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| author | Junio C Hamano <gitster@pobox.com> | 2019-11-11 13:33:46 +0900 |
|---|---|---|
| committer | Junio C Hamano <gitster@pobox.com> | 2019-11-11 13:33:46 +0900 |
| commit | 8ac8a3d8dd001611b77776fa19ef37c2c87cd987 (patch) | |
| tree | efa3af5ebd3192b611732553c90c2d9baaa1e398 /MyFirstObjectWalk.html | |
| parent | 79d41dbe0b2fcbbc1cd5f6c188073a0956174289 (diff) | |
| download | git-htmldocs-8ac8a3d8dd001611b77776fa19ef37c2c87cd987.tar.gz | |
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+<div id="header">
+<h1>My First Object Walk</h1>
+</div>
+<div id="content">
+<div class="sect1">
+<h2 id="_what_8217_s_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>Documentation/technical/api-revision-walking.txt</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><code>git checkout -b revwalk origin/master</code></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><code>/*
+ * "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;
+}</code></pre>
+</div></div>
+<div class="admonitionblock">
+<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).</p></div>
+<div class="listingblock">
+<div class="content">
+<pre><code>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);
+
+ ...
+}</code></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><code>int cmd_walken(int argc, const char **argv, const char *prefix);</code></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><code>{ "walken", cmd_walken, RUN_SETUP },</code></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><code>BUILTIN_OBJS += builtin/walken.o</code></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><code>$ echo DEVELOPER=1 >>config.mak
+$ make
+$ GIT_TRACE=1 ./bin-wrappers/git walken</code></pre>
+</div></div>
+<div class="admonitionblock">
+<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">
+<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">https://github.com/nasamuffin/git/tree/revwalk</a>.</td>
+</tr></table>
+</div>
+<div class="sect2">
+<h3 id="_code_struct_rev_cmdline_info_code"><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>Documentation/technical/api-allocation-growing.txt</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="_code_struct_rev_info_code"><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 class="paragraph"><p>For our first example within <code>git walken</code>, we don’t intend to use any other
+components within Git, and we don’t have any configuration to do. However, we
+may want to add some later, so for now, we can add an empty placeholder. Create
+a new function in <code>builtin/walken.c</code>:</p></div>
+<div class="listingblock">
+<div class="content">
+<pre><code>static void init_walken_defaults(void)
+{
+ /*
+ * We don't actually need the same components `git log` does; leave this
+ * empty for now.
+ */
+}</code></pre>
+</div></div>
+<div class="paragraph"><p>Make sure to add a line invoking it inside of <code>cmd_walken()</code>.</p></div>
+<div class="listingblock">
+<div class="content">
+<pre><code>int cmd_walken(int argc, const char **argv, const char *prefix)
+{
+ init_walken_defaults();
+}</code></pre>
+</div></div>
+</div>
+<div class="sect3">
+<h4 id="_configuring_from_code_gitconfig_code">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>:</p></div>
+<div class="listingblock">
+<div class="content">
+<pre><code>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);
+}</code></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><code>int cmd_walken(int argc, const char **argv, const char *prefix)
+{
+ ...
+
+ git_config(git_walken_config, NULL);
+
+ ...
+}</code></pre>
+</div></div>
+</div>
+<div class="sect3">
+<h4 id="_setting_up_code_rev_info_code">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:</p></div>
+<div class="listingblock">
+<div class="content">
+<pre><code>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);
+
+ ...
+}</code></pre>
+</div></div>
+</div>
+<div class="sect3">
+<h4 id="_tweaking_code_rev_info_code_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><code>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);
+}</code></pre>
+</div></div>
+<div class="admonitionblock">
+<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><code> struct setup_revision_opt opt;
+
+ memset(&opt, 0, sizeof(opt));
+ opt.def = "HEAD";
+ opt.revarg_opt = REVARG_COMMITTISH;
+ setup_revisions(argc, argv, rev, &opt);</code></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><code>int cmd_walken(int argc, const char **argv, const char *prefix)
+{
+ ...
+
+ final_rev_info_setup(&rev);
+
+ ...
+}</code></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_code_rev_info_code_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><code>static void walken_commit_walk(struct rev_info *rev)
+{
+ if (prepare_revision_walk(rev))
+ die(_("revision walk setup failed"));
+}</code></pre>
+</div></div>
+<div class="admonitionblock">
+<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><code>static void walken_commit_walk(struct rev_info *rev)
+{
+ struct commit *commit;
+ struct strbuf prettybuf = STRBUF_INIT;
+
+ ...
+
+ while ((commit = get_revision(rev))) {
+ if (!commit)
+ continue;
+
+ strbuf_reset(&prettybuf);
+ pp_commit_easy(CMIT_FMT_ONELINE, commit, &prettybuf);
+ puts(prettybuf.buf);
+ }
+ strbuf_release(&prettybuf);
+}</code></pre>
+</div></div>
+<div class="admonitionblock">
+<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><code>$ make
+$ ./bin-wrappers/git walken</code></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>init_grep_defaults()</code> to <code>init_walken_defaults()</code> and add
+<code>grep_config()</code> to <code>git_walken_config()</code>:</p></div>
+<div class="listingblock">
+<div class="content">
+<pre><code>static void init_walken_defaults(void)
+{
+ init_grep_defaults(the_repository);
+}
+
+...
+
+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);
+}</code></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><code>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);
+
+ ...
+}</code></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">
+<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">
+<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><code>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;
+
+ ...
+}</code></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><code>$ make
+$ ./bin-wrappers/git walken > commit-date.txt</code></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><code>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;
+
+ ...
+}</code></pre>
+</div></div>
+<div class="listingblock">
+<div class="content">
+<pre><code>$ make
+$ ./bin-wrappers/git walken > author-date.txt</code></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><code>$ diff -u commit-date.txt author-date.txt</code></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><code>static void final_rev_info_setup(int argc, const char **argv, const char *prefix,
+ struct rev_info *rev)
+{
+ ...
+
+ rev->reverse = 1;
+
+ ...
+}</code></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_filtered()</code>, which are a superset of the
+arguments to the unfiltered version.</p></div>
+<div class="ulist"><ul>
+<li>
+<p>
+<code>struct list_objects_filter_options *filter_options</code>: This is a struct which
+ stores a filter-spec as outlined in <code>Documentation/rev-list-options.txt</code>.
+</p>
+</li>
+<li>
+<p>
+<code>struct rev_info *revs</code>: This is the <code>rev_info</code> used for the walk.
+</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>
+<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 this <code>traverse_commit_list_filtered()</code> uses 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><code>static int commit_count;
+static int tag_count;
+static int blob_count;
+static int tree_count;</code></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><code>static void walken_show_commit(struct commit *cmt, void *buf)
+{
+ commit_count++;
+}</code></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><code>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));
+ }
+}</code></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><code>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;</code></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>:</p></div>
+<div class="listingblock">
+<div class="content">
+<pre><code> 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);
+}</code></pre>
+</div></div>
+<div class="admonitionblock">
+<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><code> if (1) {
+ add_head_to_pending(&rev);
+ walken_object_walk(&rev);
+ } else {
+ final_rev_info_setup(argc, argv, prefix, &rev);
+ walken_commit_walk(&rev);
+ }</code></pre>
+</div></div>
+<div class="admonitionblock">
+<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><code>Object walk completed. Found 55733 commits, 100274 blobs, 0 tags, and 104210 trees.</code></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">
+<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>First, we’ll need to <code>#include "list-objects-filter-options.h</code>" and set up the
+<code>struct list_objects_filter_options</code> at the top of the function.</p></div>
+<div class="listingblock">
+<div class="content">
+<pre><code>static void walken_object_walk(struct rev_info *rev)
+{
+ struct list_objects_filter_options filter_options = {};
+
+ ...</code></pre>
+</div></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. Replace 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><code> if (0) {
+ /* Unfiltered: */
+ trace_printf(_("Unfiltered object walk.\n"));
+ traverse_commit_list(rev, walken_show_commit,
+ walken_show_object, NULL);
+ } else {
+ trace_printf(
+ _("Filtered object walk with filterspec 'tree:1'.\n"));
+ parse_list_objects_filter(&filter_options, "tree:1");
+
+ traverse_commit_list_filtered(&filter_options, rev,
+ walken_show_commit, walken_show_object, NULL, NULL);
+ }</code></pre>
+</div></div>
+<div class="paragraph"><p><code>struct list_objects_filter_options</code> 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><code>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);
+
+ ...</code></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><code> ...
+
+ traverse_commit_list_filtered(&filter_options, rev,
+ walken_show_commit, walken_show_object, NULL, &omitted);
+
+ ...</code></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><code> /* 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);</code></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><code>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));
+
+ ...
+}</code></pre>
+</div></div>
+<div class="admonitionblock">
+<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><code>$ GIT_TRACE=1 ./bin-wrappers/git walken | head -n 10</code></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><code> ...
+
+ rev->tree_objects = 1;
+ rev->blob_objects = 1;
+ rev->tag_objects = 1;
+ rev->tree_blobs_in_commit_order = 1;
+ rev->reverse = 1;
+
+ ...</code></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><code>$ make
+$ GIT_TRACE=1 ./bin-wrappers git walken | tail -n 10</code></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></div>
+</div>
+</div>
+</div>
+<div id="footnotes"><hr /></div>
+<div id="footer">
+<div id="footer-text">
+Last updated
+ 2019-11-11 13:29:13 JST
+</div>
+</div>
+</body>
+</html>
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