[CMake] cmake deletes cache when source patch contains symlink to complier directory

[Michael Wild]

Hi Russ

I can't claim to fully understand your explanation, but I tried your  
steps on Mac OS X 10.6 and CMake-2.8 and I can reproduce this error.  
Really, really weird stuff!

Bet it took you quite some time, effort and nerves to track this one  
down...

Michael

On 25. Nov, 2009, at 17:35 , Russell Yanofsky wrote:

> (I tried to create a Mantis bug, but Mantis won't let me log in, and
> when I try to reset the password, I see "This account is protected.  
> You
> are not allowed to access this until the account protection is  
> lifted."
> I tried creating a completely new account, and I see the same message
> with that.)
>
> Simple set of steps to reproduce this bug:
>
>        mkdir /usr/bug_test
>        ln -s /usr test_symlink
>        cd test_symlink/bug_test
>        echo "project(test)" > CMakeLists.txt
>        cmake -DCMAKE_C_COMPILER=/usr/bin/gcc .
>        cmake .
>
> The bug is that the second CMake command outputs:
>
>        You have changed variables that require your cache to be
>        deleted. Configure will be re-run and you may have to reset  
> some
>        variables.
>
> This warning is invalid because no variables were changed, and it is a
> problem because after CMake deletes the cache, the configuration can  
> be
> hosed.
>
> The reason why for the warning and CMake deleting the cache on the
> second invocation is that the first invocation incorrectly saves
> different CMAKE_C_COMPILER paths to CMakeCache.txt and
> CMakeFiles/CMakeCCompiler.cmake files. The CMakeCCompiler file has the
> original path:
>
>        SET(CMAKE_C_COMPILER "/usr/bin/gcc")
>
> But the cache looks something like:
>
>        CMAKE_C_COMPILER:FILEPATH=/home/russ/test_symlink/bin/gcc
>
> The reason for the discrepency is that cache path gets transformed  
> with
> the SystemTools::CollapseFullPath function, while the CCompiler path
> uses the original path specified on the command line.
>
> This bug is difficult to work around because the
> SystemTools::CollapseFullPath uses logic that resolves symlinks in
> reverse, which is something that would require some path crawling
> outside of CMake.
>
> I think a safe and straightforward fix would be to make cmake use
> SystemTools::CollapseFullPath for the path it writes to
> CMakeFiles/CMakeCCompiler.cmake, but I didn't see an obvious way of
> implementing this, because the file is generated by CMake code, and  
> not
> C++ code. I wonder if it makes sense to expose the CollapseFullPath
> function to CMake scripts so CMake code can use it.
>
> This bug just started happening for me recently, after my project  
> layout
> changed, but I tested with a few CMake versions and the behavior is  
> the
> same in CMake 2.8.0, 2.6.4, 2.6.2, and 2.6.0.
>
> I do want to say thanks to CMake developers for making this powerful  
> and
> well thought-out tool. I've had my share of learning experiences with
> CMake but given complexity of what CMake does, I've been amazed to  
> never
> have encountered any real bugs until now, and this bug just seems like
> an unhandled corner case.
>
> - Russ
>
> _______________________________________________
> Powered by www.kitware.com
>
> Visit other Kitware open-source projects at http://www.kitware.com/opensource/opensource.html
>
> Please keep messages on-topic and check the CMake FAQ at: http://www.cmake.org/Wiki/CMake_FAQ
>
> Follow this link to subscribe/unsubscribe:
> http://www.cmake.org/mailman/listinfo/cmake