Chromium is a big project, and it makes sense to modularize. Many
sub-teams work largely independently, such as the sandbox and V8 teams,
and the WebKit
code is merged on regular schedules. It makes sense to modularize such
that these teams can work independently, yet still share common code.
For an overview of the modules that exist, please see Chromium code layout.
Say you need to make a change that adds or changes something in base for the benefit of something in chrome.
First you make your change, get it reviewed and checked in. At
this point, Chromium is still pulling a specific, older revision of base
without your changes. It does this by pulling in a specific version of webkit, and then specifying that it should use the version of base that the webkit module is currently using (see the "From" syntax below). Nobody will see your change because of this.
Then you update the webkit module to pull in your new version of base. First you need to figure out which version your new version is. It's a good idea to first do svn update in base/ to make sure everything is up-to-date, then do svn info. You'll see a Revision line that tells you the current revision number of the current module. Put this revision number into the webkit/DEPS file for the base module. Get this change reviewed and submitted.
At this point, chrome is still pulling a specific version of webkit that pulls base without your change. Repeat the same procedure as before, but this time, make chrome pull the latest version of webkit.
Along with this deps file changes should go your corresponding changes
to Chromium. This allows you to atomically pull in the most recent
version and update Chromium to be compatible with it.
Important tip: On the Chromium buildbot
page one of the builders is called chrome-release-webkit-latest. This
pulls Chromium with the most recent version of WebKit. If your change
introduced incompatibilities between base and chrome,
this will be red until you check in your corresponding changes to
Chromium. On the other hand, if you didn't do anything incompatible, if
this builder is red, that means it's a bad time to sync WebKit to the
latest Chromium! You should make sure the problem is fixed and the
builder is green before submitting your change!
Each project depends on a variety of modules. For example, the
WebKit project depends on the networking code, our shared base code,
and some third-party libraries. Chromium and V8 in turn both depend on
WebKit. (In practice, Chromium depends on everything since we are in the
Chromium tree). Each project must list the projects it depends on, and
which version it wants to pull. We do not support transitive
dependencies, so each project must list all of the projects it depends
on.
A user's client specifies a set of target solutions to check out.
(Yes, please excuse the Visual Studio jargon.) In the simplest case, a user's client
only specifies a single solution, say the chrome solution. The root
directory of each solution has a text file named DEPS that defines
the set of dependencies for the project. Users can override these dependencies if
desired.
The contents of the DEPS file is a python associative array, which looks something like the following:
deps = {
"breakpad" : "http://google-breakpad.googlecode.com/svn/trunk@189",
"webkit" : "http://src.chromium.org/chrome/trunk/src/webkit@3395",
"v8" : "http://v8.googlecode.com/svn/trunk@77829",
...
}
The DEPS file can easily be used to express a dependency on a subversion tag and other subversion servers:
deps = {
...
"breakpad" : "http://google-breakpad.googlecode.com/svn/tags/1.0.29.0",
...
}
While it is possible for a DEPS file to specify that the trunk of a dependency
be used, it is intended that a DEPS file instead specify a known good revision
number or tag. This ensures that Chromium developers, for example, are insulated
from activity on the trunk of a dependency. When the maintainer(s) of the dependency
decide to make Chromium use the new revision of the dependency, they just need to
contribute a change to the chrome/DEPS file.
Given that dependencies are not computed recursively, it can be a pain to maintain
complex dependency trees manually, especially if modules have multiple overlapping
dependencies. To simplify such situations, gclient supports the From keyword
which can be used to express a dependency in terms of the DEPS file of another
module. For example, you could have:
deps = {
"breakpad" : "http://google-breakpad.googlecode.com/svn/trunk@189",
"webkit" : "http://src.chromium.org/chrome/trunk/src/webkit@3395",
"v8" : From("webkit"),
...
}
The use of the From keyword above indicates that the version of "v8" to be pulled
should be determined by inspecting webkit/DEPS.
Some modules like webkit have very large amounts of test data. For these
modules, a good convention is to define a separate module for the test data. In
the case of the webkit module, webkit/DEPS can define a dependency on
the webkit test data module, but chrome/DEPS can exclude this dependency.
As a result, Chromium developers are exempted from having to checkout the WebKit
test data by default, but this also does not inconvenience WebKit developers who
prefer to check out the test data. Also, by having the test data in a parallel
directory, the cost of manually updating just the webkit directory is minimized.
To facilitate working with a bunch of separately versioned modules, some tooling is
needed.
Checkout the depot_tools package, which includes gclient, gcl, and svn:
$ git clone https://chromium.googlesource.com/chromium/tools/depot_tools
Add the depot_tools directory to your PATH.
To checkout Chromium the first time, you need to first configure your client:
$ gclient config http://src.chromium.org/chrome/trunk/src/chrome
This command generates a basic .gclient file suitable for checking out
the specified version of chrome and its dependencies. See the Advanced
Usage section below for details on customizing your .gclient file.
To actually checkout Chromium and to later update your working copy:
$ gclient sync
or,
$ gclient update
This checks out Chromium and each of the dependencies listed in the
chrome/DEPS file. The dependencies are placed alongside the chrome
directory.
When run, the gclient sync command looks for a .gclient file in the
current directory or in a parent directory (walking up the parent directory chain
until it finds one). It is an error if that file is not found.
The contents of a default .gclient file looks something like:
solutions = [
{ "name" : "chrome",
"url" : "http://src.chromium.org/chrome/trunk/src/chrome",
"custom_deps" : {}
}
]
An element of the solutions array (a solution) describes a repository
directory that will be checked out into your working copy. Each solution may optionally
define additional dependencies (via its DEPS file) to be checked out alongside the
solution's directory. A solution may also specify custom dependencies (via the
custom_deps property) that override or augment the dependencies specified by the
DEPS file.
Users can edit this file to add new solutions or alter dependencies for a particular
solution.
For example, a V8 developer may wish to checkout the V8 trunk alongside a stable version
of Chromium. So, they might setup a .gclient file like so:
solutions = [
{ "name" : "chrome",
"url" : "http://src.chromium.org/chrome/trunk/src/chrome@5000",
"custom_deps" : {
"v8" : "http://v8.googlecode.com/svn/trunk"
}
}
]
The above specifies that a particular revision of Chromium, r5000, should be used
instead of the Chromium trunk. It then specifies that the V8 trunk should be used
instead of the version of V8 specified by chrome/DEPS.
The V8 example is somewhat simple since V8 does not itself have other dependencies.
For modules like webkit, specifying custom dependencies for each of WebKit's dependencies
is tedious and could potentially get out of sync with what other developers are using
(as specified in webkit/DEPS). So, an alternative approach to working with the
trunk of another module is to set up a second solution:
solutions = [
{ "name" : "chrome",
"url" : "http://src.chromium.org/chrome/trunk/src/chrome@5000",
"custom_deps" : {
"skia" : None,
"webkit" : None,
"v8" : None
}
},
{ "name" : "webkit",
"url" : "http://src.chromium.org/chrome/trunk/src/webkit",
"custom_deps" : {}
}
]
The above .gclient file specifies that the svn URLs given in chrome/DEPS for
the skia, webkit, and v8 modules should be ignored. Those modules are then fetched
according to webkit/DEPS. Both chrome/DEPS and webkit/DEPS, in this
example, specify other common dependencies such as base. The tool will complain if
the two solutions specify conflicting dependencies. A user must explicitly ignore a
dependency that conflicts.
