all: tests, tweaks for lazy extension decoding

Add a test to confirm that extensions are lazily decoded when we expect.

Drop the UnmarshalDefaultResolver flag. I added it thinking for some
reason that internal/impl couldn't depend on protoregistry; since it can
(and does), it's simpler to just test if the resolver is the expected
value.

Use a default set of options when lazily unmarshaling extensions.

Change-Id: Ied7666ffdc3bf90630260a80c9568d9a945048bc
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/218038
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
diff --git a/internal/impl/codec_extension.go b/internal/impl/codec_extension.go
index 1098159..efeea2f 100644
--- a/internal/impl/codec_extension.go
+++ b/internal/impl/codec_extension.go
@@ -136,7 +136,7 @@
 			wtyp := wire.Type(tag & 7)
 			var out unmarshalOutput
 			var err error
-			val, out, err = f.lazy.xi.funcs.unmarshal(b, val, num, wtyp, unmarshalOptions{})
+			val, out, err = f.lazy.xi.funcs.unmarshal(b, val, num, wtyp, lazyUnmarshalOptions)
 			if err != nil {
 				panic(errors.New("decode failure in lazy extension decoding: %v", err))
 			}
@@ -227,3 +227,34 @@
 		return f.typ.ValueOf(fn())
 	})
 }
+
+// IsLazy reports whether a field is lazily encoded.
+// It is exported for testing.
+func IsLazy(m pref.Message, fd pref.FieldDescriptor) bool {
+	var mi *MessageInfo
+	var p pointer
+	switch m := m.(type) {
+	case *messageState:
+		mi = m.messageInfo()
+		p = m.pointer()
+	case *messageReflectWrapper:
+		mi = m.messageInfo()
+		p = m.pointer()
+	default:
+		return false
+	}
+	xd, ok := fd.(pref.ExtensionTypeDescriptor)
+	if !ok {
+		return false
+	}
+	xt := xd.Type()
+	ext := mi.extensionMap(p)
+	if ext == nil {
+		return false
+	}
+	f, ok := (*ext)[int32(fd.Number())]
+	if !ok {
+		return false
+	}
+	return f.typ == xt && f.lazy != nil && atomic.LoadUint32(&f.lazy.atomicOnce) == 0
+}