8344030: Improved handling of TOOLCHAIN_PATH

[magicus]

This bug report was triggered by the discussion in #21999.

In TOOLCHAIN_PRE_DETECTION, we prepend $TOOLCHAIN_PATH to the actual PATH, and then restore this in TOOLCHAIN_POST_DETECTION. This is likely a remnant of older times, before we started using our own UTIL_LOOKUP* functions.

Instead, we should update UTIL_LOOKUP_TOOLCHAIN_PROGS to use $TOOLCHAIN_PATH:$PATH as 3rd variable to UTIL_LOOKUP_PROGS (if it is not given).

The risk we need to be aware of is if some tool we're running assume that the TOOLCHAIN_PATH is properly included on the PATH, e.g. because it tries to execute a helper command in turn. This could specifically be a problem for Visual Studio, and we need to double-check that removing TOOLCHAIN_PATH from the PATH does not break anything. (In fact, I believe we already append TOOLCHAIN_PATH to the PATH specifically for Windows already, so this could very well be done twice...)

With these changes in place, the hack introduced by JDK-8343883 can be changed to a simple call to UTIL_LOOKUP_TOOLCHAIN_PROGS.

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Issue

• JDK-8344030: Improved handling of TOOLCHAIN_PATH (Bug - P4)

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