Fix GRPC 500 from BlobsResolver

Passing an invalid ref into the BlobsResolver was resulting in a 500
because of a GRPC::InvalidArgument error raised from Gitaly. This commit
instead checks that the ref is valid and raises a GraphQL ArgumentError
if it is not, which will be properly returned to the client by GraphQL.

Changelog: fixed

app/graphql/resolvers/blobs_resolver.rb

@@ -30,8 +30,17 @@ module Resolvers
       return [] if repository.empty?
 
       ref ||= repository.root_ref
+      validate_ref(ref)
 
       repository.blobs_at(paths.map { |path| [ref, path] })
     end
+
+    private
+
+    def validate_ref(ref)
+      unless Gitlab::GitRefValidator.validate(ref)
+        raise Gitlab::Graphql::Errors::ArgumentError, 'Ref is not valid'
+      end
+    end
   end
 end

spec/graphql/resolvers/blobs_resolver_spec.rb

@@ -68,6 +68,28 @@ RSpec.describe Resolvers::BlobsResolver do
             )
           end
         end
+
+        context 'when specifying an invalid ref' do
+          let(:ref) { 'ma:in' }
+
+          it 'raises an ArgumentError' do
+            expect { resolve_blobs }.to raise_error(
+              Gitlab::Graphql::Errors::ArgumentError,
+              'Ref is not valid'
+            )
+          end
+        end
+
+        context 'when passing an empty ref' do
+          let(:ref) { '' }
+
+          it 'raises an ArgumentError' do
+            expect { resolve_blobs }.to raise_error(
+              Gitlab::Graphql::Errors::ArgumentError,
+              'Ref is not valid'
+            )
+          end
+        end
       end
     end
   end