[CMake] CMake internal cache deletion seems to nullify command-line cache var settings

[Wheeler, Frederick W (GE Global Research)]

I think I found a CMake bug where cache variables set on the
command-line (CMAKE_INSTALL_PREFIX:PATH) are not set as expected when
the cache is deleted by cmake because a variable like
CMAKE_C_COMPILER:FILEPATH is set.  I've reduced this down as much as I
could.  I'm demonstrating the bug using the ITK source tree, but I doubt
that matters.

My workaround is to always remove the cache file before running cmake.

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# make a build dir
rm -rf $DEV/cmake_debug
mkdir $DEV/cmake_debug
cd $DEV/cmake_debug

# system info
% uname -a
Linux tanaga 2.6.18-164.el5 #1 SMP Tue Aug 18 15:51:48 EDT 2009 x86_64
x86_64 x86_64 GNU/Linux
% /local/wheeler/dev/cmake-2.8.3/bin/cmake --version
cmake version 2.8.3

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# Step 1

Remove the cache file and run CMake, setting CMAKE_INSTALL_PREFIX on
the command-line.  CMAKE_INSTALL_PREFIX gets set correctly.

rm -f CMakeCache.txt
/local/wheeler/dev/cmake-2.8.3/bin/cmake \
  -G"Unix Makefiles" \
  -DCMAKE_INSTALL_PREFIX:PATH=/local/wheeler/dev/itk_usr_gcc_rel \
  -DCMAKE_C_COMPILER:FILEPATH=gcc \
  /local/wheeler/dev/itk_src
grep CMAKE_INSTALL_PREFIX:PATH CMakeCache.txt

CMAKE_INSTALL_PREFIX:PATH=/local/wheeler/dev/itk_usr_gcc_rel

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# Step 2

Do not remove the cache file, but re-run cmake, but without setting
CMAKE_INSTALL_PREFIX:PATH or CMAKE_C_COMPILER:FILEPATH.  No problem,
the setting of CMAKE_INSTALL_PREFIX is preserved.

# rm -f CMakeCache.txt
/local/wheeler/dev/cmake-2.8.3/bin/cmake \
  -G"Unix Makefiles" \
  /local/wheeler/dev/itk_src
grep CMAKE_INSTALL_PREFIX:PATH CMakeCache.txt

CMAKE_INSTALL_PREFIX:PATH=/local/wheeler/dev/itk_usr_gcc_rel

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# Step 3

Do not remove the cache file, but re-run cmake setting
CMAKE_INSTALL_PREFIX:PATH and CMAKE_C_COMPILER:FILEPATH.
CMAKE_INSTALL_PREFIX is no longer set the way I want it.

# rm -f CMakeCache.txt
/local/wheeler/dev/cmake-2.8.3/bin/cmake \
  -G"Unix Makefiles" \
  -DCMAKE_INSTALL_PREFIX:PATH=/local/wheeler/dev/itk_usr_gcc_rel \
  -DCMAKE_C_COMPILER:FILEPATH=gcc \
  /local/wheeler/dev/itk_src
grep CMAKE_INSTALL_PREFIX:PATH CMakeCache.txt

CMAKE_INSTALL_PREFIX:PATH=/usr/local

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In Step 3 I get this warning from CMake.  Should be no problem since
CMAKE_INSTALL_PREFIX is explicitly set on the command line.

You have changed variables that require your cache to be deleted.
Configure will be re-run and you may have to reset some variables.
The following variables have changed:
CMAKE_C_COMPILER= gcc

It seems that when CMake clears the cache because I set
CMAKE_C_COMPILER:FILEPATH it reverts CMAKE_INSTALL_PREFIX:PATH to its
default value and does not honor the -DCMAKE_INSTALL_PREFIX:PATH=... on
the cmake command-line.  Also note that I am not changing the value of
CMAKE_C_COMPILER:FILEPATH.

If this is not a bug, then I suggest the warning cmake issues (above)
also explain that even variables settings made with the -D option may
not be honored.

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