[CMake] Behaviour of CACHE PATH variables

[Roger Leigh]

On Wed, Mar 25, 2015 at 11:29:28AM -0400, Brad King wrote:
> On 03/25/2015 10:15 AM, Roger Leigh wrote:
> > Question: Why is TESTDIR2 transformed from "t2" to
> > "/home/rleigh/pathtest/test/t2" (which is
> > "$PROJECT_BINARY_DIR/$TESTDIR2)?
> 
> It looks like the behavior has been this way since 2003:
> 
>  ENH: When fixing cache value with uninitialized type, collapse
>  full paths for PATH and FILEPATH. Closes Bug #82 - Specifying
>  relative path when entering path can break things
>  http://cmake.org/gitweb?p=cmake.git;a=commitdiff;h=a6567329
> 
> but was added without documentation back in our wild west days.
> 
> > Copying the GNUInstallDirs behaviour of wrapping in an "if (NOT DEFINED foo)" test
> > mitigates the problem.
> > 
> > For my usage, I'm defining project-specific install dirs derived from the
> > toplevel GNUInstallDirs dirs.  I'd quite like to be able to specify a relative
> > (or absolute) path, and then later prepend the install prefix / appropriate
> > GNUInstallDirs toplevel variable.  Having the current working directory added
> > by default screws this up quite badly!
> 
> From the commit message above I think the intended behavior is
> exactly to transform relative paths.  The behavior has been this
> way for so long I don't think we can change it now.  We can update
> the documentation to describe the behavior, but I think using the
> approach in GNUInstallDirs is the solution for you.

OK.  It's a shame this can't be fixed, but entirely understandable.

A comment explaining the behaviour would be appreciated; it will certainly
save the next person encountering this special case from a good bit of
headscratching!


Thanks,
Roger

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