Make message generation idempotent

proto2ros/cmake/proto2ros_generate.cmake

@@ -52,7 +52,10 @@ function(proto2ros_generate target)
 
   set(BASE_PATH "${CMAKE_CURRENT_BINARY_DIR}/proto2ros_generate")
   set(OUTPUT_PATH "${BASE_PATH}/${ARG_PACKAGE_NAME}")
-  file(REMOVE_RECURSE "${OUTPUT_PATH}")
+  # NOTE Do not wipe OUTPUT_PATH here. Generated files are both build outputs
+  # and configure dependencies (via rosidl_generate_interfaces), so deleting and
+  # regenerating them can trigger an endless reconfigure loop. Orphaned files
+  # are pruned below, once the manifest is known.
   file(MAKE_DIRECTORY "${OUTPUT_PATH}")
 
   foreach(path ${ARG_PROTO_DESCRIPTORS})
@@ -140,6 +143,16 @@ function(proto2ros_generate target)
   file(STRINGS "${OUTPUT_PATH}/manifest.txt" output_files)
   file(RENAME "${OUTPUT_PATH}/manifest.txt" "${OUTPUT_PATH}/manifest.orig.txt")
 
+  # Prune orphaned files left over from previous generations
+  set(_keep_files ${output_files}
+    "${OUTPUT_PATH}/manifest.txt" "${OUTPUT_PATH}/manifest.orig.txt")
+  file(GLOB_RECURSE _existing_files "${OUTPUT_PATH}/*")
+  foreach(_existing ${_existing_files})
+    if(NOT _existing IN_LIST _keep_files)
+      file(REMOVE "${_existing}")
+    endif()
+  endforeach()
+
   add_custom_command(
     OUTPUT ${output_files}
     COMMAND

proto2ros/proto2ros/cli/generate.py

@@ -26,6 +26,24 @@
 )
 
 
+def write_text_if_changed(path: pathlib.Path, content: str) -> None:
+    """Write ``content`` to ``path`` only if it has changed.
+
+    Avoids bumping the file's modification time when the generated content is
+    unchanged. This is important because generated files, e.g. ``.msg``, are
+    both build outputs and CMake configure-time dependencies; rewriting them
+    unconditionally can cause an endless reconfigure loop.
+
+    """
+    if path.exists():
+        try:
+            if path.read_text() == content:
+                return
+        except (OSError, UnicodeDecodeError):
+            pass
+    path.write_text(content)
+
+
 def do_generate(args: argparse.Namespace) -> int:
     """Primary function to execute conversion of protobufs to ros msgs."""
     # Fetch baseline configuration.
@@ -101,7 +119,10 @@ def do_generate(args: argparse.Namespace) -> int:
     # Write message specifications to .py file.
     specifications_python_file = args.output_directory / "specifications.py"
     if not args.dry:
-        specifications_python_file.write_text(dump_specifications_python_module(message_specifications, config) + "\n")
+        write_text_if_changed(
+            specifications_python_file,
+            dump_specifications_python_module(message_specifications, config) + "\n",
+        )
     files_written.append(specifications_python_file)
 
     messages_output_directory = args.output_directory / "msg"
@@ -112,13 +133,14 @@ def do_generate(args: argparse.Namespace) -> int:
     for message_specification in message_specifications:
         message_output_file = which_message_specification(message_specification, messages_output_directory)
         if args.force_message_gen or not args.dry:
-            message_output_file.write_text(dump_message_specification(message_specification) + "\n")
+            write_text_if_changed(message_output_file, dump_message_specification(message_specification) + "\n")
         files_written.append(message_output_file)
 
     # Write Python conversion APIs .py file.
     conversions_python_file = args.output_directory / "conversions.py"
     if not args.dry:
-        conversions_python_file.write_text(
+        write_text_if_changed(
+            conversions_python_file,
             dump_conversions_python_module(message_specifications, known_message_specifications, config) + "\n",
         )
     files_written.append(conversions_python_file)
@@ -133,8 +155,8 @@ def do_generate(args: argparse.Namespace) -> int:
             known_message_specifications,
             config,
         )
-        conversions_hpp_file.write_text(hpp_content + "\n")
-        conversions_cpp_file.write_text(cpp_content + "\n")
+        write_text_if_changed(conversions_hpp_file, hpp_content + "\n")
+        write_text_if_changed(conversions_cpp_file, cpp_content + "\n")
     files_written.append(conversions_hpp_file)
     files_written.append(conversions_cpp_file)
 

proto2ros/proto2ros/cli/test_generate.py

@@ -0,0 +1,41 @@
+# Copyright (c) 2025 Robotics and AI Institute LLC dba RAI Institute. All rights reserved.
+
+"""Unit tests for proto2ros.cli.generate."""
+
+import pathlib
+
+from proto2ros.cli.generate import write_text_if_changed
+
+
+def test_write_text_if_changed_creates_file(tmp_path: pathlib.Path) -> None:
+    """A missing file is created with the requested content."""
+    target = tmp_path / "out.txt"
+    write_text_if_changed(target, "hello\n")
+    assert target.read_text() == "hello\n"
+
+
+def test_write_text_if_changed_is_noop_when_unchanged(tmp_path: pathlib.Path) -> None:
+    """Rewriting identical content must not touch the file (stable mtime).
+
+    Generated files are both build outputs and CMake configure-time
+    dependencies; bumping their mtime on every generation causes an endless
+    CMake/Ninja reconfigure loop. This guards that regression.
+    """
+    target = tmp_path / "out.txt"
+    target.write_text("same\n")
+    before_ns = target.stat().st_mtime_ns
+
+    write_text_if_changed(target, "same\n")
+
+    assert target.read_text() == "same\n"
+    assert target.stat().st_mtime_ns == before_ns, "unchanged content must not rewrite the file"
+
+
+def test_write_text_if_changed_updates_on_difference(tmp_path: pathlib.Path) -> None:
+    """Differing content is written out."""
+    target = tmp_path / "out.txt"
+    target.write_text("old\n")
+
+    write_text_if_changed(target, "new\n")
+
+    assert target.read_text() == "new\n"