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GN support in the build system

GN (Generated Ninja) is a third-party build tool used by chromium and some related projects that are vendored in mozilla-central. Rather than requiring GN to build or writing our own build definitions for these projects, we have support in the build system for translating GN configuration files into moz.build files. In most cases these moz.build files will be like any others in the tree (except that they shouldn't be modified by hand), however those updating vendored code or building on platforms not supported by Mozilla automation may need to re-generate these files. This is a two-step process, described below.

Generating GN configs as JSON

The first step must take place on a machine with access to the GN tool, which is specified in a mozconfig with:

export GN=<path/to/gn>

With this specified, and the tree configured, run:

$ ./mach build-backend -b GnConfigGen

This will run gn gen on projects found in GN_DIRS specified in moz.build files, injecting variables from the current build configuration and specifying the result be written to a JSON file in $objdir/gn-output. The file will have a name derived from the arguments passed to gn gen, for instance x64_False_x64_linux.json.

If updating upstream sources or vendoring a new project, this step must be performed for each supported build configuration. If adding support for a specific configuration, the generated configuration may be added to existing configs before re-generating the moz.build files, which should be found under the gn-configs directory under the vendored project's top-level directory.

Generating moz.build files from GN JSON configs

Once the relevant JSON configs are present under a project's gn-configs directory, run:

$ ./mach build-backend -b GnMozbuildWriter

This will combine the configuration files present into a set of moz.build files that will build the given project. Once the result is verified, the resulting moz.build files should be checked in and should build like any other part of mozilla-central.