CMAKE_INSTALL_PREFIX can be set incorrectly after error

[Helveg]

Describe the bug If you first hit the "NEST must be installed in a virtual environment or with CMAKE_INSTALL_PREFIX", then switch to a venv and rerun, CMAKE_INSTALL_PREFIX will have been set to /usr/local/:

(3.8.10) robin@neurocomp:~/nest-simulator/build$ cmake ..

...

-- Found Python: /home/robin/.pyenv/versions/3.8.10/bin/python3.8 (found suitable version "3.8.10", minimum required is "3.8") found components: Interpreter Development.Module 
CMake Error at cmake/ProcessOptions.cmake:415 (message):
  No virtual Python environment found and no installation prefix specified.
  Please either build and install NEST in a virtual Python environment or
  specify CMake option -DCMAKE_INSTALL_PREFIX=<nest_install_dir>.
Call Stack (most recent call first):
  CMakeLists.txt:143 (nest_process_with_python)


-- Configuring incomplete, errors occurred!
See also "/home/robin/nest-simulator/build/CMakeFiles/CMakeOutput.log".
See also "/home/robin/nest-simulator/build/CMakeFiles/CMakeError.log".
(3.8.10) robin@neurocomp:~/nest-simulator/build$ pyenv virtualenv virtual3.8
(3.8.10) robin@neurocomp:~/nest-simulator/build$ pyenv local virtual3.8 
(virtual3.8) robin@neurocomp:~/nest-simulator/build$ cmake ..
...

-- Found PythonInterp: /home/robin/.pyenv/versions/virtual3.8/bin/python (found version "3.8.10") 

--------------------------------------------------------------------------------
NEST Configuration Summary
--------------------------------------------------------------------------------

Build type          : 
Target System       : Linux
Cross Compiling     : FALSE
C compiler          : GNU 9.3.0 (/usr/bin/cc)
C compiler flags    :  -O2 -Wall -fopenmp -fdiagnostics-color=auto
C++ compiler        : GNU 9.3.0 (/usr/bin/c++)
C++ compiler flags  :  -std=c++11 -O2 -Wall -fopenmp -fdiagnostics-color=auto
Build dynamic       : ON

Built-in modules    : models
User modules        : None
Python bindings     : Yes (Python 3.8.10: /home/robin/.pyenv/versions/3.8.10/bin/python3.8)
       Includes     : /home/robin/.pyenv/versions/3.8.10/include/python3.8
       Libraries    : 

Cython bindings     : Yes (Cython 0.29.25: /home/robin/.pyenv/versions/3.8.10/bin/cython)
MPI4PY header	    : NO 
Use threading       : Yes (OpenMP: -fopenmp)
Use GSL             : No
Use Readline        : Yes (GNU Readline 8.0)
    Includes        : /usr/include
    Libraries       : /usr/lib/x86_64-linux-gnu/libreadline.so;/usr/lib/x86_64-linux-gnu/libncurses.so

Use libltdl         : Yes (LTDL 2.4.6)
    Includes        : /usr/include
    Libraries       : /usr/lib/x86_64-linux-gnu/libltdl.so

Use doxygen         : No
Use MPI             : No
Detailed timers     : No

Use MUSIC           : No
Use libneurosim     : No
Use Boost           : No
Use SIONlib         : No

--------------------------------------------------------------------------------

The NEST executable will be installed to:
  /usr/local/bin/

NEST dynamic libraries and user modules will be installed to:
  /usr/local/lib/nest/

Documentation and examples will be installed to:
  /usr/local/share/doc/nest/

PyNEST will be installed to:
    /usr/local/lib/python3.8/site-packages

To Reproduce Steps to reproduce the behavior:

1. cmake .. in non-virtual Python env.
2. Switch to virtual env
3. cmake ..

Expected behavior The CMAKE_INSTALL_PREFIX should be unset when the error is thrown so that on the next cmake run CMake doesn't presume the user has set it.

[med-ayssar]

Hey @Helveg, maybe you might want to take a look at this PR 2245.

[github-actions[bot]]

Issue automatically marked stale!

[heplesser]

@Helveg Is this still relevant now that #2245 is merged?

[Helveg]

I would close it until something resurfaces, it's probably just a CMAKE learning curve to make a habit of clearing your build cache