Fix shelltool readExitCode to wait on the live stdout signal so it is not gated on the 100ms poll

tool/shelltool/localshell_internal_test.go

@@ -3,9 +3,12 @@
 package shelltool
 
 import (
+	"context"
+	"io"
 	"slices"
 	"strings"
 	"testing"
+	"time"
 )
 
 func TestPreserveEnvironmentValuesKeepsOnlyAllowlist(t *testing.T) {
@@ -78,6 +81,68 @@ func TestHeadTailBuffer_MultiByteUTF8OverflowPreservesHeadTailOrder(t *testing.T
 	}
 }
 
+// scriptedReader delivers a fixed sequence of chunks, sleeping delay before
+// each so successive stdout chunks land on distinct readLoop signals.
+type scriptedReader struct {
+	chunks [][]byte
+	delay  time.Duration
+	i      int
+}
+
+func (r *scriptedReader) Read(p []byte) (int, error) {
+	if r.i >= len(r.chunks) {
+		return 0, io.EOF
+	}
+	if r.delay > 0 {
+		time.Sleep(r.delay)
+	}
+	n := copy(p, r.chunks[r.i])
+	r.i++
+	return n, nil
+}
+
+// TestReadExitCodeWakesOnLiveStdoutSignal exercises the exit-code read path
+// when the sentinel and the exit-code line arrive in two separate stdout
+// reads. readExitCode must wake on the live stdout signal for the second read
+// rather than waiting out the 100ms fallback poll.
+func TestReadExitCodeWakesOnLiveStdoutSignal(t *testing.T) {
+	s := &persistentSession{stdoutSignal: newSignal()}
+	s.readerWG.Add(1)
+
+	reader := &scriptedReader{
+		chunks: [][]byte{
+			[]byte("hello\nSENTINEL"),
+			[]byte("_0\n"),
+		},
+		delay: 5 * time.Millisecond,
+	}
+	go s.readLoop(reader, &s.stdoutBuf, true)
+
+	start := time.Now()
+	idx, rc, timedOut, overflow, err := s.waitForSentinel(context.Background(), []byte("SENTINEL"), 0, 1<<20)
+	elapsed := time.Since(start)
+	s.readerWG.Wait()
+
+	if err != nil {
+		t.Fatalf("waitForSentinel: %v", err)
+	}
+	if timedOut || overflow {
+		t.Fatalf("timedOut=%v overflow=%v, want both false", timedOut, overflow)
+	}
+	if idx < 0 {
+		t.Fatalf("sentinel not found: idx=%d", idx)
+	}
+	if rc != 0 {
+		t.Fatalf("rc = %d, want 0", rc)
+	}
+	// The exit-code line arrives ~5ms after the sentinel on a live signal.
+	// Before the fix readExitCode discarded that signal and only noticed the
+	// line on the 100ms fallback poll, so elapsed would be ~100ms.
+	if elapsed >= 80*time.Millisecond {
+		t.Fatalf("readExitCode gated on 100ms poll: elapsed %v", elapsed)
+	}
+}
+
 func TestResolvedShellArgvIncludesExtraArgv(t *testing.T) {
 	shell := resolvedShell{
 		binary:    "/custom/bash",

tool/shelltool/session.go

@@ -313,7 +313,6 @@ func (s *persistentSession) readExitCode(ctx context.Context, afterIdx int) (int
 			tail = snapshotRange(s.stdoutBuf, afterIdx, lenTail)
 		}
 		signal := s.stdoutSignal
-		s.stdoutSignal = newSignal()
 		s.bufferGate.Unlock()
 
 		if nl := bytes.IndexByte(tail, '\n'); nl >= 0 {