Add support for automatic cancellation of running/pending builds

[oldmantaiter]

By using two new variables (flagged as EXPERIMENTAL), new builds
for a branch will check to see whether it should cancel any preceding
builds.

Cancelling pending builds uses the same logic that killing zombie processes
does, and cancelling running builds uses the same logic that cancelling from
the UI does.

Deals, in part, with the ideas discussed in #1980

[CLAassistant]

All committers have signed the CLA.

[oldmantaiter]

@bradrydzewski - Have you had time to take a look at this? I've been running it in my stack since I opened the PR and it doesn't seem to have any issues that I've noticed.

[bradrydzewski]

Sorry for the delayed reply. Auto-cancel is something that would be implemented at the repository-level as opposed to a global setting. It would also only cancel pending builds (not yet possible in Drone) as opposed to running builds. The planned design is based on how other systems handle this feature (see below screenshot for prior art).

The ability to cancel pending builds should land in 0.9 with our new queue system, which could then be used to implement auto-cancel for pending builds.

[oldmantaiter]

@bradrydzewski - Thanks for the update. I've seen a couple threads asking for auto-cancel of running builds, so I am wondering if once those 0.9 features land I could take a crack at implementing auto-cancel of running builds? I think if people understand that once enabled it is not responsible for gracefully stopping the running build, then maybe it can be labelled as an "at your own risk" feature that will only be allowed in the community edition and hidden behind a server config enabling the server to look for the repo config variable that allows cancelling running builds for the same branch. What do you think?

[casiodk]

how far is this feature from release?

[damm]

Crickets this feature would be quite helpful as one of our tests spawns a bunch of docker containers (using dind) and if I push too fast I can take down our drone host easily

32gigs of ram it's not huge but would be helpful if it would auto cancel the previous build in favor of a new build if i push into a branch.

[thunderball1]

Is this going to be merged?

[bradrydzewski]

@thunderball1 see comment here #2267 (comment) tldr we want per-repository implementation, not global

[oldmantaiter]

@bradrydzewski Is there a branch for the new queuing system you mentioned in #2267 (comment) ?