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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</code> <code>log</code>.</p> |
| </div> |
| <div class="sect2"> |
| <h3 id="_related_reading">Related Reading</h3> |
| <div class="ulist"> |
| <ul> |
| <li> |
| <p><code>Documentation/user-manual.adoc</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</code> <code>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" |
| #include "trace.h" |
| |
| int cmd_walken(int argc, const char **argv, const char *prefix, struct repository *repo) |
| { |
| 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>(), defined in <code>trace.h</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_version</code>():</p> |
| </div> |
| <div class="listingblock"> |
| <div class="content"> |
| <pre>int cmd_walken(int argc, const char **argv, const char *prefix, struct repository *repo);</pre> |
| </div> |
| </div> |
| <div class="paragraph"> |
| <p>Include the command in <code>git.c</code> in <code>commands</code>[] near the entry for <code>version</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 an entry for the new command in the both the Make and Meson build system, |
| before the entry for <code>worktree</code>:</p> |
| </div> |
| <div class="ulist"> |
| <ul> |
| <li> |
| <p>In the <code>Makefile</code>:</p> |
| </li> |
| </ul> |
| </div> |
| <div class="listingblock"> |
| <div class="content"> |
| <pre>... |
| BUILTIN_OBJS += builtin/walken.o |
| ...</pre> |
| </div> |
| </div> |
| <div class="ulist"> |
| <ul> |
| <li> |
| <p>In the <code>meson.build</code> file:</p> |
| </li> |
| </ul> |
| </div> |
| <div class="listingblock"> |
| <div class="content"> |
| <pre>builtin_sources = [ |
| ... |
| 'builtin/walken.c', |
| ... |
| ]</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.adoc</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</code> <code>rev_cmdline_info</code></h3> |
| <div class="paragraph"> |
| <p>The definition of <code>struct</code> <code>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>alloc.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.adoc</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.adoc</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 #include`s <code>in</code> `revision.<code>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</code> <code>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</code> <code>rev_info</code> have a mirror in <code>Documentation/rev-list-options.adoc</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</code> <code>log</code> <code>--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</code> <code>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</code> <code>config</code>). This is done by providing a |
| callback to <code>repo_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, |
| const struct config_context *ctx, void *cb) |
| { |
| /* |
| * For now, we don't have any custom configuration, so fall back to |
| * the default config. |
| */ |
| return git_default_config(var, value, ctx, cb); |
| }</pre> |
| </div> |
| </div> |
| <div class="paragraph"> |
| <p>Make sure to invoke <code>repo_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, struct repository *repo) |
| { |
| ... |
| |
| repo_config(repo, 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</code> <code>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, struct repository *repo) |
| { |
| /* This can go wherever you like in your declarations.*/ |
| struct rev_info rev; |
| ... |
| |
| /* This should go after the repo_config() call. */ |
| repo_init_revisions(repo, &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, struct repository *repo) |
| { |
| ... |
| |
| 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>#include "pretty.h" |
| |
| ... |
| |
| 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</code> <code>log</code> <code>--author=</code><em><pattern></em>. We can add a filter by |
| modifying <code>rev_info.grep_filter</code>, which is a <code>struct</code> <code>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, |
| const struct config_context *ctx, void *cb) |
| { |
| grep_config(var, value, ctx, cb); |
| return git_default_config(var, value, ctx, 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</code> <code>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</code> <code>grep_context</code> and |
| <code>enum</code> <code>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</code> <code>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</code> <code>log</code> <code>--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</code> <code>help</code> <code>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</code> <code>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</code> <code>pack-objects</code> prepares all kinds of |
| objects for packing into a bitmap or packfile. The work we are interested in |
| resides in <code>builtin/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</code> <code>rev_info</code> <code>*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</code> <code>show_commit</code>: A callback which will be used to handle each |
| individual commit object.</p> |
| </li> |
| <li> |
| <p><code>show_object_fn</code> <code>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</code> <code>*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</code> <code>oidset</code> <code>*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</code> <code>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</code> <code>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.adoc</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")); |
| |
| 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-</code>><code>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</code> <code>commit</code> <code>--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</code> <code>log</code> <code>--filter=</code><em><spec></em> <code>--filter-print-omitted</code>. To do this, |
| change <code>traverse_commit_list</code>() to <code>traverse_commit_list_filtered</code>(), which is |
| able to populate an <code>omitted</code> list. Asking for this list of filtered objects |
| may cause performance degradations, however, because in this case, despite |
| filtering objects, the possibly much larger set of all reachable objects must |
| be processed in order to populate that list.</p> |
| </div> |
| <div class="paragraph"> |
| <p>First, add the <code>struct</code> <code>oidset</code> and related items we will use to iterate it:</p> |
| </div> |
| <div class="listingblock"> |
| <div class="content"> |
| <pre>#include "oidset.h" |
| |
| ... |
| |
| 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>Replace the call to <code>traverse_commit_list</code>() with |
| <code>traverse_commit_list_filtered</code>() and pass a pointer to the <code>omitted</code> oidset |
| defined and initialized above:</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>#include "hex.h" |
| |
| ... |
| |
| 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 2>&1 | head -n 10</pre> |
| </div> |
| </div> |
| <div class="paragraph"> |
| <p>Take a look at the top commit with <code>git</code> <code>show</code> and the object ID you printed; it |
| should be the same as the output of <code>git</code> <code>show</code> <code>HEAD</code>.</p> |
| </div> |
| <div class="paragraph"> |
| <p>Next, let’s change a setting on our <code>struct</code> <code>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-</code>><code>tree_objects</code> and <code>rev-</code>><code>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 2>&1 | 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</code> <code>show</code> <em><oid></em> with that OID should give you again |
| the same results as <code>git</code> <code>show</code> <code>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="footer"> |
| <div id="footer-text"> |
| Last updated 2025-06-30 16:59:08 -0700 |
| </div> |
| </div> |
| </body> |
| </html> |
