Building geckodriver

geckodriver is written in Rust, a systems programming language from Mozilla. Crucially, it relies on the webdriver crate to provide the HTTPD and do most of the heavy lifting of marshalling the WebDriver protocol. geckodriver translates WebDriver commands, responses, and errors to the Marionette protocol, and acts as a proxy between WebDriver and Marionette.

geckodriver is an optional build component when building Firefox, which means it is not built by default when invoking ./mach build. To enable building of geckodriver, ensure to put this in your mozconfig:

ac_add_options --enable-geckodriver

Because we use geckodriver in testing, particularly as part of the Web Platform Tests, it is built by default in the Firefox CI. A regular -b do -p all -u none -t none try syntax will build geckodriver on all the supported platforms. The build will be part of the B task.

If you use artifact builds you may also build geckodriver using cargo:

% cd testing/geckodriver
% cargo build
…
   Compiling geckodriver v0.21.0 (file:///home/ato/src/gecko/testing/geckodriver)
…
    Finished dev [optimized + debuginfo] target(s) in 7.83s

Because all Rust code in central shares the same cargo workspace, the binary will be put in the $(topsrcdir)/target directory.

You can run your freshly built geckodriver this way:

% ./mach geckodriver -- --other --flags

And run its unit tests like this:

% ./mach test testing/geckodriver

Or by invoking cargo in its subdirectory:

% cd testing/geckodriver
% cargo test

Making geckodriver part of the official build

There is a long-term intention to include geckodriver as part of the regular Firefox build.

When we migrated geckodriver from GitHub into central it was originally enabled by default in all local builds. Since this was one of the very first Rust components to land in central, support for Rust was a little bit experimental and we discovered a couple of different problems:

(1) Not all developers had Rust installed, and support for Rust in the tree was experimental. This caused some compile errors in particular on Windows, and at the time developers were not happy with requiring Rust for building Firefox.

(2) The additional build time induced by including a fairly large Rust component in the default build was considered too high.

At the time of writing, Rust support in central has improved vastly. We also require Rust for regular Firefox builds, which addresses part of issue 1.

Our working theory is that we now build so much Rust code in central that many of the dependencies geckodriver relies on and that takes up a lot of time to build will have been built as dependencies for other Rust components in the tree, effectively making the additional time it takes to build geckodriver less prominent than it was when we originally tried including it in the default build.

This work is tracked as part of https://bugzilla.mozilla.org/show_bug.cgi?id=1471281.