From a92d1dee6d0a2f03d70a92dad5192e6bd806552e Mon Sep 17 00:00:00 2001
From: "Ronny.Herrgesell" <ronny.herrgesell@jtl-software.com>
Date: Thu, 17 Sep 2026 11:37:36 +0200
Subject: [PATCH] fix: don't call cmd.Wait concurrently with stdout reader in
 RunWithPrompts

Wait() closes the stdout pipe as soon as the child exits, racing our own
Read on the same fd and occasionally truncating the last chunk of output.
Fixes flaky examples/06_prompts.clitest under the new Go 1.27 CI matrix
(golang:1.27-alpine), reproduced and verified fixed against that exact image.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
---
 internal/runner/prompts.go | 65 ++++++++++++++++++--------------------
 1 file changed, 31 insertions(+), 34 deletions(-)

diff --git a/internal/runner/prompts.go b/internal/runner/prompts.go
index b74aa5e..300d2ed 100644
--- a/internal/runner/prompts.go
+++ b/internal/runner/prompts.go
@@ -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
@@ -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{
@@ -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
-	}
-}