PYCBC-1643: Update build system mechanism to find Python3

Changes
=======
* Add function to attempt to determine the local Python3 version
* Use local Python3 if found, otherwise default to version CMake is able
  to find

Change-Id: Ibcaca99b723fa9e5ecf4e36bd11ab7603393dda1
Reviewed-on: https://review.couchbase.org/c/couchbase-python-client/+/222172
Reviewed-by: Dimitris Christodoulou <dimitris.christodoulou@couchbase.com>
Tested-by: Build Bot <build@couchbase.com>

Author: thejcfactor

CMakeLists.txt

@@ -20,6 +20,21 @@ include(FetchContent)
 
 set(CMAKE_CXX_STANDARD 17)
 
+function(get_python_version)
+  if(DEFINED ENV{PYCBC_PYTHON3_EXECUTABLE})
+    message(STATUS "Using python3 executable from environment: $ENV{PYCBC_PYTHON3_EXECUTABLE}")
+    set(PYTHON3_EXE $ENV{PYCBC_PYTHON3_EXECUTABLE})
+  else()
+    message(STATUS "Using default python3 executable")
+    set(PYTHON3_EXE "python3")
+  endif()
+
+  execute_process(COMMAND ${PYTHON3_EXE} -c "import platform;print(platform.python_version())"
+                  OUTPUT_VARIABLE local_python_version)
+  string(STRIP "${local_python_version}" PYTHON_VERSION)
+  set(LOCAL_PYTHON_VERSION "${PYTHON_VERSION}" PARENT_SCOPE)
+endfunction()
+
 if(WIN32)
   # cmake-format: off
   # MultiThreaded$<$<CONFIG:Debug>:Debug>DLL for /MD compile flag
@@ -33,13 +48,17 @@ endif()
 project(couchbase_client)
 set(CMAKE_EXPORT_COMPILE_COMMANDS ON)
 
-if(PYTHON_VERSION_EXACT)
-  set(Python_FIND_VIRTUALENV FIRST)
-  message("finding python version ${PYTHON_VERSION_EXACT}")
+get_python_version()
+message(STATUS "LOCAL_PYTHON_VERSION=${LOCAL_PYTHON_VERSION}")
+
+if(LOCAL_PYTHON_VERSION)
+  set(Python3_FIND_VIRTUALENV FIRST)
+  message("finding python version ${LOCAL_PYTHON_VERSION}")
+  find_package(Python3 ${LOCAL_PYTHON_VERSION} EXACT COMPONENTS Interpreter Development.Module)
 else()
-  set(PYTHON_VERSION_EXACT 3.6)
+  find_package(Python3 COMPONENTS Interpreter Development.Module)
 endif()
-find_package(Python3 ${PYTHON_VERSION_EXACT} COMPONENTS Interpreter Development.Module)
+
 
 if(WIN32)
   set(PYCBC_C_MOD_SUFFIX ".pyd")

pycbc_build_setup.py

@@ -146,11 +146,23 @@ def create_cmake_config(cls,  # noqa: C901
                 os.environ.get('CMAKE_COMMON_VARIABLES', '').split(' ')
                 if x])
 
-        python3_executable = env.pop('PYCBC_PYTHON3_EXECUTABLE', None)
+        python3_find_strategy = env.get('PYCBC_PYTHON3_FIND_STRATEGY', 'LOCATION')
+        cmake_config_args += [f'-DPython3_FIND_STRATEGY={python3_find_strategy}']
+
+        python3_rootdir = env.get('PYCBC_PYTHON3_ROOT_DIR', None)
+        if python3_rootdir:
+            cmake_config_args += [f'-DPython3_ROOT_DIR={python3_rootdir}']
+
+        python3_executable = env.get('PYCBC_PYTHON3_EXECUTABLE', None)
+        if python3_executable is None and sys.executable:
+            # if sys.executable determines the path we want to use that to determine the
+            # Python version in our get_python_version CMake function.
+            python3_executable = sys.executable
+            env['PYCBC_PYTHON3_EXECUTABLE'] = python3_executable
         if python3_executable:
             cmake_config_args += [f'-DPython3_EXECUTABLE={python3_executable}']
 
-        python3_include = env.pop('PYCBC_PYTHON3_INCLUDE_DIR', None)
+        python3_include = env.get('PYCBC_PYTHON3_INCLUDE_DIR', None)
         if python3_include:
             cmake_config_args += [f'-DPython3_INCLUDE_DIR={python3_include}']