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

[Russell Yanofsky]

Hi Michael,

Thanks for trying this out. It wasn't too hard to debug after I noticed
the discrepancy in the CMake output. The simple explanation of the bug
is that when CMake runs, it writes the CMAKE_C_COMPILER value out to two
different files:

   CMakeCache.txt
   CMakeFiles/CMakeCCompiler.cmake

The problem is it writes out different paths to each file, because
SystemTools::CollapseFullPath is called on the first path and not the
second. I think a good fix would be to call CollapseFullPath on both
paths, but I'm not sure how to do this because CollapseFullPath is a C++
function, and the second file isn't generated by C++.

- Russ

On Wed, 2009-11-25 at 18:04 +0100, Michael Wild wrote:
> 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
> >
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