re-enable copy of python files, but be careful that the copy is only … (pytorch#14982)

Summary:
…done once

This allow no-op build to work correctly even when BUILD_CAFFE2_OPS is on.
Pull Request resolved: pytorch#14982

Differential Revision: D13413960

Pulled By: zdevito

fbshipit-source-id: 6e5412a8c375af8a47c76f548cdd31cff15f3853

Author: zdevito

caffe2/CMakeLists.txt

@@ -471,7 +471,7 @@ endif()
 # rebuild step. The long-term fix should be to clean up these rules so they
 # only rerun when needed.
 
-if (BUILD_PYTHON AND BUILD_CAFFE2_OPS)
+if (BUILD_PYTHON)
   # Python site-packages
   # Get canonical directory for python site packages (relative to install
   # location).  It varies from system to system.
@@ -628,37 +628,22 @@ if (BUILD_PYTHON AND BUILD_CAFFE2_OPS)
   endif()
 
   # Finally, Copy all python files to build directory
-  # Generate and create all needed __init__.py files, if they aren't already
-  # present in the current source tree.
-  message(STATUS "Automatically generating missing __init__.py files.")
-  caffe_autogen_init_py_files()
-
   # Create a custom target that copies all python files.
   file(GLOB_RECURSE PYTHON_SRCS RELATIVE ${PROJECT_SOURCE_DIR}
        "${PROJECT_SOURCE_DIR}/caffe2/*.py")
-  add_custom_target(python_copy_files ALL)
-  if(MSVC OR CMAKE_GENERATOR MATCHES "Ninja")
-    # ninja fails when the command line is too long so we split
-    # the target into several. This would be beneficial for VS also
-    # since it build targets in parallel but not custom commands
-    foreach(python_src ${PYTHON_SRCS})
-      get_filename_component(dir ${python_src} DIRECTORY)
-      string(SHA1 name_hash "${python_src}")
-      # get_filename_component(name_we ${python_src} NAME_WE)
-      add_custom_target(python_copy_files_${name_hash}
-          COMMAND ${CMAKE_COMMAND} -E copy
-          ${PROJECT_SOURCE_DIR}/${python_src} ${CMAKE_BINARY_DIR}/${dir})
-      add_dependencies(python_copy_files python_copy_files_${name_hash})
-    endforeach()
-  else()
-    foreach(python_src ${PYTHON_SRCS})
-      get_filename_component(dir ${python_src} DIRECTORY)
-      add_custom_command(
-          TARGET python_copy_files PRE_BUILD
-          COMMAND ${CMAKE_COMMAND} -E copy
-          ${PROJECT_SOURCE_DIR}/${python_src} ${CMAKE_BINARY_DIR}/${dir})
-    endforeach()
-  endif()
+
+  set(build_files)
+  foreach(python_src ${PYTHON_SRCS})
+    add_custom_command(OUTPUT ${CMAKE_BINARY_DIR}/${python_src}
+                       DEPENDS ${PROJECT_SOURCE_DIR}/${python_src}
+                       COMMAND ${CMAKE_COMMAND} -E copy
+                       ${PROJECT_SOURCE_DIR}/${python_src}
+                       ${CMAKE_BINARY_DIR}/${python_src})
+    list(APPEND build_files ${CMAKE_BINARY_DIR}/${python_src})
+  endforeach()
+
+  add_custom_target(python_copy_files ALL DEPENDS ${build_files})
+
 
   # Install commands
   # Pick up static python files