RFR: Section on backporting [v7]

[Jesper Wilhelmsson]

On Thu, 2 Dec 2021 19:34:09 GMT, Jesper Wilhelmsson <jwilhelm at openjdk.org> wrote:

>> Guidance on backporting.
>> Also moved "Backing out a change" to the testing section where I believe it fits better. This section is unchanged.
>
> Jesper Wilhelmsson has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Fixes after review

If it's desired to redo the backport filing a REDO bug might be sufficient, but if it's found to be too complex to fix whatever issues caused the backout a REDO bug won't be filed, leaving us in a situation where the main issue looks like it has been backported to 11u when in fact the change is not available in 11u.

Even if the REDO bug is filed, keeping the backported-by-link to the backed out backport would cause confusion as the main issue still looks like it's already backported. And if the second backport is pushed to the same update release there will be two backport issues with the same fix version. In this case JBS will only show one of them in the list of backports, and it's unlikely to be the correct one.

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PR: https://git.openjdk.java.net/guide/pull/66