[backport] Add option to toggle the formatting via clang-format in th…

…e generator cmake macro (#282) (#308)

cmake/podioMacros.cmake

@@ -105,7 +105,8 @@ function(PODIO_GENERATE_DICTIONARY dictionary)
   set_target_properties(${dictionary} PROPERTIES CXX_CLANG_TIDY "")
 endfunction()
 
-
+set(PODIO_USE_CLANG_FORMAT AUTO CACHE STRING "Try to use clang-format to format the code generated by podio")
+set_property(CACHE PODIO_USE_CLANG_FORMAT PROPERTY STRINGS AUTO ON OFF)
 #---------------------------------------------------------------------------------------------------
 #---PODIO_GENERATE_DATAMODEL( datamodel YAML_FILE RETURN_HEADERS RETURN_SOURCES
 #      OUTPUT_FOLDER      output_directory
@@ -139,11 +140,31 @@ function(PODIO_GENERATE_DATAMODEL datamodel YAML_FILE RETURN_HEADERS RETURN_SOUR
   ENDIF()
 
   set(CLANG_FORMAT_ARG "")
-  find_program(CLANG_FORMAT_EXE NAMES "clang-format")
-  find_file(CLANG_FORMAT_FILE .clang-format PATH ${PROJECT_SOURCE_DIR} NO_DEFAULT_PATH)
-  if(CLANG_FORMAT_EXE AND CLANG_FORMAT_FILE)
-    message(STATUS "Found .clang-format file and clang-format executable. Will pass it to podio class generator")
-    set(CLANG_FORMAT_ARG "--clangformat")
+  if (PODIO_USE_CLANG_FORMAT STREQUAL AUTO OR PODIO_USE_CLANG_FORMAT)
+    find_program(CLANG_FORMAT_EXE NAMES "clang-format")
+    find_file(CLANG_FORMAT_FILE .clang-format PATH ${PROJECT_SOURCE_DIR} NO_DEFAULT_PATH)
+    # only newer versions of clang-format know about fallback-style.
+    execute_process(COMMAND ${CLANG_FORMAT_EXE} --fallback-style OUTPUT_VARIABLE CLANG_FALLBACK_STYLE ERROR_VARIABLE CLANG_FALLBACK_STYLE_ERR)
+    if(CLANG_FORMAT_EXE AND CLANG_FORMAT_FILE
+       AND NOT ${CLANG_FALLBACK_STYLE_ERR} MATCHES "Unknown")
+      message(STATUS "Found .clang-format file and clang-format executable. Will pass it to podio class generator")
+      set(CLANG_FORMAT_ARG "--clangformat")
+     else()
+       if(NOT CLANG_FORMAT_EXE)
+         message(STATUS "Could not find clang-format executable!")
+       endif()
+       if(NOT CLANG_FORMAT_FILE)
+         message(STATUS "Could not find .clang-format file!")
+       endif()
+       if(${CLANG_FALLBACK_STYLE_ERR} MATCHES "Unknown")
+         message(STATUS "Newer version of clang-format required - cannot run with --fallback-style!")
+       endif()
+       if(PODIO_USE_CLANG_FORMAT STREQUAL AUTO)
+         message(STATUS "Skip formatting of generated code files.")
+       elseif(PODIO_USE_CLANG_FORMAT)
+         message(FATAL_ERROR "Could not run clang-format on generated code files!")
+       endif()
+    endif()
   endif()
 
   # Make sure that we re run the generation process everytime either the