RFR: 8282225: GHA: Allow one concurrent run per PR only [v2]

[Magnus Ihse Bursie]

On Tue, 22 Feb 2022 16:18:24 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
>
> Aleksey Shipilev has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Also specialize by workflow

The two reviewer rule is specific to hotspot code. In build code, we're okay with a single reviewer and no 24h waiting (with the risk of having to amend the fix later), since so few people are available to review build code.

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