Test that pixad writes only its own errors to stderr (closes #359)

Both tests run pixad as a child process. They fail against this code:
every start writes the dependency-injection library's event log to
stderr, and a startup error is printed twice inside its frames.

Model: opus-5-5

internal/app/app_internal_test.go

@@ -0,0 +1,65 @@
+package app
+
+import (
+	"bytes"
+	"context"
+	"errors"
+	"io"
+	"testing"
+
+	"go.uber.org/fx"
+)
+
+var errTestStopFailed = errors.New("test stop hook failed")
+
+// TestRunAppPrintsStopErrorOnce checks that when a stop hook fails, runApp
+// prints the hook's error once and returns exit status 1.
+func TestRunAppPrintsStopErrorOnce(t *testing.T) {
+	t.Parallel()
+
+	app := fx.New(
+		fx.NopLogger,
+		fx.Invoke(func(lifecycle fx.Lifecycle, shutdowner fx.Shutdowner) {
+			lifecycle.Append(fx.Hook{
+				// Ask to shut down as soon as the app has started, so
+				// that runApp goes on to stop it.
+				OnStart: func(context.Context) error { return shutdowner.Shutdown() },
+				OnStop:  func(context.Context) error { return errTestStopFailed },
+			})
+		}),
+	)
+
+	var stderr bytes.Buffer
+
+	exitCode := runApp(app, &stderr)
+	if exitCode != 1 {
+		t.Errorf("runApp() = %d, want 1", exitCode)
+	}
+
+	want := errTestStopFailed.Error() + "\n"
+	if stderr.String() != want {
+		t.Errorf("stderr = %q, want %q", stderr.String(), want)
+	}
+}
+
+// TestRunAppReturnsShutdownExitCode checks that runApp returns the exit
+// code a shutdown request names.
+func TestRunAppReturnsShutdownExitCode(t *testing.T) {
+	t.Parallel()
+
+	app := fx.New(
+		fx.NopLogger,
+		fx.Invoke(func(lifecycle fx.Lifecycle, shutdowner fx.Shutdowner) {
+			lifecycle.Append(fx.Hook{
+				OnStart: func(context.Context) error {
+					return shutdowner.Shutdown(fx.ExitCode(3))
+				},
+			})
+		}),
+	)
+
+	exitCode := runApp(app, io.Discard)
+	if exitCode != 3 {
+		t.Errorf("runApp() = %d, want 3", exitCode)
+	}
+}