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fix: sync stderr after command completion to fix CI flake #95

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Merged
blaspat merged 1 commit into from
Jun 25, 2026
Merged

fix: sync stderr after command completion to fix CI flake #95

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17 changes: 17 additions & 0 deletions internal/exec/shell.go
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Original file line number Diff line number Diff line change
Expand Up @@ -345,6 +345,20 @@ func (s *Session) Run(ctx context.Context, cmd string) (string, string, int, err
return "", "", -1, res.err
}

// Wait for the STDBUF marker to appear in stderrBuf.
// This ensures captureStderr has flushed all stderr from the
// command before we read, avoiding a race where stdout signals
// completion before the stderr pipe has finished delivering.
stdbufMarker := fmt.Sprintf("__HERMES_STDBUF_%d__", seq)
for {
s.stderrMu.Lock()
done := strings.Contains(s.stderrBuf.String(), stdbufMarker)
s.stderrMu.Unlock()
if done {
break
}
time.Sleep(time.Millisecond)
}
// Read the stderr produced since fromPos.
s.stderrMu.Lock()
stderrContent := s.stderrBuf.String()[fromPos:]
Expand Down Expand Up @@ -394,6 +408,9 @@ func (s *Session) buildFrame(seq uint64, userCmd string) string {
// deliberately use $? *after* eval so a pipeline like
// `false | true` reports 0.
b.WriteString("__hermes_ec=$?\n")
// STDBUF marker: written to stderr after the user command so Run
// can wait for stderr to drain before reading the buffer.
fmt.Fprintf(&b, "echo '__HERMES_STDBUF_%d__' >&2\n", seq)
fmt.Fprintf(&b, "printf '%%s\\n' '__HERMES_END_%d__'\n", seq)
b.WriteString("printf 'EXIT %d\\n' \"$__hermes_ec\"\n")
fmt.Fprintf(&b, "printf '%%s%%s%%s\\n' '__HERMES_CWD_%s__' \"$(pwd -P)\" '__HERMES_CWD_%s__'\n", s.id, s.id)
Expand Down
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