tree 96c06dc3f48c930eb1caa3b468eb00bdbdb308c0
parent d0489c3863f6b2a0d0b0e15880217da3fd4e6d8f
author Tomasz Śniatowski <tsniatowski@vewd.com> 1613390790 +0100
committer Tomasz Śniatowski <tsniatowski@vewd.com> 1614034138 +0100

Add a -t missingdeps tool to detect some classes of build flakes

The tool looks for targets that depend on a generated file, but do not
properly specify a dependency on the generator. It needs to be run after
a successful build, and will list all potential flakes that may have
broken the build, but didn't due to accidental build step ordering.

The search relies on the correctness of depfile and generator output
information, but these are usually easier to get right than dependencies.

The errors found can usually be verified as actual build flakes by trying
to build the listed problematic files alone in a clean build directory.
Such builds usually fail with a compile job lacking a generated file.

There is some overlap between this tool and 'gn check', but not everyone
uses gn, not everyone using gn uses gn check, and most importantly, gn
check is more about modularity, and less about actual build-time deps
without flakes.

The tool needs to be run after a build completes and depfile data is
collected. It may take several seconds to process, up to a dozen or
two on a large, chromium-sized build.