[Ruby] Fix for FieldDescriptor.get(msg).

This fix is similar to the previous bug found in
Message.[]. The fix is the same: we need to handle
arrays and maps properly.

Fixes: https://github.com/protocolbuffers/protobuf/issues/8325

ruby/ext/google/protobuf_c/defs.c

@@ -960,16 +960,14 @@ static VALUE FieldDescriptor_subtype(VALUE _self) {
 static VALUE FieldDescriptor_get(VALUE _self, VALUE msg_rb) {
   FieldDescriptor* self = ruby_to_FieldDescriptor(_self);
   const upb_msgdef *m;
-  const upb_msgdef *msg = Message_Get(msg_rb, &m);
-  VALUE arena = Message_GetArena(msg_rb);
-  upb_msgval msgval;
+
+  Message_Get(msg_rb, &m);
 
   if (m != upb_fielddef_containingtype(self->fielddef)) {
     rb_raise(cTypeError, "get method called on wrong message type");
   }
 
-  msgval = upb_msg_get(msg, self->fielddef);
-  return Convert_UpbToRuby(msgval, TypeInfo_get(self->fielddef), arena);
+  return Message_getfield(msg_rb, self->fielddef);
 }
 
 /*

ruby/ext/google/protobuf_c/message.c

@@ -292,7 +292,7 @@ static void Message_setfield(upb_msg* msg, const upb_fielddef* f, VALUE val,
   upb_msg_set(msg, f, msgval, arena);
 }
 
-static VALUE Message_getfield(VALUE _self, const upb_fielddef* f) {
+VALUE Message_getfield(VALUE _self, const upb_fielddef* f) {
   Message* self = ruby_to_Message(_self);
   // This is a special-case: upb_msg_mutable() for map & array are logically
   // const (they will not change what is serialized) but physically

ruby/ext/google/protobuf_c/message.h

@@ -63,6 +63,9 @@ const upb_msg* Message_GetUpbMessage(VALUE value, const upb_msgdef* m,
 // object will reference |arena| and ensure that it outlives this object.
 VALUE Message_GetRubyWrapper(upb_msg* msg, const upb_msgdef* m, VALUE arena);
 
+// Gets the given field from this message.
+VALUE Message_getfield(VALUE _self, const upb_fielddef* f);
+
 // Implements #inspect for this message, printing the text to |b|.
 void Message_PrintMessage(StringBuilder* b, const upb_msg* msg,
                           const upb_msgdef* m);

ruby/tests/well_known_types_test.rb

@@ -193,4 +193,12 @@ class TestWellKnownTypes < Test::Unit::TestCase
     assert_equal false, s['b'][:y]
     assert_equal false, s[:b]['y']
   end
+
+  def test_b8325
+    value_field = Google::Protobuf::ListValue.descriptor.lookup("values")
+    proto = Google::Protobuf::ListValue.new(
+        values: [Google::Protobuf::Value.new(string_value: "Hello")]
+    )
+    assert_equal '[<Google::Protobuf::Value: string_value: "Hello">]', value_field.get(proto).inspect
+  end
 end