the first commit printed is the |
|
initial commit, <code>e83c5163</code>.</p> |
|
</div> |
|
</div> |
|
</div> |
|
</div> |
|
<div class="sect1"> |
|
<h2 id="_wrapping_up">Wrapping Up</h2> |
|
<div class="sectionbody"> |
|
<div class="paragraph"> |
|
<p>Let’s review. In this tutorial, we:</p> |
|
</div> |
|
<div class="ulist"> |
|
<ul> |
|
<li> |
|
<p>Built a commit walk from the ground up</p> |
|
</li> |
|
<li> |
|
<p>Enabled a grep filter for that commit walk</p> |
|
</li> |
|
<li> |
|
<p>Changed the sort order of that filtered commit walk</p> |
|
</li> |
|
<li> |
|
<p>Built an object walk (tags, commits, trees, and blobs) from the ground up</p> |
|
</li> |
|
<li> |
|
<p>Learned how to add a filter-spec to an object walk</p> |
|
</li> |
|
<li> |
|
<p>Changed the display order of the filtered object walk</p> |
|
</li> |
|
</ul> |
|
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