Compute the payload size of packed varint fields once per serialization

[merlimat]

Problem

Serializing a packed varint field walks its elements twice just to size it. getSerializedSize() sums the element sizes to size the payload, then _writeTo() sums them again to write the payload's length prefix. Packed is the default for repeated scalars in proto3, so this affects every proto3 message with a repeated integer or enum field, plus proto2 fields marked [packed = true].

Example

The generated _writeTo() for repeated int64 x_int64 = 2 [packed = true] repeats the loop that getSerializedSize() just ran:

int _xInt64sSize = 0;
for (int i = 0; i < _xInt64sCount; i++) {
    long _item = xInt64s[i];
    _xInt64sSize += LightProtoCodec.computeVarInt64Size(_item);
}
_i = LightProtoCodec.writeRawVarInt(_a, _i, _xInt64sSize);

Change

getSerializedSize() now keeps the payload size in a new field, and _writeTo() uses it:

int _xInt64sSize = _xInt64sPackedSize;
if (_xInt64sSize < 0) {
    // compute as before
    _xInt64sPackedSize = _xInt64sSize;
}

Why the cached value stays correct:

• Every serialization runs getSerializedSize() just before _writeTo().
• Every change to the elements (addX(), clearX(), clear(), copyFrom()) resets _cachedSize, so getSerializedSize() recomputes the payload size whenever the elements changed.
• The exception is parseFrom(), which caches the message size without computing any field. So parsing the field resets its payload size to -1, and _writeTo() computes it and keeps it when it's unknown.

clear() is unchanged. Resetting the payload size there instead cost every parse one store per packed field, whether the field was present or not: +3.5% on a 10-element parse of a message with 7 packed fields.

Fixed-width packed fields already size their payload as count × width. So only messages with packed varint fields change; Pulsar's generated code is byte-identical.

JDK 26, one packed int64 field, 3 interleaved rounds (median ns/op):

Benchmark	Elements	Before	After	Δ
writeTo, size already cached	10	34.1	28.3	−17%
	1200	4761	3840	−19%
writeTo after a change	10	43.8	37.8	−14%
	1200	5883	4808	−18%
parse	10	39.7	40.2	+1%
	1200	5062	5059	0%

Testing

The new PackedSizeTest changes the elements after the size was cached in six ways: adding, clearX(), clear(), parsing, re-serializing a parsed message and then adding, and copyFrom(). Each case runs on both write paths (heap array, and the NIO view of a direct buffer) and is compared byte for byte with protobuf-java. Without the parse-time reset, the parse case fails on both paths. mvn verify passes (554 tests).