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3d4fda9a0d
stackTun.Read always returned exactly one packet per call regardless
of how many the caller's buf could hold. amneziawg-go's
RoutineReadFromTUN sizes its buffers to device.BatchSize(), which on
Linux is the UDP bind's own batch size (128, conn.IdealBatchSize) --
so real batch capacity was already there and going unused on the
download path.
The UDP bind's Send/Receive both genuinely batch via recvmmsg/
sendmmsg (conn/bind_std.go, confirmed in vendored source). The upload
path exploits this end to end: bind.Receive returns up to 128
datagrams per syscall, decrypt processes them as a batch, and
stackTun.Write already loops over its whole buf. The download path
never reached that batching at all: capped to 1 packet at the TUN
read step, every downstream stage (peer lookup, per-peer staging,
eventual UDP send) paid a full cycle per packet instead of amortizing
it across up to 128.
Have Read block for the first packet, then opportunistically drain
whatever's already buffered (non-blocking), up to len(buf). This is
the second half of the throughput-asymmetry fix (see 6436fd9c, which
fixed the channel being fully unbuffered and blocking the producer on
every packet) -- confirmed live on a real test connection: download
went from 30-40 Mbit/s to 130-250 after the channel-buffering fix,
with a real-network-plausible sequential-speedtest gap remaining
against upload's 300+. This closes the remaining structural gap
between the two directions' per-packet processing cost.
Regression tests confirm the drain behavior directly (not just "it
doesn't crash"): stashed this fix alone and re-ran both new tests to
confirm they fail with the exact expected message first.