How the build process works

Let’s take a closer look at Prelude repository. It’s based on Chromium codebase version 71.0.3559.3 with some modifications, plus the folder prelude which contains all the examples.

Chromium source repository alone does not have everything. The build process pulls in hundreds of other projects, e.g., skia, as dependencies. Therefore, it has to know what to import and where to put it for later use. Such information is stored in a file named DEPS at the root directory.

Open DEPS, variable vars stores three types of information:

1. some gn flags: 'checkout_nacl': True,
2. server urls for core repositories: 'chromium_git': 'https://chromium.googlesource.com',
3. revision for cour components: 'skia_revision': 'f610bae66163723d8fc3463546b4110c86345d79',

Next, variable deps specifies information about libaries Chromium depends on. For example:

  'src/third_party/breakpad/breakpad':
    (Var("chromium_git")) + '/breakpad/breakpad.git@a61afe7a3e865f1da7ff7185184fe23977c2adca',

This means grab breakpad from: https://chromium.googlesource.com/breakpad/breakpad.git and sync to revision a61afe7a3e865f1da7ff7185184fe23977c2adca. The code should be downloaded to src/third_party/breakpad/breakpad.

DEPS also defines a list of hooks for extra works. For example, the following hook gets the gn executable we used in the last post:

  {
    'action': [
      'vpython',
      'src/third_party/depot_tools/download_from_google_storage.py',
      '--no_resume',
      '--no_auth',
      '--bucket',
      'chromium-gn',
      '-s',
      'src/buildtools/win/gn.exe.sha1'
    ],
    'pattern':
      '.',
    'name':
      'gn_win',
    'condition':
      'host_os == "win"'
  },

action specifies the command to run; condition dictates that this hook should only run on windows machines. You can find similar hooks to get gn for mac and linux.

These tools and external source code must be in place before build start. For Chromium, it is done when you run fetch chromium. In Prelude, all these nuts and bolts are already in repository (windows only), so that you can start using gn and fire up building process immediately.

Once the build is complete, all the artifacts (i.e., executables, dlls) are in out/debug folder, or whatever folder you specified in command gn gen. Examples of components built in Prelude include base library, v8 engine, ui service. We will get to these later. Many executables built in Prelude are unit tests. It is included because unit tests are one of the best resources to learn how the code works.

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