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Caleb JasikandClaude Opus 5.5
61947ff1c2
Passthrough.Flush hands its whole lane to a new tio.BatchWriter in one WriteBatch call when the queue implements it, and NewSingleQueue exposes WriteBatch when its source does. On darwin, tun.WriteBatch writes up to 64 packets per sendmsg_x on the utun control socket, which is a connected datagram socket, so xnu takes the batched sendit_x path rather than one sendit per message. utun registers no send_list handler, so the kernel still passes each packet to utun on its own: this saves syscalls and fd locking, not per-packet kernel work. Error handling follows what sendmsg_x reports: - A short count with no error (a packet past the send buffer mid-call, or a later send failing with ENOBUFS): the remainder goes out in the next sendmsg_x. - EMSGSIZE: only returned when the first packet is too large, before the kernel takes any, so that packet alone goes through Write, which reports it, and the rest are batched again. - ENOSYS, EPERM, EOPNOTSUPP: sendmsg_x is private API, so WriteBatch logs once and falls back to writev per packet for good. - Anything else: sendmsg_x returns the error without the count it took, so the call is reported rather than retried, which could deliver packets twice. One loss mode has no userspace fix. sendit_x counts each packet as sent once it is copied into an mbuf; if a later allocation fails, it frees the copied prefix unsent, returns its length, and sendmsg_x drops the ENOBUFS because the count is nonzero. That looks exactly like a real short count, so under mbuf exhaustion WriteBatch silently drops packets that writev would have delivered. The iOS tun path (tun_ios.go) is unchanged. Inside a macOS Network Extension, download goes from 640 to 832 Mbps against the recvmsg_x change alone in three paired rounds, at 15% less CPU per Mbps; the likely cause is that its utun charges more per write than a raw one. Raw dnclient already runs download near line rate, and there the CPU difference is within noise. TestTunWriteBatch* round-trip through a connected AF_UNIX datagram socketpair, which takes sendmsg_x the same way, over both the batched and the fallback path: order, AF prefixes, skipped invalid packets, and an oversize packet mid-call and first-in-call. Dropping Namelen from msghdrX fails them. Mutation testing with gremlins leaves only equivalent survivors in WriteBatch, plus which error is reported first when several fail. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>