As I've written before, Mercurial is a highly extensible version control system. You can do things with Mercurial you can't do in other version control systems.
In this post, I'll outline some of the cool things Mozilla could do with Mercurial. But first, I want to outline some features of Mercurial that many don't know exist.
pushkey and listkeys command
The Mercurial wire protocol (how two Mercurial peer repositories talk to each other over a network) contains two very useful commands: pushkey and listkeys. These commands allow the storage and listing of arbitrary key-value pair metadata in the repository.
This generic storage mechanism is how Mercurial stores and synchronizes bookmarks and phases information, for example.
By implementing a Mercurial extension, you can have Mercurial store key-value data for any arbitrary data namespace. You can then write a simple extension that synchronizes this data as part of the push and pull operations.
Extending the wire protocol
For cases where you want to transmit arbitrary data to/from Mercurial servers and where the pushkey framework isn't robust enough, it's possible to implement custom commands in the Mercurial wire protocol.
A server installs an extension making the commands available. A client installs an extension knowing how to use the commands. Arbitrary data is transferred or custom actions are performed.
When it comes to custom commands, the sky is really the limit. You could do pretty much anything from transfer extra data types (this is how the largefiles extension works) to writing commands that interact with remote agents.
Custom revision set queries and templating
Mercurial offers a rich framework for querying repository data and for formatting data. The querying is called revision sets and the later templates. If you are unfamiliar with the feature, I encourage you to run hg help revset and hg help templates right now to discover the awesomeness.
As I've demonstrated, you can do some very nifty things with custom revision sets and templating!
Now that you know some ways Mercurial can be extended, let's talk about some cool use cases at Mozilla. I want to be clear that I'm not advocating we do these things, just that they are possible and maybe they are a little cool.
Storing pushlog data
Mozilla records information about who pushed what changesets where and when in what's called the pushlog. The pushlog data is currently stored in a SQLite database inside the repository on the server. The data is made available via a HTTP+JSON API.
We could go a step further and make the pushlog data available via listkeys so Mercurial clients could download pushlog data with the same channel used to pull core repository data. (Currently, we have to open a new TCP connection and talk to the HTTP+JSON API.) This would make fetching of pushlog data faster, especially for clients on slow connections.
I concede this is an iterative improvement and adds little value beyond what we currently have. But if I were designing pushlog storage from scratch, this is how I'd do it.
Storing a changeset's automation results
The pushkey framework could be used to mark specific changesets as passing automation. When release automation or a sheriff determines that a changeset/push is green, they could issue an authenticated pushkey command to the Mercurial server stating such. Clients could then easily obtain a list of all changesets that are green.
Why stop there? We could also record automation failures in Mercurial as well. Depending on how complex this gets, we may outgrow pushkey and require a separate command. But that's all doable.
Anyway, clients could download automation results for a specific changeset as part of the repository data. The same extension that pulls down that data could also monkeypatch the bisection algorithm used by hg bisect to automatically skip over changesets that didn't pass automation. You'll never bisect a backed out changeset again!
If this automation data were stored on the Try repository, the autoland tool would just need to query the Mercurial repo to see which changesets are candidates for merging into mainline - there would be no need for a separate database and web service!
Marking a changeset as reviewed
Currently, Mozilla's review procedure is very patch and Bugzilla centric. But it doesn't have to be that way. (I argue it shouldn't be that way.)
Imagine a world where code review is initiated by pushing changesets to a special server, kind of like how Try magically turns pushes into automation jobs.
In this world, reviews could be initiated by issuing a pushkey or custom command to the server. This could even initiate server-side static analysis that would hold off publishing the review unless static analysis checks passed!
Granted review could be recorded by having someone issue a pushkey command to mark a changeset as reviewed. The channel to the Mercurial server is authenticated via SSH, so the user behind the current SSH key is the reviewer. The Mercurial server could store this username as part of the repository data. The autoland tool could then pull down the reviewer data and only consider changesets that have an appropriate reviewer.
It might also be possible to integrate crypto magic into this workflow so reviewers could digitally sign a changeset as reviewed. This could help with the verification of the Firefox source code that Brendan Eich recently outlined.
Like the automation data above, no separate database would be required: all data would be part of the repository. All you need to build is a Mercurial extension.
Encouraging best practices
Mozillians have written a handful of useful Mercurial extensions to help people become more productive. We have also noticed that many developers are still (unknowingly?) running old, slow, and buggy Mercurial releases. We want people to have the best experience possible. How do we do that?
One idea is to install an extension on the server that strongly recommands or even requires users follow best practices (minimal HG version, installed extensions, etc).
I have developed a proof-of-concept that does just this.
Rich querying of metadata
When you start putting more metadata into Mercurial (or at least write Mercurial extensions to aggregate this metadata), all kinds of interesting query opportunities open up. Using revsets and templates, you can do an awful lot to use Mercurial as a database of sorts to extract useful reports.
We could enable a lot of new and useful scenarios by extending Mercurial. We could accomplish this without introducing new services and tools into our already complicated infrastructure and workflows.
The possibilities I've suggested are by no means exhaustive. I encourage others to dream up new and interesting ideas. Who knows, maybe some of them may actually happen.