Extract to function, protect against infinite loop

Seems like under windows, the introduced relative path search can cause
an infinite loop if it doesn't match. Test for that, may need
adjustment.

Signed-off-by: Kimball Thurston <kdt3rd@gmail.com>

PyIlmBase/CMakeLists.txt

@@ -55,48 +55,39 @@ endif()
 # for our other dependency - boost.
 # Boost Python has some .. annoyances in that the python module
 # has version names attached to it
+function(PYILMBASE_EXTRACT_REL_SITEARCH varname pyver pyexe pysitearch)
+  get_filename_component(_exedir ${pyexe} DIRECTORY)
+  string(FIND ${pysitearch} ${_exedir} _findloc)
+  string(LENGTH ${_exedir} _elen)
+  while(_findloc EQUAL -1 AND _elen GREATER 0)
+    get_filename_component(_nexedir ${_exedir} DIRECTORY)
+    string(FIND ${pysitearch} ${_nexedir} _findloc)
+    if (_nexedir STREQUAL _exedir)
+        message(FATAL_ERROR "Unable to get parent directory for ${_exedir}")
+    else()
+        set(_exedir ${_nexedir})
+    endif()
+    string(LENGTH ${_exedir} _elen)
+  endwhile()
+  math(EXPR _elen "${_elen}+1")
+  string(SUBSTRING ${pysitearch} ${_elen} -1 _reldir)
+  set(${varname} ${_reldir} CACHE STRING "Destination sub-folder (relative) for the python ${pyver} modules")
+endfunction()
+
 if (TARGET Python2::Python)
   set(PYILMBASE_BOOST_PY2_COMPONENT "python${Python2_VERSION_MAJOR}${Python2_VERSION_MINOR}")
   message(STATUS "Found Python2 libraries: ${Python2_VERSION_MAJOR}${Python2_VERSION_MINOR}")
   # we can't just use the Python2_SITEARCH variable as that then will
   # ignore CMAKE_INSTALL_PREFIX. Could extract this to a function somewhere
   # if it is generally useful
-  get_filename_component(_exedir ${Python2_EXECUTABLE} DIRECTORY)
-  string(FIND ${Python2_SITEARCH} ${_exedir} _findloc)
-  string(LENGTH ${_exedir} _elen)
-  while(_findloc EQUAL -1 AND _elen GREATER 0)
-    get_filename_component(_exedir ${_exedir} DIRECTORY)
-    string(FIND ${Python2_SITEARCH} ${_exedir} _findloc)
-    string(LENGTH ${_exedir} _elen)
-  endwhile()
-  math(EXPR _elen "${_elen}+1")
-  string(SUBSTRING ${Python2_SITEARCH} ${_elen} -1 _reldir)
-  set(PyIlmBase_Python2_SITEARCH_REL ${_reldir} CACHE STRING "Destination sub-folder (relative) for the python 2 modules")
-  unset(_elen)
-  unset(_exedir)
-  unset(_findloc)
-  unset(_reldir)
+  pyilmbase_extract_rel_sitearch(PyIlmBase_Python2_SITEARCH_REL 2 ${Python2_EXECUTABLE} ${Python2_SITEARCH})
   message(STATUS "  -> Installing to: \${CMAKE_INSTALL_PREFIX}/${PyIlmBase_Python2_SITEARCH_REL}")
 endif()
 if (TARGET Python3::Python)
   set(PYILMBASE_BOOST_PY3_COMPONENT "python${Python3_VERSION_MAJOR}${Python3_VERSION_MINOR}")
   message(STATUS "Found Python3 libraries: ${Python3_VERSION_MAJOR}${Python3_VERSION_MINOR}")
   # and figure out the install root here
-  get_filename_component(_exedir ${Python3_EXECUTABLE} DIRECTORY)
-  string(FIND ${Python3_SITEARCH} ${_exedir} _findloc)
-  string(LENGTH ${_exedir} _elen)
-  while(_findloc EQUAL -1 AND _elen GREATER 0)
-    get_filename_component(_exedir ${_exedir} DIRECTORY)
-    string(FIND ${Python3_SITEARCH} ${_exedir} _findloc)
-    string(LENGTH ${_exedir} _elen)
-  endwhile()
-  math(EXPR _elen "${_elen}+1")
-  string(SUBSTRING ${Python3_SITEARCH} ${_elen} -1 _reldir)
-  set(PyIlmBase_Python3_SITEARCH_REL ${_reldir} CACHE STRING "Destination sub-folder (relative) for the python 3 modules")
-  unset(_elen)
-  unset(_exedir)
-  unset(_findloc)
-  unset(_reldir)
+  pyilmbase_extract_rel_sitearch(PyIlmBase_Python3_SITEARCH_REL 3 ${Python3_EXECUTABLE} ${Python3_SITEARCH})
   message(STATUS "  -> Installing to: \${CMAKE_INSTALL_PREFIX}/${PyIlmBase_Python3_SITEARCH_REL}")
 endif()
 # different flavors of O.S. have multiple versions of python