Gn Deps Debugging

GN Dependency Debugging

Fix Chromium GN dependency metadata, include visibility, and DEPS rule failures by correcting the dependency graph accurately, not by adding broad dependencies that only make errors disappear.

Core principles

1. Read the exact error first. Identify the including source file, the included header, the failing target, and the diagnostic category before editing.
2. Find both owning targets. Determine which GN target owns the including file and which GN target owns the included header.
3. Prefer the narrowest dependency. Add the dependency to the smallest target that needs it, not a parent or aggregate target.
4. Preserve platform conditions. If the failing include is in a platform-specific file such as *_win.cc, add the dependency inside the matching platform block such as if (is_win).
5. Use deps for implementation-only dependencies. If a symbol is used only in .cc files, keep the dependency private.
6. Use public_deps only for public API exposure. If a public header exposes a type from another target in its declarations or inline code, downstream consumers may need that dependency transitively.
7. Prefer forward declarations in headers. If a header only needs a pointer, reference, or return type declaration, forward declare and move the include to the .cc file when possible.
8. Do not paper over architecture issues. Circular dependencies usually indicate the need to split an interface target, move shared abstractions, or invert dependencies.
9. Avoid // nogncheck. Use it only for known GN limitations with conditional includes, and include the required bug reference if the local codebase already documents one.

Workflow

1. Capture the failing command and output.

   • If the user provided a truncated error, ask for or rerun the relevant gn check or build command when an output directory and target are known.
   • Do not start a broad build just to reproduce unless the output directory and target are established.

2. Classify the failure.

   • Read common-errors.md and map the diagnostic to one of the supported categories.
   • Extract the including file, included header, and target names from the diagnostic.

3. Find the target that owns the including file.

   • Inspect nearby BUILD.gn files first.
   • Use GN queries when an output directory is known:

     gn refs out/Default //path/to/file.cc --all

   • Prefer the most specific non-aggregate target that directly lists the file or owns the source set containing it.
   • Check whether the file is platform-specific, such as *_win.cc, *_mac.mm, *_linux.cc, *_android.cc, or *_ios.mm. Platform-specific files usually require platform-specific dependencies.

4. Find the target that owns the included header.

   • Search for the header in BUILD.gn sources lists.
   • If several targets list it, prefer the public/interface target intended for consumers.
   • Check whether the header belongs to a generated target, a public variable, or a private implementation target.

5. Choose the correct fix.

   • Read dependency-patterns.md for deps vs public_deps, forward declaration, DEPS, and cycle handling rules.
   • If the include appears only in .cc, add a private deps entry.
   • If the include appears only in a platform-specific source file, add the private deps entry inside the matching platform condition, for example if (is_win).
   • If the include appears in a public header, first try to remove the include using a forward declaration. If the header must expose the dependency, add or promote the dependency to public_deps.
   • If a DEPS file blocks the include, verify whether the dependency direction is allowed by the architecture before adding include rules.
   • If adding the dependency creates a cycle, stop and redesign the dependency boundary instead of adding broader deps. When an output directory is known, use gn path <out_dir> //target_a //target_b to inspect the dependency chain that connects the targets.

6. Validate the narrow change.

   • Run git cl format if BUILD.gn or DEPS files were edited.
   • Re-run the smallest relevant gn check or build command that reproduced the issue.
   • If the first fix reveals additional dependency errors, repeat the workflow one error at a time.