Add test for handler crash with stderr capture

Adds TestHandler_Crash to verify the runner correctly handles handlers that crash while processing requests. The test confirms ErrHandlerClosed is returned and that panic messages from stderr are captured in the error.

Author: stringintech

runner/handler_test.go

@@ -5,6 +5,7 @@ import (
 	"errors"
 	"fmt"
 	"os"
+	"strings"
 	"testing"
 	"time"
 )
@@ -16,14 +17,17 @@ const (
 	// envTestHelperName specifies which helper function to execute in subprocess mode.
 	envTestHelperName = "TEST_HELPER_NAME"
 
+	// Handler simulation test identifiers.
 	helperNameNormal       = "normal"
 	helperNameUnresponsive = "unresponsive"
+	helperNameCrash        = "crash"
 )
 
 // testHelpers maps helper names to functions that simulate different handler behaviors.
 var testHelpers = map[string]func(){
 	helperNameNormal:       helperNormal,
 	helperNameUnresponsive: helperUnresponsive,
+	helperNameCrash:        helperCrash,
 }
 
 // TestMain allows the test binary to serve two purposes:
@@ -135,6 +139,49 @@ func helperUnresponsive() {
 	}
 }
 
+// TestHandler_Crash tests that the runner correctly handles a handler that crashes
+// while processing a request
+func TestHandler_Crash(t *testing.T) {
+	h, err := newHandlerForTest(t, helperNameCrash, 0)
+	if err != nil {
+		t.Fatalf("Failed to create handler: %v", err)
+	}
+	defer h.Close()
+
+	// Send a request to the handler
+	request := `{"id":1,"method":"test"}`
+	if err := h.SendLine([]byte(request)); err != nil {
+		t.Fatalf("Failed to send request: %v", err)
+	}
+
+	// Try to read the response - should get ErrHandlerClosed
+	_, err = h.ReadLine()
+
+	if err == nil {
+		t.Fatal("Expected error from crashed handler, got nil")
+	}
+
+	// Verify it's the handler closed error we expect
+	if !errors.Is(err, ErrHandlerClosed) {
+		t.Errorf("Expected ErrHandlerClosed, got: %v", err)
+	}
+
+	// Verify the error message contains the panic string from stderr
+	if !strings.Contains(err.Error(), "simulated handler crash") {
+		t.Errorf("Expected error to contain 'simulated handler crash', got: %v", err)
+	}
+}
+
+// helperCrash simulates a handler that crashes while processing a request,
+// triggering the ErrHandlerClosed error in the runner.
+func helperCrash() {
+	// Read requests from stdin but panic instead of responding
+	scanner := bufio.NewScanner(os.Stdin)
+	for scanner.Scan() {
+		panic("simulated handler crash")
+	}
+}
+
 // newHandlerForTest creates a Handler that runs a test helper as a subprocess.
 // The helperName identifies which helper to run (e.g., "normal", "crash", "hang").
 // The timeout parameter sets the per-request timeout (0 uses default).