[Format] Clarify that a compressed IPC buffer can be empty for empty data

[pitrou]

Describe the bug, including details regarding any error messages, version, and platform.

The Flatbuffers spec stipulates that every compressed buffer should be prefixed with the 8-byte uncompressed length.

However, in the C++ IPC writer, we actually write an empty compressed buffer (without length prefix) if the uncompressed data is empty:

arrow/cpp/src/arrow/ipc/writer.cc

Lines 243 to 246 in 8dd9eba

	if (out_->body_buffers[i]->size() > 0) {
	RETURN_NOT_OK(CompressBuffer(*out_->body_buffers[i], options_.codec.get(),
	&out_->body_buffers[i]));
	}

and, of course, we also handle this case in the C++ IPC reader:

arrow/cpp/src/arrow/ipc/reader.cc

Lines 513 to 515 in 8dd9eba

	if (buf == nullptr || buf->size() == 0) {
	return buf;
	}

We should make it clear in the Flatbuffers spec that this case is allowed.

Component(s)

Format

[pitrou]

Thanks @Hind-M for noticing this. Also cc @alamb @paleolimbot @zeroshade .