Minor cvd display fixes

Author: jmacnak

base/cvd/cuttlefish/host/commands/display/main.cpp

@@ -36,6 +36,24 @@
 namespace cuttlefish {
 namespace {
 
+static const char kUsage[] =
+    R"(
+
+Interact with a Cuttlefish virtual device's displays.
+
+usage: cvd display <subcommand> <args>
+
+Commands:(cvd display help <subcommand> for more information):
+
+    add: Adds and connects displays.
+
+    list: Lists all of the displays currently connected.
+
+    remove: Disconnects and removes displays.
+
+    screenshot: Screenshots the contents of a given display.
+)";
+
 static const char kAddUsage[] =
     R"(
 
@@ -81,6 +99,11 @@ Result<int> GetInstanceNum(std::vector<std::string>& args) {
 }
 
 Result<int> DoHelp(std::vector<std::string>& args) {
+  if (args.empty()) {
+    std::cerr << kUsage << std::endl;
+    return 0;
+  }
+
   static const android::base::NoDestructor<
       std::unordered_map<std::string, std::string>>
       kSubCommandUsages({

base/cvd/cuttlefish/host/libs/vm_manager/crosvm_display_controller.cpp

@@ -107,7 +107,9 @@ Result<int> CrosvmDisplayController::RunCrosvmDisplayCommand(
 
   Result<std::string> res = RunAndCaptureStdout(std::move(command));
   if (res.ok()) {
-    *stdout_str = CF_EXPECT(std::move(res));
+    if (stdout_str != nullptr) {
+      *stdout_str = std::move(*res);
+    }
     return 0;
   } else {
     LOG(ERROR) << "Failed to run crosvm display command:\n"

e2etests/cvd/BUILD.bazel

@@ -65,6 +65,17 @@ cvd_command_boot_test(
     target = "aosp_cf_x86_64_only_phone-userdebug",
 )
 
+cvd_command_boot_test(
+    name = "add_displays",
+    branch = "aosp-android-latest-release",
+    cvd_command = [
+        "display",
+        "add",
+        "--display=width=500,height=500",
+    ],
+    target = "aosp_cf_x86_64_only_phone-userdebug",
+)
+
 cvd_command_boot_test(
     name = "list_displays",
     branch = "aosp-android-latest-release",