Add back _refresh_cache_if_necessary

Author: larryliu0820

third-party/CMakeLists.txt

@@ -16,7 +16,7 @@ endif()
 
 FetchContent_gflags()
 # Create a symlink from the gflags source directory to third-party/gflags
-message(STATUS "gflags source dir: ${gflags_SOURCE_DIR}")
+message(STATUS "Creating a symlink from ${gflags_SOURCE_DIR} to third-party/gflags")
 execute_process(
   COMMAND ${CMAKE_COMMAND} -E create_symlink "${gflags_SOURCE_DIR}" "${CMAKE_CURRENT_SOURCE_DIR}/gflags"
 )

tools/cmake/FetchContent.cmake

@@ -8,7 +8,30 @@
 # Make the content available at configure time so that BUCK can use it.
 include(FetchContent)
 
+# Some third-party libraries need to be materialized in the source tree for
+# buck. Unfortunately, FetchContent bakes in generator information in the
+# fetched content. Thus, if the generator is changed, it causes build failures.
+# So, if the generator has changed, then nuke the content.
+function(_refresh_cache_if_necessary DIR)
+  set(_generator_stamp_file "${DIR}/.executorch_cmake_generator_stamp")
+  set(_current_generator "${CMAKE_GENERATOR}")
+
+  if(EXISTS "${_generator_stamp_file}")
+    file(READ "${_generator_stamp_file}" _previous_generator)
+    string(STRIP "${_previous_generator}" _previous_generator)
+    if(NOT _previous_generator STREQUAL _current_generator)
+      file(REMOVE_RECURSE ${DIR})
+    endif()
+  endif()
+
+  file(WRITE "${_generator_stamp_file}" "${_current_generator}")
+endfunction()
+
 function(FetchContent_gflags)
+  if(NOT ${gflags_SOURCE_DIR} STREQUAL "")
+    _refresh_cache_if_necessary(${gflags_SOURCE_DIR})
+  endif()
+
   FetchContent_Declare(
     gflags
     GIT_REPOSITORY https://github.com/gflags/gflags.git