Only regenerate ROOT dictionaries when needed

cmake/modules/ROOTTargetMacros.cmake

@@ -84,7 +84,9 @@ function(generate_target_and_root_library target)
     endif()
     list(APPEND headers ${habs})
     get_filename_component(hName ${habs} NAME)
-    configure_file(${habs} "${CMAKE_BINARY_DIR}/include/${hName}")
+    execute_process(
+      COMMAND "${CMAKE_COMMAND}" -E copy_if_different "${habs}" "${CMAKE_BINARY_DIR}/include/${hName}"
+      OUTPUT_QUIET ERROR_QUIET)
   endforeach()
 
 
@@ -249,12 +251,44 @@ function(make_target_root_dictionary target)
 
   set(includeDirs $<REMOVE_DUPLICATES:$<TARGET_PROPERTY:${target},INCLUDE_DIRECTORIES>>)
 
-  # add a custom command to generate the dictionary using rootcling
+  # Write the list of input headers to a cmake file at configure time.
+  # The build-time stamp script reads this file to compute content hashes,
+  # avoiding semicolon/shell-escaping issues when passing long lists via -D.
+  set(_headersListFile ${CMAKE_CURRENT_BINARY_DIR}/${dictionary}_headers_list.cmake)
+  set(_headersListContent "set(DICT_HEADERS\n")
+  foreach(_h ${headers})
+    string(APPEND _headersListContent "  \"${_h}\"\n")
+  endforeach()
+  string(APPEND _headersListContent ")\n")
+  file(WRITE "${_headersListFile}.tmp" "${_headersListContent}")
+  execute_process(
+    COMMAND "${CMAKE_COMMAND}" -E copy_if_different
+      "${_headersListFile}.tmp" "${_headersListFile}"
+    OUTPUT_QUIET ERROR_QUIET)
+  file(REMOVE "${_headersListFile}.tmp")
+
+  set(stampFile ${CMAKE_CURRENT_BINARY_DIR}/${dictionary}_headers.stamp)
+
+  # Step 1: Update stamp only when header CONTENT changes (not just mtime).
+  # This prevents rootcling from rerunning after git pull or cmake reconfigure
+  # when header content is actually unchanged.
+  # cmake-format: off
+  add_custom_command(
+    OUTPUT ${stampFile}
+    COMMAND ${CMAKE_COMMAND}
+      "-DHEADERS_LIST_FILE=${_headersListFile}"
+      "-DSTAMP_FILE=${stampFile}"
+      -P "${CMAKE_SOURCE_DIR}/cmake/modules/update_dict_stamp.cmake"
+    DEPENDS ${headers}
+    VERBATIM
+    COMMENT "Checking header content for ${dictionary}")
+  # cmake-format: on
+
+  # Step 2: Run rootcling only when stamp changes (i.e., header CONTENT changed).
   # cmake-format: off
-  set(space " ")
-  #message(STATUS " Adding dictionary ${dictionaryFile} to target ${target}")
   add_custom_command(
     OUTPUT ${dictionaryFile} ${pcmFile} ${rootmapFile}
+    BYPRODUCTS ${CMAKE_CURRENT_BINARY_DIR}/${pcmBase}
     VERBATIM
     COMMAND ${CMAKE_COMMAND} -E env "LD_LIBRARY_PATH=${LD_LIBRARY_PATH}:$ENV{LD_LIBRARY_PATH}"
     ${ROOT_rootcling_CMD}
@@ -267,7 +301,7 @@ function(make_target_root_dictionary target)
     ${headers}
     COMMAND ${CMAKE_COMMAND} -E copy_if_different ${CMAKE_CURRENT_BINARY_DIR}/${pcmBase} ${pcmFile}
     COMMAND_EXPAND_LISTS
-    DEPENDS ${headers})
+    DEPENDS ${stampFile})
   # cmake-format: on
 
   # add dictionary source to the target sources and suppress warnings

cmake/modules/update_dict_stamp.cmake

@@ -0,0 +1,48 @@
+# Helper script invoked at build time to update a ROOT dictionary stamp file
+# only when the content of the input headers has actually changed.
+#
+# This breaks the mtime-based dependency chain for rootcling: rather than
+# rebuilding dictionaries whenever any header has a newer mtime (e.g., after
+# git pull or cmake reconfigure), rootcling only reruns when header content
+# actually differs.
+#
+# Arguments (passed via -D flags):
+#   HEADERS_LIST_FILE  -- path to a cmake file that sets DICT_HEADERS to the
+#                         list of input header paths
+#   STAMP_FILE         -- path to the stamp file to (conditionally) update
+
+if(NOT DEFINED HEADERS_LIST_FILE OR NOT DEFINED STAMP_FILE)
+  message(FATAL_ERROR
+    "update_dict_stamp.cmake requires HEADERS_LIST_FILE and STAMP_FILE variables")
+endif()
+
+if(NOT EXISTS "${HEADERS_LIST_FILE}")
+  message(FATAL_ERROR
+    "Headers list file missing: ${HEADERS_LIST_FILE}\n"
+    "Re-run cmake to regenerate it: cmake -S <src> -B <build>")
+endif()
+
+include("${HEADERS_LIST_FILE}") # sets DICT_HEADERS
+
+# Compute combined SHA256 hash of all input headers.
+set(_combined "")
+foreach(_h IN LISTS DICT_HEADERS)
+  if(NOT EXISTS "${_h}")
+    message(FATAL_ERROR "Header not found: ${_h}")
+  endif()
+  file(SHA256 "${_h}" _h_hash)
+  string(APPEND _combined "${_h}:${_h_hash}\n")
+endforeach()
+string(SHA256 _new_hash "${_combined}")
+set(_new_content "${_new_hash}\n")
+
+# Only rewrite stamp file when the hash has changed.
+# Preserving the mtime when content is unchanged prevents rootcling from
+# rerunning (the dictionary custom command depends on this stamp's mtime).
+set(_old_content "")
+if(EXISTS "${STAMP_FILE}")
+  file(READ "${STAMP_FILE}" _old_content)
+endif()
+if(NOT _new_content STREQUAL _old_content)
+  file(WRITE "${STAMP_FILE}" "${_new_content}")
+endif()