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| <h1 id="crater"><a class="header" href="#crater">Crater</a></h1> |
| <p><a href="https://github.com/rust-lang/crater">Crater</a> is a tool for compiling and running tests for <em>every</em> crate on <a href="https://crates.io">crates.io</a> (and a few on GitHub). |
| It is mainly used for checking the extent of breakage when implementing potentially breaking changes and ensuring lack of breakage by running beta vs stable compiler versions.</p> |
| <p>Essentially it runs some <code>cargo</code> command on every crate twice; once against the “start” toolchain and again against the “end” toolchain. |
| For example, “start” could be the stable release, and “end” could be beta. |
| If it passes in “start” but fails with “end”, then that is reported as a regression.</p> |
| <p>There is a bot called <a href="https://github.com/rust-lang/crater/blob/master/docs/bot-usage.md">craterbot</a> which is used to run crater on hardware managed by the rust-lang organization.</p> |
| <p>Crater is run by the release team during the beta cycle. |
| If there are any regressions that look like they are caused by Cargo, they should contact the Cargo team to decide how to handle it.</p> |
| <h2 id="running-crater"><a class="header" href="#running-crater">Running crater</a></h2> |
| <p>If you have a change that you want to test before the beta release, or you want to test behavior that is not normally exercised by crater, you can do a manual run of crater. |
| Roughly the steps are:</p> |
| <ol> |
| <li> |
| <p>Create a branch with your changes.</p> |
| <p>In your clone of cargo, make the changes to incorporate whatever new thing you want to test and push it to a branch on your fork on GitHub.</p> |
| </li> |
| <li> |
| <p>Get a clone of <a href="https://github.com/rust-lang/rust">https://github.com/rust-lang/rust</a></p> |
| </li> |
| <li> |
| <p>Create a branch in your rust-lang/rust clone to add your changes.</p> |
| </li> |
| <li> |
| <p>Change the <code>src/tools/cargo</code> submodule to point to your new branch.</p> |
| <p>Modify <code>.gitmodules</code> to point to your clone and branch of cargo with the changes you want to test. |
| For example:</p> |
| <pre><code class="language-bash">git submodule set-url src/tools/cargo https://github.com/ehuss/cargo.git |
| git submodule set-branch --branch my-awesome-feature src/tools/cargo |
| git submodule update --remote src/tools/cargo |
| git add .gitmodules src/tools/cargo |
| git commit |
| </code></pre> |
| </li> |
| <li> |
| <p>Create a PR on rust-lang/rust.</p> |
| <p>Push your submodule changes to GitHub and make a PR. |
| Start the PR title with <code>[EXPERIMENT]</code> to make it clear what the PR is for and assign yourself or @ghost.</p> |
| </li> |
| <li> |
| <p>Make a “try” build.</p> |
| <p>A “try” build creates a full release of x86_64-unknown-linux-gnu and stores it on rust-lang servers. |
| This can be done with a comment <code>@bors try</code> on the PR (all Cargo team members should have permission to do this).</p> |
| </li> |
| <li> |
| <p>Run crater.</p> |
| <p>Look at the <a href="https://github.com/rust-lang/crater/blob/master/docs/bot-usage.md">craterbot</a> docs to determine the command that you want to run. |
| There are different modes like <code>check-only</code>, <code>build-and-test</code>, <code>rustdoc</code>, etc.</p> |
| <p>You can also choose how many crates to run against. |
| If you are uncertain if your cargo changes will work correctly, it might be a good idea to run against <code>top-100</code> first to check its behavior. |
| This will run much faster. |
| You can do a full run afterwards.</p> |
| <p>After the try build finishes (which should take a couple hours), ask someone to make a crater run. |
| The Cargo team does not have that permission, so just ask someone on Zulip. |
| They will need to write a comment to <code>@craterbot</code> with the command that you have specified.</p> |
| </li> |
| <li> |
| <p>Wait.</p> |
| <p>Crater can take anywhere from a few hours to a few weeks to run depending on how long the <a href="https://crater.rust-lang.org/">craterbot queue</a> is and which mode you picked and the priority of your job. |
| When the crater run finishes, craterbot will post a comment to the PR with a link to a report of the results.</p> |
| </li> |
| <li> |
| <p>Investigate the report.</p> |
| <p>Look through the report which contains links to build logs for any regressions or errors.</p> |
| </li> |
| <li> |
| <p>Close the PR.</p> |
| <p>Whenever you are done doing crater runs, close your PR.</p> |
| </li> |
| </ol> |
| <h2 id="advanced-crater-modes"><a class="header" href="#advanced-crater-modes">Advanced crater modes</a></h2> |
| <p>Crater only has a few built-in modes, such as running <code>cargo check</code> or <code>cargo test</code>. |
| You can pass extra flags with <code>+cargoflags</code>.</p> |
| <p>More complex tests can be accomplished by customizing Cargo to perform whatever actions you want. |
| Since crater essentially runs <code>cargo check</code>, you can modify the <code>check</code> command to perform whichever actions you want. |
| For example, to test <code>cargo fix --edition</code>, <a href="https://github.com/ehuss/cargo/commit/6901690a6f8d519efb4fabf48c1c2b94af0c3bd8">this commit</a> intercepted <code>cargo check</code> and modified it to instead:</p> |
| <ol> |
| <li>Only run on crates with the 2018 edition.</li> |
| <li>Run <code>cargo fix --edition</code>.</li> |
| <li>Modify the manifest to switch to the 2021 edition.</li> |
| <li>Run <code>cargo check</code> to verify.</li> |
| </ol> |
| <p>If you need to compare the before and after of a command that is not part of crater’s built-in modes, that can be more difficult. |
| Two possible options:</p> |
| <ul> |
| <li>Work with the infra team to add a new mode.</li> |
| <li>Build two custom try builds. |
| Each one should modify the <code>cargo check</code> command as described above. |
| The “start” build should perform whichever action you want with an otherwise unmodified cargo. |
| The “end” build should perform whichever action you want with your modified cargo. |
| Then, in the <code>@craterbot</code> command, specify the start and end hashes of the two try builds.</li> |
| </ul> |
| <h2 id="limitations"><a class="header" href="#limitations">Limitations</a></h2> |
| <p>There are some limitations of crater to consider when running Cargo:</p> |
| <ul> |
| <li>A crater run without regressions is not a green light to move forward. |
| <ul> |
| <li>A large portion of Rust code is not tested, such as closed-source projects or things otherwise not collected by crater.</li> |
| <li>Many crates can’t build in crater’s environment or are otherwise broken.</li> |
| <li>Some crates have flaky tests.</li> |
| </ul> |
| </li> |
| <li>Crater runs in an isolated environment. |
| <ul> |
| <li>It only runs on Linux x86-64.</li> |
| <li>It does not have network access.</li> |
| <li>The crate source is in a read-only mount.</li> |
| </ul> |
| </li> |
| <li>Crater does several steps before running the test (using its own copy of the stable toolchain): |
| <ul> |
| <li>It generates a lockfile using <code>generate-lockfile</code> and includes <code>-Zno-index-update</code> to prevent index updates (which makes it run much faster).</li> |
| <li>All dependencies are downloaded ahead-of-time with <code>cargo fetch</code>.</li> |
| </ul> |
| </li> |
| <li>The built-in modes pass several flags to cargo such as <code>--frozen</code> or <code>--message-format=json</code>. |
| It will sometimes use <code>--all-targets</code> and sometimes not. |
| Check the <a href="https://github.com/rust-lang/crater/blob/master/src/runner/test.rs">crater source</a> for more details on how it works.</li> |
| </ul> |
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