RFR: 8315060: Out of tree incremental build fails with ccache

[Kim Barrett]

On Fri, 25 Aug 2023 22:31:31 GMT, Erik Joelsson <erikj at openjdk.org> wrote:

> After [JDK-8313374](https://bugs.openjdk.org/browse/JDK-8313374), out of tree incremental builds with ccache started failing. The rewriting of the generated dependency (*.d) files creates object file paths with `/../` in them which make does not match to other rules. I don't think we should apply the rewriting when the build dir is outside CCACHE_BASEDIR. 
> 
> During review of [JDK-8313374](https://bugs.openjdk.org/browse/JDK-8313374) I also noted that the CCACHE_BASEDIR should point to WORKSPACE_ROOT rather than TOPDIR for ccache to work uniformly for OracleJDK and OpenJDK builds.
> 
> Both of these issues are addressed with this patch. I have tried various scenarios and it seems to be working for me, but would appreciate some more verification by people using ccache.

Looks good to me, though I'm not an expert in this part of the build system.
It does fix the problem I encountered.

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Marked as reviewed by kbarrett (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/15434#pullrequestreview-1596735017