This documentation is about testing FOG itself

This document contains information about how FOG tests itself, how to add new tests, how and what to log, and stuff like that. If you’re interested in learning how to test instrumentation you added, you’ll want to read the instrumetnation testing docs instead.

Given the multiple API languages, processes, and dependencies, testing FOG is a matter of choosing the right tool for the situation.

One Big Command

To run all the things, here’s the tl;dr:

./mach build && ./mach lint -W -w -o --fix && ./mach rusttests && ./mach gtest "FOG*" && python3 ./mach python-test toolkit/components/glean/tests/pytest && ./mach test toolkit/components/glean/tests/xpcshell && ./mach telemetry-tests-client toolkit/components/telemetry/tests/marionette/tests/client/test_fog* --gecko-log "-" && ./mach test toolkit/components/glean/tests/browser --headless


An often-overlooked first line of testing is “what do the logs say?”. To turn on logging for FOG, use any of the following:

  • Run Firefox with RUST_LOG="fog_control,fog,glean_core".

    • On some platforms this will use terminal colours to indicate log level.

  • Run Firefox with MOZ_LOG="timestamp,sync,glean::*:5,fog::*:5,fog_control::*:5,glean_core::*:5".

  • Set the following prefs:

    • logging.config.timestamp to true

    • logging.config.sync to true

    • logging.fog_control::* to 5

    • logging.fog::* to 5

    • logging.glean::* to 5

    • logging.glean_core::* to 5

    • logging.config.clear_on_startup to false (or all these prefs will be cleared on startup)

For more information on logging in Gecko, see the Gecko Logging docs.

User-destined logs (of the “You did something wrong” variety) might output to the Browser Console if they originate in JS land. Open via Ctrl+Shift+J, or Cmd+Shift+J.


At present, Rust logging in non-main processes just doesn’t work.

What to log, and to where?

FOG covers a lot a ground (languages, layers): where you are determines what logging you have available.

Here are some common situtations for logging:

JS to C++

If your logging is aimed at the user using the JS API (e.g. because the type provided isn’t convertable to the necessary C++ type) then use the Browser Console via FOG’s Common’s LogToBrowserConsole.


If you are in C++ and didn’t come from JS, use MOZ_LOG with module fog.


Use the logging macros from log, e.g. log::info! or log::error!. Remember that, no matter the log level, log::debug! and log::trace! will not appear in non-debug builds

If you are logging due to a situation caused by and fixable by a developer using the API, use log::error!(...). Otherwise, use a quieter level.


about:glean is a page in a running Firefox that allows you to debug the Glean SDK in Firefox Desktop. It does this through the displayed user interface (just follow the instructions).


To keep in accordance with Mozilla’s various and several Coding Styles, we rely on mach lint.

To lint the code in the “usual” way, automatically fixing where possible, run: ./mach lint -W -w -o --fix

This should keep you from checking in code that will automatically be backed out.


Not all of our Rust code can be tested in a single fashion, unfortunately.

Using rusttests (Treeherder symbol Br (a build task))

If the crate you’re testing has no Gecko symbols you can write standard Rust tests.

This supports both unit tests (inline in the file under test) and integration tests (in the tests/ folder in the crate root). Metric type tests are currently written as unit tests inline in the file, as they require access to the metric ID, which should only be exposed in tests.

To run FOG’s rusttests suite use mach rusttests

If the crate uses only a few Gecko symbols, they may use the with_gecko feature to conditionally use them. This allows the crate to test its non-Gecko-adjacent code using Rust tests. (You will need to cover the Gecko-adjacent code via another means.)

Note: Some FOG rusttests panic on purpose. They print stack traces to stdout. If the rusttests fail and you see a stack trace, double-check it isn’t from a purposefully-panicking test.

Note: If a test fails, it is very likely they’ll poison the test lock. This will cause all subsequent tests that attempt to take the test lock (which is all of them) to also fail due to PoisonErrors. They can be safely ignored.

Using gtest (Treeherder symbol GTest (a build task))

Because Gecko symbols aren’t built for the rusttests build, any test that is written for code that uses Gecko symbols should be written as a gtest in toolkit/components/glean/tests/gtest/. You can write the actual test code in Rust. It needs to be accompanied by a C++ GTest that calls a C FFI-exported Rust function. See Testing & Debugging Rust Code for more. See toolkit/components/glean/tests/gtest/TestFog.cpp and toolkit/components/glean/tests/gtest/test.rs for an example.

By necessity these can only be integration tests against the compiled crate.

Note: When adding a new test file, don’t forget to add it to toolkit/components/glean/tests/gtest/moz.build and use the FOG prefix in your test names (e.g. TEST(FOG, YourTestName) { ... }).

To run FOG’s Rust gtest suite use mach gtest FOG.*

Python (Treeherder symbol py3(fp) aka source-test-python-fog)

The Glean Parser has been augmented to generate FOG-specific APIs for Glean metrics. This augmentation is tested by running:

mach test toolkit/components/glean/tests/pytest

These tests require Python 3+. If your default Python is Python 2, you may need to instead run:

python3 mach python-test toolkit/components/glean/tests/pytest

These tests check the code generator output against known good file contents. If you change the code generator the files will need an update. Run the test suite with the UPDATE_EXPECT environment variable set to do that automatically:

UPDATE_EXPECT=1 mach test toolkit/components/glean/tests/pytest

C++ (Treeherder symbol GTest (a build task))

To test the C++ parts of FOG’s implementation (like metric types) you should use gtest. FOG’s gtest tests are in gtest/.

You can either add a test case to an existing file or add a new file. If you add a new file, remember to add it to the moz.build) or the test runner won’t be able to find it.

All tests should start with FOG so that all tests are run with ./mach gtest FOG*.

JS (Treeherder symbol X(Xn) for some number n)

To test the JS parts of FOG’s implementation (like metric types) you should use xpcshell. FOG’s xpcshell tests are in xpcshell/.

You can either add a test case to an existing file or add a new file. If you add a new file, remember to add it to the xpcshell.ini or the test runner will not be able to find it.

To run FOG’s JS tests, run: ./mach test toolkit/components/glean/tests/xpcshell

Non-content-process multiprocess (Browser Chrome Mochitests with Treeherder symbol M(bcN) for some number N)

To test e.g. the GPU process support you need a full Firefox browser: xpcshell doesn’t have the flexibility. To test that and have access to privileged JS (i.e. Glean and FOG APIs), we use browser-chrome-flavoured mochitests you can find in browser/.

If you need to add a new test file, remember to add it to the browser.ini manifest, or the test runner will not be able to find it.

To run FOG’s browser chrome tests, run: ./mach test toolkit/components/glean/tests/browser

Integration (Marionette, borrowing telemetry-tests-client Treeherder symbol tt(c))

To test pings (See bug 1681742) or anything that requires one or more full browsers running, we use the telemetry-tests-client suite in toolkit/components/telemetry/tests/marionette/tests/client/.

For more information on this suite, look to Firefox Telemetry’s Test Documentation and Marionette’s Documentation.

To run these integration tests, run: ./mach telemetry-tests-client toolkit/components/telemetry/tests/marionette/tests/client/

To capture the Firefox under test’s logs, use the --gecko-log parameter. For example, to echo to stdout: ./mach telemetry-tests-client toolkit/components/telemetry/tests/marionette/tests/client/test_fog* --gecko-log "-"

Note: Running the tt(c) suite in this way ignored skip directives in the manifest. This means that you might run tests that are not expected to succeed on your platform. Check toolkit/components/telemetry/tests/marionette/tests/client/manifest.ini for details.