Start discussion for auto-cancel functionality in github service (#180)

* Start discussion for auto-cancel functionality in github service

* Updated few parts after discussion

* Added suggestion to expose cancel functionality through API

Author: lotas

README.md

@@ -65,3 +65,4 @@ See [mechanics](mechanics.md) for more detail.
 | RFC#169 | [Easy Taskcluster Setup](rfcs/0169-Easy-Taskcluster-setup.md)                                                                                                                          |
 | RFC#175 | [Restricted Roles for Github Pull Requests](rfcs/0175-restricted-pull-requests.md)                                                                                                     |
 | RFC#177 | [Skip CI in github integration](rfcs/0177-Skip-ci-integrations.md)                                                                                                                     |
+| RFC#180 | [Github cancel previous tasks](rfcs/0180-Github-cancel-previous-tasks.md)                                                                                                              |

rfcs/0180-Github-cancel-previous-tasks.md

@@ -0,0 +1,38 @@
+# RFC 180 - Github cancel previous tasks
+* Comments: [#180](https://github.com/taskcluster/taskcluster-rfcs/pull/180)
+* Proposed by: @lotas
+
+# Summary
+
+Allow to automatically cancel previous pipelines for github `push` and `pull_request` events.
+
+## Motivation
+
+This will allow to save computing resources and give more flexibility for the Taskcluster users on Github.
+When several pushes happen to the same branch within a short amount of time, like hotfixes or rebases,
+only the last commit and pipelines matters.
+By cancelling previous task groups and tasks we could save significant amount of compute resources and developer hours by not running redundant tasks.
+
+# Details
+
+## Github hooks
+
+This will only apply to non-default branches (and possible protected branches?),
+to allow all pushes to the default branch to have their own builds.
+
+To be flexible and allow some projects to still run checks for all events, we could add new key to the `.taskcluster.yml` configuration.
+We can introduce top-level configuration option `authCancelPreviousChecks: true` (having `true` by default) to control if this behaviour is desired.
+
+Github's webhook handler upon receiving `push` event would check if there are other task groups existing for given branch, that are not HEAD.
+If `authCancelPreviousChecks` is set to `true` it will cancel them.
+
+Cancellation will happen for all the non-resolved tasks within the same `taskGroupId`. However, due to the fact that tasks can create their own sub-tasks, there might be cases where running tasks would still manage to create some tasks that might not be cancelled.
+
+## Github API
+
+To make it easier for developers to cancel running builds associated with some specific branch we can expose new API endpoint. `github.cancelBuilds({ organization, repository, branch })` would be able to find all task groups associated with this branch, find all non-resolved tasks within those groups and cancel those.
+
+
+# Implementation
+
+* Original request issue [#5621](https://github.com/taskcluster/taskcluster/issues/5621)

rfcs/README.md

@@ -53,3 +53,4 @@
 | RFC#169 | [Easy Taskcluster Setup](0169-Easy-Taskcluster-setup.md)                                                                                                                          |
 | RFC#175 | [Restricted Roles for Github Pull Requests](0175-restricted-pull-requests.md)                                                                                                     |
 | RFC#177 | [Skip CI in github integration](0177-Skip-ci-integrations.md)                                                                                                                     |
+| RFC#180 | [Github cancel previous tasks](0180-Github-cancel-previous-tasks.md)                                                                                                              |