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


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="glean=info,fog=info,glean_core=info".

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

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

  • Set the following prefs:

    • logging.config.timestamp to true

    • logging.fog::* to 5

    • logging.glean::* to 5

    • logging.glean_core::* to 5

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


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

Using rusttests

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).

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.)

Using gtest

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/gtest/.

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/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.*