Writing to a pipe whose reader has already exited raises SIGPIPE, which
kills the whole process instead of throwing. MacNodeHostWorker already
guarded its stdin pipe with F_SETNOSIGPIPE; the codex App Server client
and the MLX TTS helper transport did not, so a child exiting mid-write
could take down the app. Suppressing the signal exposed that an
undelivered App Server request write was reported as appServerUnavailable
even though the frame was provably never sent, so it now requeues once
onto a fresh child instead of failing the caller.
Test-side pipe write ends whose readers are spawned children (or a
readability handler that can close the pipe mid-test) get the same
suppression so a racing reader exit fails the assertion instead of
killing swiftpm-testing-helper with signal 13, which is what caused the
macos-swift CI lane's intermittent unrelated-test crashes (e.g. PR
#126559, run 32341197738 job 96340683947).
Peter Steinberger