RFR: 8282225: GHA: Allow one concurrent run per PR only

[Magnus Ihse Bursie]

On Tue, 22 Feb 2022 07:05:40 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

> Since last year, GHA allows concurrency control over GHA runs:
>  https://github.blog/changelog/2021-04-19-github-actions-limit-workflow-run-or-job-concurrency/
>  https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions#concurrency
> 
> Our GHA workflows trigger on every PR update, sometimes doing multiple runs per PR. This is seldom useful and wastes resources with our very large jobs. For example, one can push a commit, quickly realize there is a mistake, push another commit, and this would do *two* GHA runs, both taking many hours.
> 
> I think we can say that only one run per branch is good, and all running/pending runs should be cancelled when a new run starts.
> 
> Additional testing:
>  - [x] Verified queued run gets cancelled on new commit
>  - [x] Verified in-progress run gets cancelled on new commit
>  - [x] Verified in-progress run gets cancelled on merge
>  - [x] Verified in-progress run gets cancelled on rebase + force-push

Looks good.

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Marked as reviewed by ihse (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/7570