Documentation/user-submodules.txt

= Gerrit Code Review - Superproject subscription to submodules updates

== Description

Gerrit supports a custom git superproject feature for tracking submodules.
This feature is useful for automatic updates on superprojects whenever
a change is merged on tracked submodules. To take advantage of this
feature, one should add submodule(s) to a local working copy of a
superproject, edit the created .gitmodules configuration file to
have a branch field on each submodule section with the value of the
submodule branch it is subscribing to, commit the changes, push and
merge the commit.

When a commit is merged to a project, the commit content is scanned
to identify if it registers git submodules (if the commit registers
any gitlinks and .gitmodules file with required info) and if so,
a new submodule subscription is registered.

When a new commit of a registered submodule is merged, Gerrit
automatically updates the subscribers to the submodule with a new
commit having the updated gitlinks.

== Git Submodules Overview

Submodules are a git feature that allows an external repository to be
attached inside a repository at a specific path. The objective here
is to provide a brief overview, further details can be found
in the official git submodule command documentation.

Imagine a repository called 'super' and another one called 'a'.
Also consider 'a' available in a running Gerrit instance on "server".
With this feature, one could attach 'a' inside of 'super' repository
at path 'a' by executing the following command when being inside
'super':
=====
git submodule add ssh://server/a a
=====

Still considering the above example, after its execution notice that
inside the local repository 'super' the 'a' folder is considered a
gitlink to the external repository 'a'. Also notice a file called
.gitmodules is created (it is a configuration file containing the
subscription of 'a'). To provide the SHA-1 each gitlink points to in
the external repository, one should use the command:
====
git submodule status
====

In the example provided, if 'a' is updated and 'super' is supposed
to see the latest SHA-1 (considering here 'a' has only the master
branch), one should then commit the modified gitlink for 'a' in
the 'super' project. Actually it would not even need to be an
external update, one could move to 'a' folder (insider 'super'),
modify its content, commit, then move back to 'super' and
commit the modified gitlink for 'a'.

== Creating a New Subscription

=== Defining the Submodule Branch

This is required because submodule subscription is actually the
subscription of a submodule project and one of its branches for
a branch of a super project.

Since Gerrit manages subscriptions in the branch scope, we could have
a scenario having a project called 'super' having a branch 'integration'
subscribed to a project called 'a' in branch 'integration', and also
having the same 'super' project but in branch 'dev' subscribed to the 'a'
project in a branch called 'local-dev'.

After adding the git submodule to a super project, one should edit
the .gitmodules file to add a branch field to each submodule
section which is supposed to be subscribed.

As the branch field is a Gerrit specific field it will not be filled
automatically by the git submodule command, so one needs to edit it
manually. Its value should indicate the branch of a submodule project
that when updated will trigger automatic update of its registered
gitlink.

The branch value could be "'.'" if the submodule project branch
has the same name as the destination branch of the commit having
gitlinks/.gitmodules file.

If the intention is to make use of the Gerrit feature described
here, one should always be sure to update the .gitmodules file after
adding submodules to a super project.

If a git submodule is added but the branch field is not added to the
.gitmodules file, Gerrit will not create a subscription for the
submodule and there will be no automatic updates to the superproject.

=== Detecting and Subscribing Submodules

Whenever a commit is merged to a project, its content is scanned
to identify if it registers any submodules (if the commit contains new
gitlinks and a .gitmodules file with all required info) and if so,
a new submodule subscription is registered.

== Automatic Update of Superprojects

After a superproject is subscribed to a submodule, it is not
required to push/merge commits to this superproject to update the
gitlink to the submodule.

Whenever a commit is merged in a submodule, its subscribed superproject
is updated.

Imagine a superproject called 'super' having a branch called 'dev'
having subscribed to a submodule 'a' on a branch 'dev-of-a'. When a commit
is merged in branch 'dev-of-a' of 'a' project, Gerrit automatically
creates a new commit on branch 'dev' of 'super' updating the gitlink
to point to the just merged commit.

=== Subscription Limitations

Gerrit will only automatically update superprojects where the
submodules are hosted on the same Gerrit instance as the
superproject. Gerrit determines this by checking the hostname of the
submodule specified in the .gitmodules file and comparing it to the
hostname from the canonical web URL.

It is currently not possible to use the submodule subscription feature
with a canonical web URL hostname that differs from the hostname of
the submodule. Instead relative submodules should be used.

The Gerrit instance administrator group should always certify to
provide the canonical web URL value in its configuration file. Users
should certify to use the correct hostname of the running Gerrit
instance to add/subscribe submodules.

=== Relative submodules

To enable easier usage of Gerrit mirrors and/or distribution over
several protocols, such as plain git and HTTP(S) as well as SSH, one
can use relative submodules. This means that instead of providing the
entire URL to the submodule a relative path is stated in the
.gitmodules file.

Gerrit will try to match the entire project name of the submodule
including directories. Therefore it is important to supply the full
path name of the Gerrit project, not only relative to the super
repository. See the following example:

We have a super repository placed under a sub directory.

  product/super_repository.git

To this repository we wish add a submodule "deeper" into the directory
structure.

  product/framework/subcomponent.git

Now we need to edit the .gitmodules to include the complete path to
the Gerrit project. Observe that we need to use two "../" to include
the complete Gerrit project path.

  path = subcomponent.git
  url = ../../product/framework/subcomponent.git
  branch = master

In contrast the following will not setup proper submodule
subscription, even if the submodule will be successfully cloned by git
from Gerrit.

  path = subcomponent.git
  url = ../framework/subcomponent.git
  branch = master

== Removing Subscriptions

If one has added a submodule subscription and drops it, it is
required to merge a commit updating the subscribed super
project/branch to remove the gitlink and the submodule section
of the .gitmodules file.

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