tree 711f22ebe9d502543ac9d453c06a80171488dc4f
parent bab3d4084ecc24a53ecd33b386625611b9ca18b1
author Damien Neil <dneil@google.com> 1565205701 -0700
committer Damien Neil <dneil@google.com> 1565288425 +0000

all: make handling of zero-value composites more consistent

We occasionally need to work with immutable, empty lists, maps, and
messages. Notably, Message.Get on an empty repeated field will return a
"frozen" empty value.

Move handling of these immutable, zero-length composites into Converter,
to unify the behavior of regular and extension fields.

Add a Zero method to Converter, MessageType, and ExtensionType, to
provide a consistent way to get an empty, frozen value of a composite
type. Adding this method to the public {Message,Extension}Type
interfaces does increase our API surface, but lets us (for example)
cleanly represent an empty map as a nil map rather than a non-nil
one wrapped in a frozenMap type.

Drop the frozen{List,Map,Message} types as no longer necessary.
(These types did have support for creating a read-only view of a
non-empty value, but we are not currently using that feature.)

Change-Id: Ia76f149d591da07b40ce75b7404a7ab8a60cb9d8
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/189339
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>