[cmake-developers] [CMake 0013257]: On-the-fly switching of CMake generators (flexible build tree recreation) results in broken partial reconfiguration

[Mantis Bug Tracker]

The following issue has been SUBMITTED. 
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http://www.itk.org/Bug/view.php?id=13257 
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Reported By:                Andreas Mohr
Assigned To:                
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Project:                    CMake
Issue ID:                   13257
Category:                   CMake
Reproducibility:            have not tried
Severity:                   major
Priority:                   normal
Status:                     new
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Date Submitted:             2012-05-30 04:45 EDT
Last Modified:              2012-05-30 04:45 EDT
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Summary:                    On-the-fly switching of CMake generators (flexible
build tree recreation) results in broken partial reconfiguration
Description: 
To investigate configuration issues with a certain CMakeCache.txt setup,
it would be *very helpful* to be able to create a new *pristine* build
directory, copy the old sufficiently complex (too many settings to try to figure
out individually) CMakeCache.txt there, adapt CMakeCache.txt's build directory
entries to the new name of the sibling directory, and then run e.g. "cmake
-DCMAKE_GENERATOR="Unix Makefiles" ." (WITHIN BUILD TREE) to let CMake
*re*create an entirely fresh build tree yet with a *slightly* different
configuration (in this case, switching the generator).

Well... this actually works. Going from a previous Ninja-based build tree, the
new build directory now actually gets a full Makefile-based hierarchy created.

Except... it falls flat on its face, since CMAKE_MAKE_PROGRAM and
CMAKE_BUILD_TOOL remain set to the old Ninja-based arguments and don't get
overridden to a Makefile-side setting when reconfiguring CMAKE_GENERATOR.
Bad doggy, no cake today.


Manually correcting the two cache variables to contain correct Makefile-side
entries does seem to make everything work as expected (successfully test-builds
an entire library target using make).

Summary:
- we want to play some thoroughly helpful tricks
- it *almost* works
- yet some stupid remaining non-overridden parts prevent it from working
- Majorly annoying bug?? Hell yeah, we've got a winner...

The question would be how to change CMake to handle it properly.
Possibly required steps:
1. add detection to correctly realize that there's a change in CMAKE_GENERATOR
setting (from old to new)
2. override CMAKE_MAKE_PROGRAM (or simply kill cache variable, to then simply
automatically have it configured later)
3. dito with CMAKE_BUILD_TOOL

I chose to initially indicate Severity "major" since it breaks an IMHO very
useful feature.

Thanks!

Steps to Reproduce: 
See initial paragraph in text above.
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Issue History 
Date Modified    Username       Field                    Change               
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2012-05-30 04:45 Andreas Mohr   New Issue                                    
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