dymga Huang

This cheatsheet provides quick tips on how to build and test code in our repository using Bazel.

Start here if you're completely new to Bazel.

The original design documents for our Bazel build can be found at the following Golinks:

This section includes steps every engineer should follow to get a consistent development experience.

Bazelisk is a wrapper for Bazel that downloads and runs the version of Bazel specified in //.bazelversion. It serves a similar purpose as nvm does for NodeJS.

Bazelisk is recommended over plain Bazel because the bazel command on our gLinux workstations is automatically updated every time a new version of Bazel is released.

To install Bazelisk, grab the latest binary for your platform from GitHub, then add it to your PATH.

Tips:

If you wish to use RBE to speed up your builds and test runs (see the --config=remote flag below) run the following command:

We use Gazelle to automatically generate BUILD.bazel files for most of our Go and TypeScript code.

Note that we occasionally edit Gazelle-generated BUILD.bazel files by hand, e.g. to mark tests as flaky.

Run make gazelle from the repository's root directory.

TypeScript support is provided via a custom Gazelle extension which can be found in //bazel/gazelle/frontend.

Tip: See here for details on how this extension decides which rule to generate for a given TypeScript file.

Buildifier is a linter and formatter for BUILD.bazel files and other Bazel files (WORKSPACE, *.bzl, etc.).

Run bazel run //:buildifier.

Our Bazel build is tested on RBE via the following tasks:

We regard the above tasks as the source of truth for build and test correctness.

As an insurance policy against RBE outages, we also have the following tasks:

The non-RBE tasks tend to be a bit more brittle than the RBE ones, which is why they are excluded from the CQ.

Use commands bazel build and bazel test to build and test Bazel targets, respectively. Examples:

Any build artifacts produced by bazel build or bazel test will be found under //_bazel_bin.

Note that it‘s not necessary to bazel build a test target before bazel test-ing it. bazel test will automatically build the test target if it wasn’t built already (i.e. if it wasn't found in the Bazel cache).

More on bazel build here.

More on bazel test here.

By default, Bazel will build and test targets on the host system (aka a local build). To build on RBE, invoke Bazel with flag --config=remote, e.g.:

This repository contains some scripted actions that shell out to Bazel, such as certain make targets (e.g. make gazelle, make buildifier) and go generate actions. These actions use the “mayberemote” configuration via the --config=mayberemote flag, e.g.:

By default, the “mayberemote” configuration does nothing. This is to support users that might not have RBE access, or when working offline (e.g. on a plane with no WiFi). To get the benefits of RBE when running scripted actions, please create a //bazel/user/bazelrc file with the following contents:

To learn more about the mayberemote configuration:

Use command bazel run to run binary Bazel targets (such as go_binary, sh_binary, etc.), e.g.:

Alternatively, you can run the Bazel-built artifact directly, e.g.:

The exact path of the binary under //_bazel_bin depends on the Bazel rule (go_binary, py_binary, etc.). As you can see, said path can be non-obvious, so it's generally recommended to use bazel run.

More on bazel run here.

Our Go codebase is built and tested using Bazel rules from the rules_go repository. The go_test rule documentation is a great read to get started.

As mentioned in the Gazelle section, all Bazel targets for Go code are generated with Gazelle.

Read go/skia-infra-bazel-backend for the full details.

On non-Bazel Go projects, developers typically use locally installed binaries such as go and gofmt for code generation and code formatting tasks. However, our Bazel build aims to be as hermetic as possible. To this end, rather than requiring the developer to install a Go SDK on their system, we provide convenience Bazel targets defined in //BUILD.bazel to invoke binaries in the Bazel-downloaded Go SDK and other Bazel-downloaded tools.

Example invocations:

Our CI tasks and Makefiles use these Bazel targets. This prevents diffs that might arise from using locally installed binaries, which might differ from system to system. Developers should always use Bazel-downloaded binaries for any tasks that produce changes in checked-in files.

Note that it might still be desirable to have a locally installed Go SDK. For example, Visual Studio Code‘s Go extension requires a locally installed Go SDK to enable autocompletion and debugging. It is the developer’s responsibility to ensure that their locally installed Go SDK matches the version used by the Bazel build, which is defined in the //WORKSPACE file.

Simply use bazel build (and optionally bazel run) as described earlier.

Tip: Start by reading the General testing tips section.

Our setup differs slightly from typical Go + Bazel projects in that we use a wrapper macro around go_test to handle manual tests. Gazelle is configured to use this macro via a gazelle:map_kind directive in //BUILD.bazel. The macro is defined in //bazel/go/go_test.bzl. Read the macro's docstring for the full details.

To mark specific Go test cases as manual, extract them out into a separate file ending with _manual_test.go within the same directory.

The go_test macro in //bazel/go/go_test.bzl places files ending with _manual_test.go in a separate go_test target, which is tagged as manual.

More on manual tests here.

The go test command supports flags such as -v to print verbose outputs, -run to run a specific test case, etc. Under Bazel, these flags can be passed to a go_test test target via --test_arg, but they need to be prefixed with -test., e.g.:

The following example shows what a typical bazel test invocation might look like while debugging a go_test target locally.

Our front-end code is built and tested using a set of custom Bazel macros built on top of rules provided by the rules_nodejs repository. All such macros are either defined in or re-exported from //infra-sk/index.bzl. This section uses the terms macro and rule interchangeably when referring to the macros exported from said file.

As mentioned in the Gazelle section, most Bazel targets for front-end code are generated with Gazelle.

Read go/skia-infra-bazel-frontend for the full details.

Simply use bazel build (and optionally bazel run) as described earlier.

Demo pages are served via a Gazelle-generated sk_demo_page_server rule.

Use bazel run to serve a demo page via its sk_demo_page_server rule, e.g.:

To rebuild the demo page automatically upon changes in the custom element‘s directory, use the demopage.sh script found in the repository’s root directory, e.g.:

This script uses entr to watch for file changes and re-execute the bazel run command as needed. The above demopage.sh invocation is equivalent to:

Install entr on a gLinux workstation with sudo apt-get install entr.

In the future, we might replace this script with ibazel, which requires changes to the sk_demo_page_server rule.

Tip: Start by reading the General testing tips section.

Front-end code testing is done via three different Bazel rules:

Gazelle decides which rule to generate for a given *_test.ts file based the following patterns:

Use bazel test to run a Karma test in headless mode:

To run a Karma test in the browser during development, use bazel run instead:

As an alternative to bazel run when debugging tests in the browser, consider using the karmatest.sh script found in the repository‘s root directory. Similarly to the demopage.sh script mentioned earlier, it watches for changes in the custom element’s directory, and relaunches the test runner when a file changes. Example usage:

As with demopage.sh, this script depends on the entr command, which can be installed on a gLinux workstation with sudo apt-get install entr.

Use bazel test to run a Puppeteer test, e.g.:

To view the screenshots captured by a Puppeteer test, use the //:puppeteer_screenshot_server target:

To extract the screenshots captured by a Puppeteer test into a directory, use the //:extract_puppeteer_screenshots target:

To step through a Puppeteer test with a debugger, run your test with bazel run, and append _debug at the end of the target name, e.g.:

This will print a URL to stdout that you can use to attach a Node.js debugger (such as the VS Code Node.js debugger, or Chrome DevTools). Your test will wait until a debugger is attached before continuing.

Example debug session with Chrome DevTools:

By default, Puppeteer starts a Chromium instance in headless mode. If you would like to run your test in headful mode, invoke your test with bazel run, and append _debug_headful at the end of the target name, e.g.:

Run your test in headful mode to visually inspect how your test interacts with the demo page under test as you step through your test code with the attached debugger.

Use bazel test to run a NodeJS test, e.g.:

The below tips apply to all Bazel test targets (e.g. go_test, karma_test, etc.).

By default, Bazel omits the standard output of tests (e.g. fmt.Println("Hello")).

Use flag --test_output=all to see the full output of your tests:

Note that Bazel runs tests in parallel, so it will only print out their output once all tests have finished running.

Flag --test_output=errors can be used to only print out the output of failing tests.

To see the tests' output in real time, use flag --test_output=streamed. Note however that this forces serial execution of tests, so this can be significantly slower.

Bazel caches successful test runs, and reports (cached) PASSED on subsequent bazel test invocations, e.g.:

To disable caching, use flag --nocache_test_results, e.g.

Flaky tests can cause the CI to fail (see Bazel CI tasks).

Tests can be marked as flaky via the flaky argument, e.g.:

Bazel will execute tests marked as flaky up to three times, and report test failure only if the three attempts fail.

Using flaky is generally discouraged, but can be useful until the root cause of the flake is diagnosed (see Debugging flaky tests) and fixed.

As a last resort, consider marking your flaky test as manual (see Manual tests).

More on the flaky attribute here.

While --nocache_test_results can be useful for debugging flaky tests, flag --runs_per_test was specifically added for this purpose. Example:

Manual tests are excluded from Bazel wildcards such as bazel test //....

To mark a test target as manual, use the manual tag, e.g.:

Note that the instructions to mark go_test targets as manual are different. See Manual Go tests for more.

Note that manual tests are excluded from the Bazel CI tasks.

More on manual tests and Bazel tags here.

By default, Bazel will report TIMEOUT if the test does not finish within 5 minutes. This can be overridden via the --test_timeout flag, e.g.

$ bazel test //go/util:slow_test --test_timeout=20

This can also be overridden via the timeout and size arguments of the test target, e.g.

More on how to handle timeouts and slow tests here.

Use flag --test_arg to pass flags to the binary produced by a test target.

For example, our go_test targets define custom command-line flags such as flag.Bool("logtostderr", ...). This flag can be enabled with --test_arg, e.g.:

As an alternative, command-line flags can be specified via the args argument of the Bazel test target, as follows:

More on test arguments here.

By default, Bazel isolates test targets from the host system's environment variables, and sets the environment with a number of variables with Bazel-specific information that some *_test rules depend on (documented here).

Use flag --test_env to specify any environment variables, e.g.

To pipe through an environment variable from the host's system:

More on the --test_env flag here.

By default, Bazel sandboxes every build step. Effectively, it runs the compile command with only the given source files for a particular rule and the specified dependencies visible, to force all dependencies to be properly listed.

For steps that have a lot of files, this can have a bit of I/O overhead. To speed this up, one can use tempfs (e.g. a RAM disk) for the sandbox by adding --sandbox_base=/dev/shm to the build command. When compiling Skia, for example, this reduces compile time by 2-3x.

Sandboxing can make diagnosing failing rules a bit harder. To see what command got run and to be able to view the sandbox after failure, add --subcommands --sandbox_debug to the command.

Bazel builds fast and correct by making use of cached outputs and reusing them when the input file is identical. This can make it hard to debug a slow or non-deterministic build.

To get a detailed log of all the actions your build is taking:

Bazel has a query feature that lets one extract information from the build graph.

There's a query and cquery variant that lets one query for the maximal set of information or the information in one specific case, respectively.

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