fix: exit rule if response type cannot be resolved (#1415)

The utility method that resolves the response type descriptor, handling LROs in the process, can return `nil` if either the response type is not resolvable in the protos (very bad) OR the RPC is an LRO that isn't annotated as required by AIP-151 with `google.longrunning.operation_info`. In these cases, anywhere the helper is used should just exit - if the type is unresolvable there are other issues, and if the LRO RPC is unannotated, then AIP-151 rules will warn on that missing annotation.

We definitely shouldn't be going into a `panic`

Updates #1399

rules/aip0133/request_required_fields.go

@@ -31,6 +31,9 @@ var requestRequiredFields = &lint.MethodRule{
 	OnlyIf: utils.IsCreateMethodWithResolvedReturnType,
 	LintMethod: func(m *desc.MethodDescriptor) []lint.Problem {
 		ot := utils.GetResponseType(m)
+		if ot == nil {
+			return nil
+		}
 		r := utils.GetResource(ot)
 		resourceMsgName := utils.GetResourceSingular(r)
 

rules/aip0134/request_required_fields.go

@@ -29,6 +29,9 @@ var requestRequiredFields = &lint.MethodRule{
 	OnlyIf: utils.IsUpdateMethod,
 	LintMethod: func(m *desc.MethodDescriptor) (problems []lint.Problem) {
 		ot := utils.GetResponseType(m)
+		if ot == nil {
+			return nil
+		}
 		it := m.GetInputType()
 
 		allowedRequiredFields := stringset.New("update_mask")

rules/aip0136/response_message_name.go

@@ -47,6 +47,12 @@ var responseMessageName = &lint.MethodRule{
 		}
 
 		response := utils.GetResponseType(m)
+		if response == nil {
+			// If the return type is not resolveable (bad) or if an LRO and
+			// missing the operation_info annotation (covered by AIP-151 rules),
+			// just exit.
+			return nil
+		}
 		requestResourceType := utils.GetResourceReference(m.GetInputType().FindFieldByName("name")).GetType()
 		responseResourceType := utils.GetResource(response).GetType()
 

rules/aip0233/resource_reference_type.go

@@ -28,6 +28,9 @@ var resourceReferenceType = &lint.MethodRule{
 	OnlyIf: func(m *desc.MethodDescriptor) bool {
 		// Return type of the RPC.
 		ot := utils.GetResponseType(m)
+		if ot == nil {
+			return false
+		}
 
 		// First repeated message field must be annotated with google.api.resource.
 		repeated := utils.GetRepeatedMessageFields(ot)

rules/internal/utils/common_lints.go

@@ -182,7 +182,11 @@ func LintMethodHasMatchingRequestName(m *desc.MethodDescriptor) []lint.Problem {
 // have a name matching the method's, with a "Response" suffix.
 func LintMethodHasMatchingResponseName(m *desc.MethodDescriptor) []lint.Problem {
 	// GetResponseType handles the LRO case.
-	if got, want := GetResponseType(m).GetName(), m.GetName()+"Response"; got != want {
+	rt := GetResponseType(m)
+	if rt == nil {
+		return nil
+	}
+	if got, want := rt.GetName(), m.GetName()+"Response"; got != want {
 		loc := locations.MethodResponseType(m)
 		suggestion := want