Return result from get_backend_name (pytorch#9465)

Author: shoumikhin

runtime/executor/method_meta.cpp

@@ -245,12 +245,15 @@ size_t MethodMeta::num_backends() const {
   return delegates ? delegates->size() : 0;
 }
 
-const char* MethodMeta::get_backend_name(size_t index) const {
-  const auto delegates = s_plan_->delegates();
-  if (delegates && index < delegates->size()) {
-    return delegates->Get(index)->id()->c_str();
-  }
-  return nullptr;
+Result<const char*> MethodMeta::get_backend_name(size_t index) const {
+  const auto count = num_backends();
+  ET_CHECK_OR_RETURN_ERROR(
+      index < count,
+      InvalidArgument,
+      "Index %zu out of range. num_backends: %zu",
+      index,
+      count);
+  return s_plan_->delegates()->Get(index)->id()->c_str();
 }
 
 size_t MethodMeta::num_instructions() const {

runtime/executor/method_meta.h

@@ -204,10 +204,10 @@ class MethodMeta final {
    * Get the backend name at the given index.
    *
    * @param[in] index The index of the backend name.
-   * @returns The backend name as a C-style string, or nullptr if the index is
-   * invalid.
+   * @returns A Result wrapping the backend name as a C-style string
+   * on success, or an error if the index is invalid.
    */
-  const char* get_backend_name(size_t index) const;
+  Result<const char*> get_backend_name(size_t index) const;
 
   /**
    * Get the number of instructions in this method.

runtime/executor/test/backend_integration_test.cpp

@@ -362,18 +362,20 @@ TEST_P(BackendIntegrationTest, GetBackendNamesSuccess) {
   EXPECT_TRUE(method_meta->uses_backend(StubBackend::kName));
 
   // Retrieve the number of backends.
-  size_t num_backends = method_meta->num_backends();
+  const size_t num_backends = method_meta->num_backends();
   EXPECT_GT(num_backends, 0u);
 
   // Iterate through each backend and verify its name.
   for (size_t i = 0; i < num_backends; ++i) {
-    const char* name = method_meta->get_backend_name(i);
-    EXPECT_NE(name, nullptr);
+    auto backend_name_result = method_meta->get_backend_name(i);
+    ASSERT_TRUE(backend_name_result.ok());
+    const char* name = backend_name_result.get();
     // For this test, we expect that the only backend is StubBackend.
     EXPECT_STREQ(name, StubBackend::kName);
   }
-  // Check that an out-of-range index returns nullptr.
-  EXPECT_EQ(method_meta->get_backend_name(num_backends), nullptr);
+  // Check that an out-of-range index returns an error.
+  auto out_of_range_result = method_meta->get_backend_name(num_backends);
+  EXPECT_FALSE(out_of_range_result.ok());
 }
 
 TEST_P(BackendIntegrationTest, FreeingProcessedBufferSucceeds) {