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+<body class="article">
+<div id="header">
+<h1>Unit Testing</h1>
+<span id="revdate">2023-12-09</span>
+</div>
+<div id="content">
+<div id="preamble">
+<div class="sectionbody">
+<div class="paragraph"><p>In our current testing environment, we spend a significant amount of effort
+crafting end-to-end tests for error conditions that could easily be captured by
+unit tests (or we simply forgo some hard-to-setup and rare error conditions).
+Unit tests additionally provide stability to the codebase and can simplify
+debugging through isolation. Writing unit tests in pure C, rather than with our
+current shell/test-tool helper setup, simplifies test setup, simplifies passing
+data around (no shell-isms required), and reduces testing runtime by not
+spawning a separate process for every test invocation.</p></div>
+<div class="paragraph"><p>We believe that a large body of unit tests, living alongside the existing test
+suite, will improve code quality for the Git project.</p></div>
+</div>
+</div>
+<div class="sect1">
+<h2 id="_definitions">Definitions</h2>
+<div class="sectionbody">
+<div class="paragraph"><p>For the purposes of this document, we&#8217;ll use <strong>test framework</strong> to refer to
+projects that support writing test cases and running tests within the context
+of a single executable. <strong>Test harness</strong> will refer to projects that manage
+running multiple executables (each of which may contain multiple test cases) and
+aggregating their results.</p></div>
+<div class="paragraph"><p>In reality, these terms are not strictly defined, and many of the projects
+discussed below contain features from both categories.</p></div>
+<div class="paragraph"><p>For now, we will evaluate projects solely on their framework features. Since we
+are relying on having TAP output (see below), we can assume that any framework
+can be made to work with a harness that we can choose later.</p></div>
+</div>
+</div>
+<div class="sect1">
+<h2 id="_summary">Summary</h2>
+<div class="sectionbody">
+<div class="paragraph"><p>We believe the best way forward is to implement a custom TAP framework for the
+Git project. We use a version of the framework originally proposed in
+<a href="https://lore.kernel.org/git/c902a166-98ce-afba-93f2-ea6027557176@gmail.com/">1</a>.</p></div>
+<div class="paragraph"><p>See the <a href="#framework-selection">Framework Selection</a> section below for the
+rationale behind this decision.</p></div>
+</div>
+</div>
+<div class="sect1">
+<h2 id="_choosing_a_test_harness">Choosing a test harness</h2>
+<div class="sectionbody">
+<div class="paragraph"><p>During upstream discussion, it was occasionally noted that <code>prove</code> provides many
+convenient features, such as scheduling slower tests first, or re-running
+previously failed tests.</p></div>
+<div class="paragraph"><p>While we already support the use of <code>prove</code> as a test harness for the shell
+tests, it is not strictly required. The t/Makefile allows running shell tests
+directly (though with interleaved output if parallelism is enabled). Git
+developers who wish to use <code>prove</code> as a more advanced harness can do so by
+setting DEFAULT_TEST_TARGET=prove in their config.mak.</p></div>
+<div class="paragraph"><p>We will follow a similar approach for unit tests: by default the test
+executables will be run directly from the t/Makefile, but <code>prove</code> can be
+configured with DEFAULT_UNIT_TEST_TARGET=prove.</p></div>
+</div>
+</div>
+<div class="sect1">
+<h2 id="framework-selection">Framework selection</h2>
+<div class="sectionbody">
+<div class="paragraph"><p>There are a variety of features we can use to rank the candidate frameworks, and
+those features have different priorities:</p></div>
+<div class="ulist"><ul>
+<li>
+<p>
+Critical features: we probably won&#8217;t consider a framework without these
+</p>
+<div class="ulist"><ul>
+<li>
+<p>
+Can we legally / easily use the project?
+</p>
+<div class="ulist"><ul>
+<li>
+<p>
+<a href="#license">License</a>
+</p>
+</li>
+<li>
+<p>
+<a href="#vendorable-or-ubiquitous">Vendorable or ubiquitous</a>
+</p>
+</li>
+<li>
+<p>
+<a href="#maintainable-extensible">Maintainable / extensible</a>
+</p>
+</li>
+<li>
+<p>
+<a href="#major-platform-support">Major platform support</a>
+</p>
+</li>
+</ul></div>
+</li>
+<li>
+<p>
+Does the project support our bare-minimum needs?
+</p>
+<div class="ulist"><ul>
+<li>
+<p>
+<a href="#tap-support">TAP support</a>
+</p>
+</li>
+<li>
+<p>
+<a href="#diagnostic-output">Diagnostic output</a>
+</p>
+</li>
+<li>
+<p>
+<a href="#runtime-skippable-tests">Runtime-skippable tests</a>
+</p>
+</li>
+</ul></div>
+</li>
+</ul></div>
+</li>
+<li>
+<p>
+Nice-to-have features:
+</p>
+<div class="ulist"><ul>
+<li>
+<p>
+<a href="#parallel-execution">Parallel execution</a>
+</p>
+</li>
+<li>
+<p>
+<a href="#mock-support">Mock support</a>
+</p>
+</li>
+<li>
+<p>
+<a href="#signal-error-handling">Signal &amp; error-handling</a>
+</p>
+</li>
+</ul></div>
+</li>
+<li>
+<p>
+Tie-breaker stats
+</p>
+<div class="ulist"><ul>
+<li>
+<p>
+<a href="#project-kloc">Project KLOC</a>
+</p>
+</li>
+<li>
+<p>
+<a href="#adoption">Adoption</a>
+</p>
+</li>
+</ul></div>
+</li>
+</ul></div>
+<div class="sect2">
+<h3 id="license">License</h3>
+<div class="paragraph"><p>We must be able to legally use the framework in connection with Git. As Git is
+licensed only under GPLv2, we must eliminate any LGPLv3, GPLv3, or Apache 2.0
+projects.</p></div>
+</div>
+<div class="sect2">
+<h3 id="vendorable-or-ubiquitous">Vendorable or ubiquitous</h3>
+<div class="paragraph"><p>We want to avoid forcing Git developers to install new tools just to run unit
+tests. Any prospective frameworks and harnesses must either be vendorable
+(meaning, we can copy their source directly into Git&#8217;s repository), or so
+ubiquitous that it is reasonable to expect that most developers will have the
+tools installed already.</p></div>
+</div>
+<div class="sect2">
+<h3 id="maintainable-extensible">Maintainable / extensible</h3>
+<div class="paragraph"><p>It is unlikely that any pre-existing project perfectly fits our needs, so any
+project we select will need to be actively maintained and open to accepting
+changes. Alternatively, assuming we are vendoring the source into our repo, it
+must be simple enough that Git developers can feel comfortable making changes as
+needed to our version.</p></div>
+<div class="paragraph"><p>In the comparison table below, "True" means that the framework seems to have
+active developers, that it is simple enough that Git developers can make changes
+to it, and that the project seems open to accepting external contributions (or
+that it is vendorable). "Partial" means that at least one of the above
+conditions holds.</p></div>
+</div>
+<div class="sect2">
+<h3 id="major-platform-support">Major platform support</h3>
+<div class="paragraph"><p>At a bare minimum, unit-testing must work on Linux, MacOS, and Windows.</p></div>
+<div class="paragraph"><p>In the comparison table below, "True" means that it works on all three major
+platforms with no issues. "Partial" means that there may be annoyances on one or
+more platforms, but it is still usable in principle.</p></div>
+</div>
+<div class="sect2">
+<h3 id="tap-support">TAP support</h3>
+<div class="paragraph"><p>The <a href="https://testanything.org/">Test Anything Protocol</a> is a text-based interface
+that allows tests to communicate with a test harness. It is already used by
+Git&#8217;s integration test suite. Supporting TAP output is a mandatory feature for
+any prospective test framework.</p></div>
+<div class="paragraph"><p>In the comparison table below, "True" means this is natively supported.
+"Partial" means TAP output must be generated by post-processing the native
+output.</p></div>
+<div class="paragraph"><p>Frameworks that do not have at least Partial support will not be evaluated
+further.</p></div>
+</div>
+<div class="sect2">
+<h3 id="diagnostic-output">Diagnostic output</h3>
+<div class="paragraph"><p>When a test case fails, the framework must generate enough diagnostic output to
+help developers find the appropriate test case in source code in order to debug
+the failure.</p></div>
+</div>
+<div class="sect2">
+<h3 id="runtime-skippable-tests">Runtime-skippable tests</h3>
+<div class="paragraph"><p>Test authors may wish to skip certain test cases based on runtime circumstances,
+so the framework should support this.</p></div>
+</div>
+<div class="sect2">
+<h3 id="parallel-execution">Parallel execution</h3>
+<div class="paragraph"><p>Ideally, we will build up a significant collection of unit test cases, most
+likely split across multiple executables. It will be necessary to run these
+tests in parallel to enable fast develop-test-debug cycles.</p></div>
+<div class="paragraph"><p>In the comparison table below, "True" means that individual test cases within a
+single test executable can be run in parallel. We assume that executable-level
+parallelism can be handled by the test harness.</p></div>
+</div>
+<div class="sect2">
+<h3 id="mock-support">Mock support</h3>
+<div class="paragraph"><p>Unit test authors may wish to test code that interacts with objects that may be
+inconvenient to handle in a test (e.g. interacting with a network service).
+Mocking allows test authors to provide a fake implementation of these objects
+for more convenient tests.</p></div>
+</div>
+<div class="sect2">
+<h3 id="signal-error-handling">Signal &amp; error handling</h3>
+<div class="paragraph"><p>The test framework should fail gracefully when test cases are themselves buggy
+or when they are interrupted by signals during runtime.</p></div>
+</div>
+<div class="sect2">
+<h3 id="project-kloc">Project KLOC</h3>
+<div class="paragraph"><p>The size of the project, in thousands of lines of code as measured by
+<a href="https://dwheeler.com/sloccount/">sloccount</a> (rounded up to the next multiple of
+1,000). As a tie-breaker, we probably prefer a project with fewer LOC.</p></div>
+</div>
+<div class="sect2">
+<h3 id="adoption">Adoption</h3>
+<div class="paragraph"><p>As a tie-breaker, we prefer a more widely-used project. We use the number of
+GitHub / GitLab stars to estimate this.</p></div>
+</div>
+<div class="sect2">
+<h3 id="_comparison">Comparison</h3>
+<div class="tableblock">
+<table rules="all"
+width="33%"
+frame="border"
+cellspacing="0" cellpadding="4">
+<col width="4%" />
+<col width="4%" />
+<col width="4%" />
+<col width="4%" />
+<col width="4%" />
+<col width="4%" />
+<col width="4%" />
+<col width="4%" />
+<col width="4%" />
+<col width="4%" />
+<col width="4%" />
+<col width="4%" />
+<col width="4%" />
+<col width="4%" />
+<col width="4%" />
+<col width="4%" />
+<col width="4%" />
+<col width="4%" />
+<col width="4%" />
+<col width="4%" />
+<col width="4%" />
+<col width="4%" />
+<col width="4%" />
+<col width="4%" />
+<col width="4%" />
+<thead>
+<tr>
+<th align="left" valign="top">Framework</th>
+<th align="left" valign="top"><a href="#license">License</a></th>
+<th align="left" valign="top"><a href="#vendorable-or-ubiquitous">Vendorable or ubiquitous</a></th>
+<th align="left" valign="top"><a href="#maintainable-extensible">Maintainable / extensible</a></th>
+<th align="left" valign="top"><a href="#major-platform-support">Major platform support</a></th>
+<th align="left" valign="top"><a href="#tap-support">TAP support</a></th>
+<th align="left" valign="top"><a href="#diagnostic-output">Diagnostic output</a></th>
+<th align="left" valign="top"><a href="#runtime--skippable-tests">Runtime- skippable tests</a></th>
+<th align="left" valign="top"><a href="#parallel-execution">Parallel execution</a></th>
+<th align="left" valign="top"><a href="#mock-support">Mock support</a></th>
+<th align="left" valign="top"><a href="#signal-error-handling">Signal &amp; error handling</a></th>
+<th align="left" valign="top"><a href="#project-kloc">Project KLOC</a></th>
+<th align="left" valign="top"><a href="#adoption">Adoption</a></th>
+</tr>
+</thead>
+<tbody>
+<tr>
+<td align="left" valign="top"><p class="table"><a href="https://lore.kernel.org/git/c902a166-98ce-afba-93f2-ea6027557176@gmail.com/">Custom Git impl.</a></p></td>
+<td align="left" valign="top"><p class="table"><span class="lime-background">GPL v2</span></p></td>
+<td align="left" valign="top"><p class="table"><span class="lime-background">True</span></p></td>
+<td align="left" valign="top"><p class="table"><span class="lime-background">True</span></p></td>
+<td align="left" valign="top"><p class="table"><span class="lime-background">True</span></p></td>
+<td align="left" valign="top"><p class="table"><span class="lime-background">True</span></p></td>
+<td align="left" valign="top"><p class="table"><span class="lime-background">True</span></p></td>
+<td align="left" valign="top"><p class="table"><span class="lime-background">True</span></p></td>
+<td align="left" valign="top"><p class="table"><span class="red-background">False</span></p></td>
+<td align="left" valign="top"><p class="table"><span class="red-background">False</span></p></td>
+<td align="left" valign="top"><p class="table"><span class="red-background">False</span></p></td>
+<td align="left" valign="top"><p class="table">1</p></td>
+<td align="left" valign="top"><p class="table">0</p></td>
+</tr>
+<tr>
+<td align="left" valign="top"><p class="table"><a href="https://github.com/silentbicycle/greatest">Greatest</a></p></td>
+<td align="left" valign="top"><p class="table"><span class="lime-background">ISC</span></p></td>
+<td align="left" valign="top"><p class="table"><span class="lime-background">True</span></p></td>
+<td align="left" valign="top"><p class="table"><span class="yellow-background">Partial</span></p></td>
+<td align="left" valign="top"><p class="table"><span class="lime-background">True</span></p></td>
+<td align="left" valign="top"><p class="table"><span class="yellow-background">Partial</span></p></td>
+<td align="left" valign="top"><p class="table"><span class="lime-background">True</span></p></td>
+<td align="left" valign="top"><p class="table"><span class="lime-background">True</span></p></td>
+<td align="left" valign="top"><p class="table"><span class="red-background">False</span></p></td>
+<td align="left" valign="top"><p class="table"><span class="red-background">False</span></p></td>
+<td align="left" valign="top"><p class="table"><span class="red-background">False</span></p></td>
+<td align="left" valign="top"><p class="table">3</p></td>
+<td align="left" valign="top"><p class="table">1400</p></td>
+</tr>
+<tr>
+<td align="left" valign="top"><p class="table"><a href="https://github.com/Snaipe/Criterion">Criterion</a></p></td>
+<td align="left" valign="top"><p class="table"><span class="lime-background">MIT</span></p></td>
+<td align="left" valign="top"><p class="table"><span class="red-background">False</span></p></td>
+<td align="left" valign="top"><p class="table"><span class="yellow-background">Partial</span></p></td>
+<td align="left" valign="top"><p class="table"><span class="lime-background">True</span></p></td>
+<td align="left" valign="top"><p class="table"><span class="lime-background">True</span></p></td>
+<td align="left" valign="top"><p class="table"><span class="lime-background">True</span></p></td>
+<td align="left" valign="top"><p class="table"><span class="lime-background">True</span></p></td>
+<td align="left" valign="top"><p class="table"><span class="lime-background">True</span></p></td>
+<td align="left" valign="top"><p class="table"><span class="red-background">False</span></p></td>
+<td align="left" valign="top"><p class="table"><span class="lime-background">True</span></p></td>
+<td align="left" valign="top"><p class="table">19</p></td>
+<td align="left" valign="top"><p class="table">1800</p></td>
+</tr>
+<tr>
+<td align="left" valign="top"><p class="table"><a href="https://github.com/rra/c-tap-harness/">C TAP</a></p></td>
+<td align="left" valign="top"><p class="table"><span class="lime-background">Expat</span></p></td>
+<td align="left" valign="top"><p class="table"><span class="lime-background">True</span></p></td>
+<td align="left" valign="top"><p class="table"><span class="yellow-background">Partial</span></p></td>
+<td align="left" valign="top"><p class="table"><span class="yellow-background">Partial</span></p></td>
+<td align="left" valign="top"><p class="table"><span class="lime-background">True</span></p></td>
+<td align="left" valign="top"><p class="table"><span class="red-background">False</span></p></td>
+<td align="left" valign="top"><p class="table"><span class="lime-background">True</span></p></td>
+<td align="left" valign="top"><p class="table"><span class="red-background">False</span></p></td>
+<td align="left" valign="top"><p class="table"><span class="red-background">False</span></p></td>
+<td align="left" valign="top"><p class="table"><span class="red-background">False</span></p></td>
+<td align="left" valign="top"><p class="table">4</p></td>
+<td align="left" valign="top"><p class="table">33</p></td>
+</tr>
+<tr>
+<td align="left" valign="top"><p class="table"><a href="https://libcheck.github.io/check/">Check</a></p></td>
+<td align="left" valign="top"><p class="table"><span class="lime-background">LGPL v2.1</span></p></td>
+<td align="left" valign="top"><p class="table"><span class="red-background">False</span></p></td>
+<td align="left" valign="top"><p class="table"><span class="yellow-background">Partial</span></p></td>
+<td align="left" valign="top"><p class="table"><span class="lime-background">True</span></p></td>
+<td align="left" valign="top"><p class="table"><span class="lime-background">True</span></p></td>
+<td align="left" valign="top"><p class="table"><span class="lime-background">True</span></p></td>
+<td align="left" valign="top"><p class="table"><span class="red-background">False</span></p></td>
+<td align="left" valign="top"><p class="table"><span class="red-background">False</span></p></td>
+<td align="left" valign="top"><p class="table"><span class="red-background">False</span></p></td>
+<td align="left" valign="top"><p class="table"><span class="lime-background">True</span></p></td>
+<td align="left" valign="top"><p class="table">17</p></td>
+<td align="left" valign="top"><p class="table">973</p></td>
+</tr>
+</tbody>
+</table>
+</div>
+</div>
+<div class="sect2">
+<h3 id="_additional_framework_candidates">Additional framework candidates</h3>
+<div class="paragraph"><p>Several suggested frameworks have been eliminated from consideration:</p></div>
+<div class="ulist"><ul>
+<li>
+<p>
+Incompatible licenses:
+</p>
+<div class="ulist"><ul>
+<li>
+<p>
+<a href="https://github.com/zorgnax/libtap">libtap</a> (LGPL v3)
+</p>
+</li>
+<li>
+<p>
+<a href="https://cmocka.org/">cmocka</a> (Apache 2.0)
+</p>
+</li>
+</ul></div>
+</li>
+<li>
+<p>
+Missing source: <a href="https://www.kindahl.net/mytap/doc/index.html">MyTap</a>
+</p>
+</li>
+<li>
+<p>
+No TAP support:
+</p>
+<div class="ulist"><ul>
+<li>
+<p>
+<a href="https://nemequ.github.io/munit/">µnit</a>
+</p>
+</li>
+<li>
+<p>
+<a href="https://github.com/google/cmockery">cmockery</a>
+</p>
+</li>
+<li>
+<p>
+<a href="https://github.com/lpabon/cmockery2">cmockery2</a>
+</p>
+</li>
+<li>
+<p>
+<a href="https://github.com/ThrowTheSwitch/Unity">Unity</a>
+</p>
+</li>
+<li>
+<p>
+<a href="https://github.com/siu/minunit">minunit</a>
+</p>
+</li>
+<li>
+<p>
+<a href="https://cunit.sourceforge.net/">CUnit</a>
+</p>
+</li>
+</ul></div>
+</li>
+</ul></div>
+</div>
+</div>
+</div>
+<div class="sect1">
+<h2 id="_milestones">Milestones</h2>
+<div class="sectionbody">
+<div class="ulist"><ul>
+<li>
+<p>
+Add useful tests of library-like code
+</p>
+</li>
+<li>
+<p>
+Integrate with
+ <a href="https://lore.kernel.org/git/20230502211454.1673000-1-calvinwan@google.com/">stdlib
+ work</a>
+</p>
+</li>
+<li>
+<p>
+Run alongside regular <code>make test</code> target
+</p>
+</li>
+</ul></div>
+</div>
+</div>
+</div>
+<div id="footnotes"><hr /></div>
+<div id="footer">
+<div id="footer-text">
+Last updated
+ 2023-12-09 17:40:53 PST
+</div>
+</div>
+</body>
+</html>
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