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Brad Fitzpatrick
0cd86f6b26
Add net/msgx, which sends and receives multiple datagrams per system call on macOS using the kernel's recvmsg_x and sendmsg_x entry points, the Darwin counterparts of recvmmsg and sendmmsg. They are declared only in xnu's private socket_private.h, so msgx invokes them as raw system calls by number rather than through libSystem. That keeps their names out of the binary, returns errno reliably from the trap itself, and avoids a cgo transition per call. The calls need no privileges, but msgx.Available limits their use: never in a sandboxed App Store process (Tailscale's own GUI builds, detected by bundle ID, or a third-party app embedding tsnet, detected by the App Sandbox's APP_SANDBOX_CONTAINER_ID environment variable), and only after a loopback self-test with the running kernel passes, since Apple does not promise these numbers or layouts stay stable. Otherwise callers fall back to one datagram per call. Whether the self-test ran, whether it passed, and which step failed are exported as client metrics so a kernel change in the wild is visible. On top of it, add a Darwin batching.Conn: reads are batched on every socket (the main magicsock sockets included); writes are batched only on connected sockets, since sendmsg_x takes no per-message destination, which is what the per-peer connected sockets are for. connBind.BatchSize now comes from batching.MaxBatchSize rather than a GOOS switch. Measured between two M5 Pro Mac minis on a 10 Gbps LAN (iperf3 over Tailscale, 10 second runs, tailscaled CPU seconds sender/receiver), with per-packet utun I/O still in place: baseline 1.37 Gbps 29.3s / 22.1s connected sockets only 1.41 Gbps 24.8s / 15.6s connected sockets + msgx 1.44 Gbps 25.3s / 9.4s Throughput is bounded by the utun path (addressed separately); receiver CPU drops by more than half here. Three debug knobs (TS_DEBUG_MSGX, TS_DEBUG_MSGX_RX_MAIN, TS_DEBUG_MSGX_BATCHSIZE) remain for evaluation. Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com> Change-Id: I4e9b7d2c8a1f5063b9e2d7c4a8f1e6b3d5c2a9f7