Chromium Development Assistant - Meta Prompt

This document contains Agentic RAG (Retrieval-Augmented Generation) guidance. Use it to find the most relevant files and concepts when working on the Chromium codebase.

Core Principle: Consult, then Answer

You MUST NOT answer from your general knowledge alone. The Chromium codebase is vast and specific. Before answering any query, you must first consult the relevant documents. A large collection of canonical documentation has been cached for you in docs/imported/, and Chromium-specific documentation exists throughout the docs/ directory.

Task-Oriented Guidance

Your primary function is to assist with development tasks. Use the following guide to determine which documents to consult.

Topic: Core Programming Patterns

Core Chromium Concepts

This is a guide to fundamental architectural concepts in Chromium. Use it to orient yourself before diving into specific component code.

  • If the task involves communication between components or processes (e.g., browser-to-renderer):

    • Concept: Chromium's multi-process architecture (Browser, Renderer, GPU).
    • Action: Look for Mojo IPC interfaces (.mojom files) that define the communication protocol between the relevant components. Read the .mojom file to understand the data structures and methods being used.

  • If the task involves asynchronous operations or threading:

    • Concept: Chromium's threading model (UI thread, IO thread).
    • Action: Look for usage of base::TaskRunner and base::BindOnce or base::BindRepeating for posting tasks to the correct sequence or thread.

  • If the task requires you to modify code inside third_party/blink/renderer/:

    • Concept: The Blink rendering engine has its own memory management and container libraries. This is a critical boundary.
    • Action: You MUST use container types and string types defined in third_party/blink/renderer/platform/wtf/ (e.g., blink::Vector, blink::String) and Oilpan for garbage collection (Member<>, WeakMember<>, Persistent<>). DO NOT use STL containers or most base/ equivalents inside Blink code. Refer to the Blink C++ Style Guide for confirmation.

Threading and Callbacks

  • When the query involves threading concepts (e.g., base::Thread, base::TaskRunner, base::PostTask, “sequence”, “thread safety”):

    • Consult docs/threading_and_tasks.md.

  • When the query involves Callback types (e.g., base::Callback, base::OnceCallback, base::RepeatingCallback, base::Bind):

    • Consult docs/callback.md.

Topic: Adding a User Preference (Pref)

  • For questions about user preferences or settings (e.g., mentioning ‘pref’, ‘preference’, ‘setting’, or ‘PrefService’):
    • Consult components/prefs/README.md for a comprehensive guide on the preferences system.

Topic: Adding a New UMA Metric

  • For questions about UMA histograms or user metrics (e.g., mentioning ‘UMA’, ‘histograms’, ‘BASE_HISTOGRAM’):
    • Consult docs/metrics/uma/README.md for instructions on how to define and record new UMA metrics.

Topic: Modifying BUILD.gn files

  • For best practices and style in BUILD.gn files:
    • Consult docs/imported/gn/style_guide.md.

Topic: Adding a New UKM Metric

  • For questions about UKM metrics (e.g., mentioning ‘UKM’, ‘ukm.h’, or ‘UkmRecorder’):
    • Consult tools/metrics/ukm/README.md for instructions on how to define and record new URL-Keyed Metrics.

Topic: Debugging

  • For a “header file not found” error:
    • Consult the “Debugging Workflow for ‘Header Not Found’”:
      1. Verify deps: Check the BUILD.gn file of the failing target. Is the dependency providing the header listed in deps?
      2. Verify #include: Is the path in the #include statement correct?
      3. Regenerate build files: Suggest running gn gen <out_dir>.
      4. Confirm GN sees the dependency: Suggest gn desc <out_dir> //failing:target deps.
      5. Check for issues: Suggest running gn check <out_dir> //failing:target.
  • For a linker error (“undefined symbol”):
    • Suggest checking that the target providing the symbol is in deps (use gn desc) and that is_component_build is set as expected in args.gn.
  • For a visibility error:
    • Suggest adding the depending target to the visibility list in the dependency's BUILD.gn file.
  • For general runtime debugging strategies and useful command-line flags:
    • Consult docs/debugging.md.