This commit makes the rarely used `raftpb.Message.Snapshot` field nullable.
In doing so, it reduces the memory size of a `raftpb.Message` message from
264 bytes to 128 bytes — a 52% reduction in size.
While this commit does not change the protobuf encoding, it does change
how that encoding is used. `(gogoproto.nullable) = false` instruct the
generated proto marshaling logic to always encode a value for the field,
even if that value is empty. `(gogoproto.nullable) = true` instructs the
generated proto marshaling logic to omit an encoded value for the field
if the field is nil.
This raises compatibility concerns in both directions. Messages encoded
by new binary versions without a `Snapshot` field will be decoded as an
empty field by old binary versions. In other words, old binary versions
can't tell the difference. However, messages encoded by old binary versions
with an empty Snapshot field will be decoded as a non-nil, empty field by
new binary versions. As a result, new binary versions need to be prepared
to handle such messages.
While Message.Snapshot is not intentionally part of the external interface
of this library, it was possible for users of the library to access it and
manipulate it. As such, this change may be considered a breaking change.
Signed-off-by: Nathan VanBenschoten <nvanbenschoten@gmail.com>
This commit removes the `XXX_NoUnkeyedLiteral`, `XXX_unrecognized`, and
`XXX_sizecache` auto-generated fields from generated protobuf structs in
the raft package. This was done for all of the same reasons CockroachDB
removed the generation of these fields in https://github.com/cockroachdb/cockroach/pull/38404.
They come with very limited advantages but moderate disadvantages.
`XXX_NoUnkeyedLiteral` and `XXX_sizecache` were only enabled recently in
cc7b4fa, and this appears to have been unintentional. Meanwhile,
`XXX_unrecognized` has been around for longer and has arguably more
reason to stay because it can assist with forwards compatibility.
However, any real mixed-version upgrade story for this package is mostly
untold at this point, and keeping this field seems just as likely to
cause unexpected bugs (e.g. a field was propagated but not updated
correctly when passed through an old version) as it seems to fix real
issues, so it also doesn't warrant its cost.
This reduces the in-memory representation size of all Raft protos.
Notably, it reduces the memory size of an `Entry` proto from *80 bytes*
to *48 bytes* and the memory size of a `Message` proto from *392 bytes*
to *264 bytes*. Both of these structs are used frequently, and often in
slices, where this wasted space really starts to add up.
This was motivated by a regression in microbenchmarks in CockroachDB due
to cc7b4fa, which was caught in https://github.com/cockroachdb/cockroach/issues/62212.
