[darc/maestro++] flow-control command

[mmitche]

During shutdown, it's easy for repos to accidentally (or purposefully) check into release branches and cause automated flow to start again. Freeze flow is intended to avoid this issue.

This could be one of two ways:

• Change subscriptions to manual mode or disable them.
• Disable default channel assignment for repos+branches.

These are both valid ways to freeze automatic flow, and have different advantages and disadvantages: https://github.com/dotnet/arcade/blob/master/Documentation/Darc.md#halting-and-restarting-dependency-flow

What I'd say is that we implement this command initially by disabling default channel assignments based on a specific channel, with options for limiting the repos that are frozen or unfrozen. Something like:

// Freeze all
darc flow-control --channel ".NET Core 3 Release" --halt

// Unfreeze all
darc flow-control --channel ".NET Core 3 Release" --resume

// Unfreeze only corefx
darc flow-control --channel ".NET Core 3 Release" --resume --repo corefx

To help out users, the command should also be clever on resume. It can look up what repos have produced builds on those default channels and produce a command line that can be used to flow those builds using the darc flow command.

[mmitche]

/cc @jcagme

[mmitche]

I think this is less intereresting in the .NET 5 timeframe given the reduced number of repos