ci, iwyu: fail job on compiler errors instead of silently logging them #36235

1. 
   David-Uka commented at 12:58 AM on September 13, 2026: none

   Follow-up to #35468 and #36228, which fixed the fatal error: '…' file not found messages reported in #35361 by generating the required headers before IWYU runs.

   Those errors were only ever printed to the raw CI logs and never failed the job, because iwyu_tool.py's non-zero exit is swallowed by || true. That is how they went unnoticed for so long.

   IWYU cannot analyse a file that fails to compile, so this turns such compiler errors into a real CI failure: after capturing IWYU output, the job fails if it contains a clang fatal error: line and prints the offending file, pointing at the GOAL header targets in ci/test/00_setup_env_native_iwyu.sh. The check lives in run_iwyu() so it covers both the enforced and non-enforced passes, since the errors originally reported came from files in the non-enforced pass.

   This locks in the recent header-generation fixes and guards against regressions such as a new capnp-generated header or a renamed target.

   I confirmed the current master iwyu job log is free of fatal error: lines, so this does not turn CI red today.

   Closes #35361.

2. ci, iwyu: fail job on compiler errors instead of silently logging them

   iwyu_tool.py's non-zero exit is swallowed by "|| true", so a compiler
   "fatal error:" (for example a generated header that was not built before
   IWYU ran) only ends up in the raw CI logs and never fails the job. This
   is how the errors reported in #35361 went unnoticed.
   
   IWYU cannot analyse a file that fails to compile, so fail the job when its
   output contains a "fatal error:" line, in both the enforced and
   non-enforced passes. This locks in the header-generation fixes from
   #35468 and #36228 and guards against regressions such as a new
   capnp-generated header or a renamed target.

   676a83f935
3. 
   DrahtBot commented at 12:59 AM on September 13, 2026: contributor

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   For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/36235.

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4. 
   hebasto commented at 9:31 AM on September 13, 2026: member

   Concept ACK.

   cc @maflcko

5. David-Uka requested review from hebasto on Sep 15, 2026

Review Requested
hebasto