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fix: race in RunWithPrompts truncates output on process exit #64

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Merged
sleipi merged 1 commit into from
Sep 17, 2026
Merged

fix: race in RunWithPrompts truncates output on process exit #64

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65 changes: 31 additions & 34 deletions internal/runner/prompts.go
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Original file line number Diff line number Diff line change
Expand Up @@ -27,6 +27,12 @@ type PromptResult struct {
TimedOut bool
}

// promptReadResult carries the outcome of draining a command's stdout.
type promptReadResult struct {
stdout string
ambiguous string
}

// promptState tracks remaining matches for a single prompt definition.
type promptState struct {
def PromptDef
Expand Down Expand Up @@ -147,21 +153,36 @@ func RunWithPrompts(command string, prompts []PromptDef, timeoutMs int, env map[
return PromptResult{Result: Result{ExitCode: -1, Stderr: fmt.Sprintf("start error: %v", err)}}
}

// Read stdout and match prompts in background
type readResult struct {
stdout string
ambiguous string
}
readDone := make(chan readResult, 1)
// Read stdout and match prompts in background. cmd.Wait must not be called
// until this goroutine has fully drained stdout: Wait() closes the stdout
// pipe as soon as the process exits, and racing it against our own Read
// here can truncate the last chunk of output.
readDone := make(chan promptReadResult, 1)
go func() {
stdout, ambiguous := readAndMatch(stdoutPipe, stdinPipe, states)
readDone <- readResult{stdout, ambiguous}
readDone <- promptReadResult{stdout, ambiguous}
}()

// Wait for process with timeout
timedOut, exitCode := waitWithTimeout(cmd, timeoutMs)
timedOut := false
var rr promptReadResult
timer := time.NewTimer(time.Duration(timeoutMs) * time.Millisecond)
select {
case rr = <-readDone:
timer.Stop()
case <-timer.C:
_ = cmd.Process.Kill()
rr = <-readDone
timedOut = true
}

rr := <-readDone
exitCode := 0
if waitErr := cmd.Wait(); waitErr != nil {
exitCode = -1
var exitErr *exec.ExitError
if errors.As(waitErr, &exitErr) {
exitCode = exitErr.ExitCode()
}
}
duration := time.Since(start).Milliseconds()

return PromptResult{
Expand All @@ -176,27 +197,3 @@ func RunWithPrompts(command string, prompts []PromptDef, timeoutMs int, env map[
TimedOut: timedOut,
}
}

// waitWithTimeout waits for process exit or kills on timeout. Returns (timedOut, exitCode).
func waitWithTimeout(cmd *exec.Cmd, timeoutMs int) (timedOut bool, exitCode int) {
timer := time.NewTimer(time.Duration(timeoutMs) * time.Millisecond)
defer timer.Stop()

doneCh := make(chan error, 1)
go func() { doneCh <- cmd.Wait() }()

select {
case waitErr := <-doneCh:
if waitErr != nil {
var exitErr *exec.ExitError
if errors.As(waitErr, &exitErr) {
return false, exitErr.ExitCode()
}
}
return false, 0
case <-timer.C:
_ = cmd.Process.Kill()
<-doneCh
return true, -1
}
}