[darc/maestro++] Create a MergeActor to merge dependency updates without opening a PR

[riarenas]

• This issue is blocking
• This issue is causing unreasonable pain

Linked to #4754 and #5136. This actor should live alongside the pull request actor and for subscriptions that have opted in, it should:

• trigger an official build for a branch with the dependency updates applied
• attempt to merge the change if the build is sucessful
• Promote the build to the target channel of the subscription that is being processed
• Cancel a duplicate official build.

And be able to tell the dependency updater service to fall back to the pull request flow if something fails.

[riarenas]

@mmitche mentioned that he's seen cases where azdo won't automatically trigger a build for a SHA that has already been built. Need to take that into account for the duplicate official build cancellation.

THe actor should probably still try and look for a duplicate build, and if it doesn't see one after some time, then there's nothing to cancel.

Need to run some experiments about this.

[lukas-lansky]

@riarenas Oh it would be so nice to have this mechanism in place.

Just out of pure curiousity, I wonder what's the relationship between the effort to move unit tests out of official builds and this issue. Do we worry about dependency upgrade breaking tests? It happens.

[mmitche]

@lukas-lansky Yes, we do worry about that. However, we mitigate this in two ways:

1. CI is still run on commit, so a repo owner can still validate quality
2. We likely will not enable this workflow for cases where we think breaking changes are likely. For instance:

• Active dev workflows (e.g. the .net 5 main branches)
• Subscriptions from tooling repos (since they bring in breaking changes)

[riarenas]

closing as we're closing the parent epic