RFR: 8351029: IncludeCustomExtension does not work on cygwin with source code below /home [v2]

[Magnus Ihse Bursie]

On Wed, 7 May 2025 22:50:52 GMT, Magnus Ihse Bursie <ihse at openjdk.org> wrote:

>> If you check out your source code in Cygwin, somewhere under /home/... (as opposed to /cygpath/...), IncludeCustomExtension does not work.
>> 
>> The problem is that TOPDIR gets a different lexical value from the spec.gmk file when setup in Makefile, even if this was the same directory. Since THIS_INCLUDE is calculated as a relative path by string substitution of $(TOPDIR), the path failed to relativize correctly.
>
> Magnus Ihse Bursie has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains three additional commits since the last revision:
> 
>  - Merge branch 'master' into fix-preinit-topdir
>  - Alternative solution
>  - 8351029: IncludeCustomExtension does not work on cygwin with source code below /home

This seems like a much safer solution. It will allow us to have two different "spellings" for TOPDIR, one that configure puts in spec.gmk, and one that is calculated by the top-level `Makefile`. I think this will make for a robust solution.

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PR Comment: https://git.openjdk.org/jdk/pull/25100#issuecomment-2860627277