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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="_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>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><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). | |
| We’ll need to include the <code>parse-options.h</code> header.</p></div> | |
| <div class="listingblock"> | |
| <div class="content"> | |
| <pre><code>#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); | |
| ... | |
| }</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>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="_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> | |
| <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>. | |
| We’ll also need to include the <code>config.h</code> header:</p></div> | |
| <div class="listingblock"> | |
| <div class="content"> | |
| <pre><code>#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); | |
| }</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. | |
| We’ll also need to include the <code>revision.h</code> header:</p></div> | |
| <div class="listingblock"> | |
| <div class="content"> | |
| <pre><code>#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); | |
| ... | |
| }</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))) { | |
| 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>grep_config()</code> to <code>git_walken_config()</code>:</p></div> | |
| <div class="listingblock"> | |
| <div class="content"> | |
| <pre><code>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()</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><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>. | |
| We’ll also need to include the <code>list-objects.h</code> header.</p></div> | |
| <div class="listingblock"> | |
| <div class="content"> | |
| <pre><code>#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); | |
| }</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>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><code> 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);</code></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><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(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 | |
| 2023-04-17 17:57:53 PDT | |
| </div> | |
| </div> | |
| </body> | |
| </html> | |