tree ef3c1f9473665b1c76d9589a12e9b75146d48bdd
parent e31c6dd73d24ea4bce7a6282c19ab38b81616ba9
author Joe Tsai <joetsai@digital-static.net> 1615459841 -0800
committer Joe Tsai <joetsai@digital-static.net> 1615862472 +0000

internal/impl: add runtime support for aberrant messages

Implement support in the protobuf runtime to better understand
message types that are not generated by the official generator.

In particular:
* Add a best-effort implementation of protobuf reflection for
"non-nullable" fields which are supposed to be represented by *T,
but are instead represented by a T. "Non-nullable" message fields
report presence based on whether the message is the zero Go value.
* We do NOT implement support for "non-nullable" fields in the
table-driven implementation since we assume that the aberrant messages
that we care about have a Marshal and Unmarshal method.
* We better handle custom messages that implement Marshal and Unmarshal,
but do NOT implement Merge. In that case, we implement merge in terms of
a back-to-back marshal and unmarshal.
* We better tolerate the situations where a protobuf message field
cannot be mapped to a Go struct field since the latter is missing.
In such cases, reflection treats the field as if it were unpopulated.
Setting such fields will panic.

This change allows the runtime to handle all message types declared
in the "go.etcd.io/etcd" and "k8s.io" modules where protobuf reflection,
Marshal, Unmarshal, Reset, Merge, and Equal all work.

The only types that still do not fully work are:
	* "k8s.io/api/authentication/v1".ExtraValue
	* "k8s.io/api/authentication/v1beta1".ExtraValue
	* "k8s.io/api/authorization/v1".ExtraValue
	* "k8s.io/api/authorization/v1beta1".ExtraValue
	* "k8s.io/api/certificates/v1".ExtraValue
	* "k8s.io/api/certificates/v1beta1".ExtraValue
	* "k8s.io/apimachinery/pkg/apis/meta/v1".MicroTime
	* "k8s.io/apimachinery/pkg/apis/meta/v1".Time
	* "k8s.io/apimachinery/pkg/apis/meta/v1".Verbs
While Marshal, Unmarshal, Reset, and Merge continue to work,
protobuf reflection and any functionality that depends on it
(e.g., prototext, protojson, Equal, etc.) will not work.

Change-Id: I67a9d2f1bec35248045ad0c16220d02fc2e0e172
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/300869
Trust: Joe Tsai <joetsai@digital-static.net>
Trust: Joe Tsai <thebrokentoaster@gmail.com>
Reviewed-by: Damien Neil <dneil@google.com>