Requests for comment/Composer managed libraries for use on WMF cluster

This is a request for comment about using Composer to manage library dependencies for MediaWiki and MediaWiki extensions on the WMF production and beta clusters and Jenkins jobs.

Background
The structured logging and patches introduce the concept of specifying external library dependencies, both required and suggested, to mediawiki/core.git via composer.json. Composer can be used by people directly consuming the git repository to install and manage these dependencies.

Problem
On the WMF production and beta clusters and the Jenkins job runners we want to avoid using Composer directly to manage library dependencies. Composer allows specifying versions of libraries that should be imported, but those versions are typically only enforced as git tags which are mutable and no checksumming and/or cryptographic signing mechanisms are provided. This would provide a potential exploit vector for attacking the WMF infrastructure as the composer.json files would be public and advertize the external dependencies to attack in order to slipstream code into the WMF environment. Even if this attack vector is unlikely to be be exploited, it's risk should be minimized. For the Jenkins environment, speed of setup for tests is also a concern. Uncached downloads/clones of external resources would expand the time needed to setup for each test.

Proposal
Both security and speed concerns can be mitigated by creating a locally hosted git repository containing a  file along with a Composer generated   and the desired libraries. This  file would be used to tell Composer the exact versions of libraries to download. Developers would manually run Composer in a checkout of this repository and then commit the downloaded content,  file and generated   to the repository for review. We would then be able to branch and use this repository as git submodule in the  branches that are deployed to production and ensure that it is checked out along with mw-core on the Jenkins nodes. By placing this submodule at  in mw-core we would be mimicking the configuration that direct users of Composer will experience. WebStart.php already includes  when present so integration with the rest of mw-core should follow from that.

It would also be possible to add this repo to the tarballs for distribution. There will probably need to be some adjustments for that process however and the final result may be that release branches update the mediawiki/core composer.json and provide a composer.lock along with a pre-populated vendor directory. This is an important use-case, but a fully formed solution for it is not presented in this RFC.

There are several use cases to consider for the general solution:

Adding/updating a library

 * Update  in
 * Run  locally to download library (and dependencies)
 * Run  to make an optimized
 * Commit changes
 * Push changes for review in gerrit

Hotfix for an external library
At some point we will run into a bug or missing feature in a Composer managed library that we need to work around with a patch. Obviously we will attempt to upstream any such fixes (otherwise what's the point of this whole exercise?). To keep from blocking things for our production cluster we would want to fork the upstream, add our patch for local use and upstream the patch. During the time that the patch was pending review in the upstream we would want to use our locally patched version in production and Jenkins.

Composer provides a solution for this with its repository package source. The Composer documentation actually gives this exact example in their discussion of the vcs repository type. We would:
 * Create a git repository tracking the external library
 * Add our patch(es)
 * Tag with a semantic version number indicating that we have amended the library (eg upstream "v1.0.1" becomes local "v1.0.1+wmf1")
 * Adjust the  file in   to reference our fork and version
 * Run Composer in  to pull in our patched version

Adding a locally developed library
The Platform Core team has been talking about extracting libraries from mw-core and/or extensions to be published externally. This may be done for any and all of the current  classes and possibly other content from core such as FormatJson.

For this use case, we would create a new gerrit repository for each exported project. The project repo would contain a  manifest describing the project correctly to be published at packagist.org like most Composer installable libraries. In the    file we would pull these libraries just like any third-party developed library. This isn't functionally much different than the way that we use git submodules today. There is one extra level of indirection when a library is changed. The repo will have to be updated with the new library version before the hash for the git submodule of   is updated in a deploy or release branch.

wmf/1.XwmfY branches
The  script (found in  ) is used to create the weekly release branches that are deployed by the "train" on each Thursday. This script would be has been updated to branch the new  repository and add the version appropriate branch as a submodule of   on the   branch. This is functionally exactly what we do for extensions today.

Updating a deployment branch
SWAT deploys often deploy bug fixes for extensions and core that can't wait for the next train release. It is a near certainty that mediawiki/core/vendor.git will have the same need. The process for updating mediawiki/core/vendor.git will be almost the same as updating an extension.


 * Follow the adding/updating library or hotfix instructions to get the changes merged into the  master branch.
 * Cherry-pick the change into the proper deployment branch
 * Merge the cherry-pick
 * Update the git submodule for  in the appropriate deployed branch
 * Pull update to deployment server
 * to deploy to cluster

Security fixes
This is a special case of upstreaming a patch. A security patch would be applied directly on the deployed branch of  as we would do for any extension. The vulnerability and patch must then be submitted upstream in a responsible manner and tracked for resolution.

Jenkins
The Jenkins jobs that checkout and run tests involving  would need to be amended to also checkout the in the appropriate location before running tests.