# Gerrit Code Review - Superproject subscription to submodules updates

version v2.13.8-1-g8656070

## 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.

When 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 by Gerrit.

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

To take advantage of this feature, one should:

1. ensure superproject subscriptions are enabled on the server via submodule.enableSuperProjectSubscriptions

2. configure the submodule to allow having a superproject subscribed

3. ensure the .gitmodules file of the superproject includes

   1. a branch field

   2. a url that starts with the `gerrit.canonicalWebUrl`

When a commit in a project is merged, Gerrit checks for superprojects that are subscribed to the the project and automatically updates those superprojects with a commit that updates the gilink for the project.

This feature is enabled by default and can be disabled via submodule.enableSuperProjectSubscriptions in the server configuration.

## 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 documentation.

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

    git submodule add ssh://server/sub sub

Still considering the above example, after its execution notice that inside the local repository 'super' the 'sub' folder is considered a gitlink to the external repository 'sub'. Also notice a file called .gitmodules is created (it is a configuration file containing the subscription of 'sub'). 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 'sub' is updated and 'super' is supposed to see the latest SHA-1 (considering here 'sub' has only the master branch), one should then commit the modified gitlink for 'sub' in the 'super' project. Actually it would not even need to be an external update, one could move to 'sub' folder (inside 'super'), modify its content, commit, then move back to 'super' and commit the modified gitlink for 'sub'.

## Creating a new subscription

### Ensure the subscription is allowed

Gerrit has a complex access control system, where different repositories can be accessed by different groups of people. To ensure that the submodule related information is allowed to be exposed in the superproject, the submodule needs to be configured to enable the superproject subscription. In a submodule client, checkout the refs/meta/config branch and edit the subscribe capabilities in the 'project.config' file:

        git fetch <remote> refs/meta/config:refs/meta/config
        git checkout refs/meta/config
        $EDITOR project.config

and add the following lines:

      [allowSuperproject "<superproject>"]
        matching = <refspec>

where the 'superproject' should be the exact project name of the superproject. The refspec defines which branches of the submodule are allowed to be subscribed to which branches of the superproject. See below for details. Push the configuration for review and submit the change:

      git add project.config
      git commit -m "Allow <superproject> to subscribe"
      git push <remote> HEAD:refs/for/refs/meta/config

After the change is integrated a superproject subscription is possible.

The configuration is inherited from parent projects, such that you can have a configuration in the "All-Projects" project like:

        [allowSuperproject "my-only-superproject"]
            matching = refs/heads/*:refs/heads/*

and then you don’t have to worry about configuring the individual projects any more. Child projects cannot negate the parent’s configuration.

### Defining the submodule branch

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 'sub' in branch 'integration', and also having the same 'super' project but in branch 'dev' subscribed to the 'sub' 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.

Whenever a commit is merged to a project, its project config is checked to see if any potential superprojects are allowed to subscribe to it. If so, the superproject is checked if a valid subscription exists by checking the .gitmodules file for the a submodule which includes a `branch` field and a url pointing to this server.

### The RefSpec in the allowSuperproject section

There are two options for specifying which branches can be subscribed to. The most common is to set `allowSuperproject.<superproject>.matching` to a Git-style refspec, which has the same syntax as the refspecs used for pushing in Git. Regular expressions as found in the ACL configuration are not supported.

The most restrictive refspec is allowing one specific branch of the submodule to be subscribed to one specific branch of the superproject:

      [allowSuperproject "<superproject>"]
        matching = refs/heads/<submodule-branch>:refs/heads/<superproject-branch>

If you want to allow for a 1:1 mapping, i.e. 'master' maps to 'master', 'stable' maps to 'stable', but not allowing 'master' to be subscribed to 'stable':

      [allowSuperproject "<superproject>"]
        matching = refs/heads/*:refs/heads/*

To allow all refs matching one pattern to subscribe to all refs matching another pattern, set `allowSuperproject.<superproject>.all` to the patterns concatenated with a colon. For example, to make a single branch available for subscription from all branches of the superproject:

      [allowSuperproject "<superproject>"]
         all = refs/heads/<submodule-branch>:refs/heads/*

To make all branches available for subscription from all branches of the superproject:

      [allowSuperproject "<superproject>"]
         all = refs/heads/*:refs/heads/*

### 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 that the URL of the submodule specified in the .gitmodules file starts with `gerrit.canonicalWebUrl`. The protocol part is ignored in this check.

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

The Gerrit instance administrator should ensure that the canonical web URL value is specified in its configuration file. Users should ensure that they use the correct hostname of the running Gerrit instance when adding submodule subscriptions.

When converting an existing submodule to use subscription by adding a `branch` field into the .gitmodules file, Gerrit does not change the revision of the submodule (i.e. update the superproject’s gitlink) until the next time the branch of the submodule advances. In other words, if the currently used revision of the submodule is not the branch’s head, adding a subscription will not cause an immediate update to the head. In this case the revision must be manually updated at the same time as adding the subscription.

### 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

To remove a subscription, either disable the subscription from the submodules configuration or remove the submodule or information thereof (such as the branch field) in the superproject.

Part of Gerrit Code Review