[mlir] Add option to control python search priming

[rkayaith]

Summary

This adds an option to configure the CMake python search priming
behaviour that was introduced in D118148. In some environments the
priming would cause the "real" search to fail. The default behaviour is
unchanged, i.e. the search will be primed.

[rkayaith]

This works around an issues I was seeing with D118148:

In D118148#3352001, @rkayaith wrote:

I started rebuilding my dev environment to get around this, and it just ended up happening again.
Here's the steps to reproduce: https://gist.github.com/rkayaith/1368238ceabf5c0c255528a0eba2e0c0
I ran that script on a live Ubuntu 20.04 USB:

$ sudo add-apt-repository universe
$ sudo apt install -y curl
$ sudo -s
$ curl https://gist.githubusercontent.com/rkayaith/1368238ceabf5c0c255528a0eba2e0c0/raw/mlir_build_2004_pyenv.sh > build.sh
$ source build.sh

Which results in the following error:

-- Could NOT find Python3 (missing: Python3_INCLUDE_DIRS Development Development.Module Development.Embed) (found suitable version "3.8", minimum required is "3.6")
CMake Error at /usr/share/cmake-3.22/Modules/FindPackageHandleStandardArgs.cmake:230 (message):
  Could NOT find Python3 (missing: Development.Module NumPy) (found suitable
  version "3.7.10", minimum required is "3.6")
Call Stack (most recent call first):
  /usr/share/cmake-3.22/Modules/FindPackageHandleStandardArgs.cmake:594 (_FPHSA_FAILURE_MESSAGE)
  /usr/share/cmake-3.22/Modules/FindPython/Support.cmake:3166 (find_package_handle_standard_args)
  /usr/share/cmake-3.22/Modules/FindPython3.cmake:490 (include)
  /home/ubuntu/llvm-project/mlir/cmake/modules/MLIRDetectPythonEnv.cmake:33 (find_package)
  /home/ubuntu/llvm-project/mlir/CMakeLists.txt:135 (mlir_configure_python_dev_packages)

I've confirmed that applying this patch and building with -DMLIR_DETECT_PYTHON_ENV_PRIME_SEARCH=0 can configure successfully in the environment mentioned above.

[stellaraccident]

mlir/CMakeLists.txt

135

Can you extend the comment a bit with " (this is a temporary measure while we debug Python detection issues on different platforms)".

mlir/CMakeLists.txt

 # Python Bindings Configuration
 # Requires:
 #   The pybind11 library can be found (set with -DPYBIND_DIR=...)
 #   The python executable is correct (set with -DPython3_EXECUTABLE=...)
 #-------------------------------------------------------------------------------
 
 set(MLIR_ENABLE_BINDINGS_PYTHON 0 CACHE BOOL
        "Enables building of Python bindings.")
+set(MLIR_DETECT_PYTHON_ENV_PRIME_SEARCH 1 CACHE BOOL
+       "Prime the python detection by searching for a full 'Development' \
+       component first (temporary while diagnosing environment specific Python \
+       detection issues)")
 if(MLIR_ENABLE_BINDINGS_PYTHON)
   include(MLIRDetectPythonEnv)
   mlir_configure_python_dev_packages()
 endif()
 
 include_directories( "include")
 include_directories( ${MLIR_INCLUDE_DIR})
 

mlir/cmake/modules/MLIRDetectPythonEnv.cmake

if(CMAKE_VERSION VERSION_LESS "3.18.0")
     message(WARNING
         "This version of CMake is not compatible with statically built Python "
         "installations. If Python fails to detect below this may apply to you. "
         "Recommend upgrading to at least CMake 3.18. "
         "Detected current version: ${CMAKE_VERSION}"
     )
     set(_python_development_component Development)
   else()
-    # Prime the search for python to see if there is a full
-    # development package. This seems to work around cmake bugs
-    # searching only for Development.Module in some environments.
+    if(MLIR_DETECT_PYTHON_ENV_PRIME_SEARCH)
+      # Prime the search for python to see if there is a full development
+      # package. This seems to work around cmake bugs searching only for
+      # Development.Module in some environments. However, in other environments
+      # it may interfere with the subsequent search for Development.Module.
       find_package(Python3 ${LLVM_MINIMUM_PYTHON_VERSION}
         COMPONENTS Development)
+    endif()
     set(_python_development_component Development.Module)
   endif()
   find_package(Python3 ${LLVM_MINIMUM_PYTHON_VERSION}
     COMPONENTS Interpreter ${_python_development_component} NumPy REQUIRED)
   unset(_python_development_component)
   message(STATUS "Found python include dirs: ${Python3_INCLUDE_DIRS}")
   message(STATUS "Found python libraries: ${Python3_LIBRARIES}")
   message(STATUS "Found numpy v${Python3_NumPy_VERSION}: ${Python3_NumPy_INCLUDE_DIRS}")