CMake check path write access

CMake configure time is often a convenient time to test if a directory or file is writable. It can be useful to immediately stop with message(FATAL_ERROR) if a path is not writable. For example, if CMAKE_INSTALL_PREFIX is not in a writable location – we want to fail right then and inform users what to do to correct the problem.

file(TOUCH) and file(MAKE_DIRECTORY) do not halt the configure step. Rather, if a path isn’t writable CMake only errors at the end of configure.

Example solution:

This snippet generates a fatal error with text telling the user what to try:

set(test_path /path/to/test/.ignore)
# could be done with random string filename in the desired path

execute_process(COMMAND ${CMAKE_COMMAND} -E touch ${test_path}
if(NOT ret EQUAL "0")
  message(FATAL_ERROR "No write access to ${test_path}
  <text for user to resolve issue>")