Chunk Index record doesn't tell me how big my chunk record is

[defunctzombie]

After reading a chunk index record, I know which message index record to read, but I don't know how big my chunk record is which means I have to go read the size of the record and then go read the record itself.

[jtbandes]

+1

[jtbandes]

Current workaround:

mcap/typescript/src/v0/Mcap0IndexedReader.ts

Lines 240 to 253 in e9250ff

	const chunkOpcodeAndLength = await this.readable.read(chunkIndex.chunkOffset, 1n + 8n);
	const chunkOpcodeAndLengthView = new DataView(
	chunkOpcodeAndLength.buffer,
	chunkOpcodeAndLength.byteOffset,
	chunkOpcodeAndLength.byteLength,
	);
	if (chunkOpcodeAndLengthView.getUint8(0) !== Opcode.CHUNK) {
	throw new Error(
	`Chunk index offset does not point to chunk record (expected opcode ${
	Opcode.CHUNK
	}, found ${chunkOpcodeAndLengthView.getUint8(0)})`,
	);
	}
	const chunkRecordLength = getBigUint64.call(chunkOpcodeAndLengthView, 1, true);

[defunctzombie]

Change offset to start_offset and end_offset so we can identify the bytes in the file containing the opcode+record lengh+record data for a chunk index record.

[defunctzombie]

chunk_start_offset - points to an opcode
chunk_end_offset - one past the end of the chunk

[start_offset, end_offset)

[defunctzombie]

Add chunk_trailer_byte_length? chunk_trailer_end_offset?

[defunctzombie]

https://github.com/foxglove/mcap/tree/roman/spec-format/docs/specification#chunk-index-op0x05