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

[Erik Joelsson]

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

I think this looks ok, and better than the other solution. Should we add a comment somewhere describing what TOPDIR_ALT is and why it's needed for future archeologists?

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Marked as reviewed by erikj (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/25100#pullrequestreview-2825771908