Decide a build step by its exit code, not by whether it printed #33
2026-08-26
docs: state that build steps fail on exit status, not on output
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test: a warning on any compile path must not fail the build
The fixture puts a #warning in each kind of source the build compiles — module interface, module implementation, C, consumer — plus an unknown -z value so ld.lld warns on the link step too, and asserts the build succeeds cold (every unit compiling, every warning emitted), succeeds with nothing to do, and succeeds again after each source is touched back into staleness. The staged copy lives outside the repo because the whole test depends on units compiling for the first time. The last pass breaks a source on purpose: "never fails" would satisfy everything above just as well, so the error path is asserted too — both that it fails and that it still reports the compiler's diagnostic.
fix: decide a build step by its exit code, not by whether it printed
RunCommand merged stderr into stdout, dropped pclose's status and returned the text; every compile, link and archive site then read "printed something" as "failed". A warning is printing something, so a translation unit that warned failed the build — but only on the run that actually recompiled it, since warnings aren't re-emitted for an object that's already up to date. The same unchanged source therefore passed or failed depending on the state of the build tree: flaky-looking tests locally, and a cold CI checkout surfacing every latent warning in a project at once as unrelated failures. RunCommand is gone, replaced by RunBuildCommand: it goes through RunCommandChecked, returns "" when the command exited 0 (so every caller's `if (!result.empty())` error path is unchanged) and hands any warnings to the new Progress::Diagnostic instead of to the error path. That also closes the quiet half of the bug — a compiler killed by the OOM killer prints nothing, so it used to read as success and leave the build carrying on with a missing object; it now reports the signal that killed it. Warnings are now shown rather than swallowed, which they weren't in either direction before: invisible on an incremental build, fatal on a cold one. Failing on them stays a project's choice, via -Werror in compileFlags.