# Managing Chromium dependencies

Chromium uses gclient (part of depot_tools) to manage dependencies. Information
is stored in DEPS file located in the root of the project. In addition to DEPS
file, gclient may read git submodules (see
[depot_tools submodules support](https://docs.google.com/document/d/1N_fseFNOj10ETZG3pZ-I30R__w96rYNtvx5y_jFGJWw/view)).

gclient supports two dependency types: git and [cipd](cipd_and_3pp.md).

[TOC]

## Adding dependencies

Add your entry in DEPS file. Then, run `gclient gitmodules` to generate
git submodules (it will contain .gitmodule change, and gitlink). Edit OWNERS
file and add gitlink path. Then, run `git add DEPS OWNERS` to stage
those files for commit, followed by `git commit`. Your change is now ready to be
sent for a review using `git cl upload`.


For example, if new dependency is "src/foo/bar.git", its gitlink path is
"foo/bar", and OWNERS entry at the top level is `per-file foo/bar=*`. You can
confirm that by running `git status`. [Example CL](https://crrev.com/c/4923074).

```
# manual edit of DEPS and OWNERS file (see changes below).

 % gclient gitmodules                    
.gitmodules and gitlinks updated. Please check `git diff --staged`and commit those staged changes (`git commit` without -a)

 % git add OWNERS DEPS # stage files

 % git diff --cached # see staged changes
diff --git a/.gitmodules b/.gitmodules
index 29c355fa92e3d..89866442d45aa 100644
--- a/.gitmodules
+++ b/.gitmodules
@@ -1,3 +1,6 @@
+[submodule "foo/bar"]
+       path = foo/bar
+       url = https://chromium.googlesource.com/foo/bar.git
 [submodule "third_party/clang-format/script"]
        path = third_party/clang-format/script
        url = https://chromium.googlesource.com/external/github.com/llvm/llvm-project/clang/tools/clang-format.git
diff --git a/DEPS b/DEPS
index 44fbc53a0d53a..05481be5066ed 100644
--- a/DEPS
+++ b/DEPS
@@ -555,6 +555,10 @@ allowed_hosts = [
 ]
 
 deps = {
+  'src/foo/bar': {
+      'url': Var('chromium_git') + '/foo/bar.git' + '@' +
+        '1111111111111111111111111111111111111111',
+  },
   'src/third_party/clang-format/script':
     Var('chromium_git') +
     '/external/github.com/llvm/llvm-project/clang/tools/clang-format.git@' +
diff --git a/OWNERS b/OWNERS
index 55bfe60fcb03b..02b4117fca1ea 100644
--- a/OWNERS
+++ b/OWNERS
@@ -37,6 +37,7 @@ per-file README.md=*
 per-file WATCHLISTS=*
 
 # git submodules
+per-file foo/bar=*
 per-file third_party/clang-format/script=*
 per-file chrome/browser/resources/preinstalled_web_apps/internal=*
 per-file chrome/installer/mac/third_party/xz/xz=*
diff --git a/foo/bar b/foo/bar
new file mode 160000
index 0000000000000..1111111111111
--- /dev/null
+++ b/foo/bar
@@ -0,0 +1 @@
+Subproject commit 1111111111111111111111111111111111111111

 % git status 
On branch test_newdep
Your branch is up to date with 'origin/main'.

Changes to be committed:
  (use "git restore --staged <file>..." to unstage)
        modified:   .gitmodules
        modified:   DEPS
        modified:   OWNERS
        new file:   foo/bar

Changes not staged for commit:
  (use "git add/rm <file>..." to update what will be committed)
  (use "git restore <file>..." to discard changes in working directory)
        deleted:    foo/bar


% # At this point, you can run gclient sync if you want to get the dependency.
% # But it's not required, and you can use `git cl upload`.

 % git commit -m "[DEPS] Example of new dependency"
[test_newdep 9731cfb680756] [DEPS] Example of new dependency
 4 files changed, 9 insertions(+)
 create mode 160000 foo/bar

 % git cl upload   
Found change with 1 commit...
Running Python 3 presubmit upload checks ...
-- snip --
remote:   https://chromium-review.googlesource.com/c/chromium/src/+/4923074 [DEPS] Example of new dependency [NEW]
-- snip --
```

## Rolling dependencies

### Using gclient

If you want to roll dependency to a specific version, you can do by running the
following:

```
gclient setdep -r {path to dependency}@{hash}
```

For example, let's say you want to roll boringssl in chromium/src to commit
e4acd6cb568214b1c7db4e59ce54ea2e1deae1f5. You would run the following:

```
gclient setdep -r src/third_party/boringssl/src@e4acd6cb568214b1c7db4e59ce54ea2e1deae1f5
```

Under the hood, gclient understands DEPS file, and knows what needs to update.
In the example above, it actually updates boringssl_revision variable that is
used in boringssl deps declaration.

Example of DEPS file:
```
vars = {
  'boringssl_git': 'https://boringssl.googlesource.com',
  'boringssl_revision': 'e4acd6cb568214b1c7db4e59ce54ea2e1deae1f5',
}
deps = {
  'src/third_party/boringssl/src':
    Var('boringssl_git') + '/boringssl.git' + '@' +  Var('boringssl_revision'),
}
```

It also updates gitlink if git submodules are used. Git status will show the
following:

```
$ git status
-- snip --
Changes to be committed:
        modified:   DEPS
        modified:   third_party/boringssl/src
```

### Using roll-dep

depot_tools also provides a `roll-dep` script which can roll the desired
repository to the latest commit on main branch. `roll-dep` handles both DEPS and
git submodules.

### Manual roll / low level roll

You can update things yourself by modifying the DEPS file directly. If git
submodules are used, you also need to update gitlinks - an entry for submodules
in the git database by using:

```
git update-index --add --cacheinfo 160000,{hash},{path}
```

git update-index instructs git to register git submodule change to the index (ie
stages submodule for commit). Particularly, --cacheinfo argument instructs git
to directly insert the specified info into the index: 160000 is gitlink mode
(used by git submodules), {hash} is a new commit hash you want to roll, and path
is relative path to git submodule.


Using the boringssl example above, the following will need to be run inside
chromium/src worktree:


```
git update-index --add --cacheinfo 160000,e4acd6cb568214b1c7db4e59ce54ea2e1deae1f5,third_party/boringssl/src
```

Once executed, `git status` will report there is an update to the submodule,
which you can commit as any other file change.

Alternatively, you can regenerate git submodules once you update DEPS file (see
section below).

## Deleting dependencies

gclient doesn't provide a way to delete dependencies. You can delete dependency
by manually editing DEPS file. If git submodules are used, you will also need to
remove it from git:

```
git rm --cache {dependency}
```

or using the example from the previous section:

```
git rm --cache third_party/boringssl/src
```

Once git-rm is executed, you can proceed with committing your change. You should
see change in DEPS, .gitmodule and {dependency}.

## Regenerating git submodules

If there are many git dependency changes to DEPS file, it may be impractical to
manually update-index. For those reasons, gclient provides a convenient way to
regenerate git modules entries and to update .gitmodules file.

Once you are done with your DEPS modifications, run the following script in the
root of you project:

```
gclient gitmodules
```

The script will create a new .gitmodules files and update all gitlinks. Please
note that old gitlinks won't be deleted, and you will need to remove them
manually (see section above for deleting dependencies).