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After copying from the backend to the client until EOF, forwardTCP shut
down the backend socket's read side. That does nothing on the wire, and
once the backend's FIN has arrived, macOS rejects shutdown(SHUT_RD) with
ENOTCONN (Linux does too once the socket reaches TIME_WAIT), so every
forwarded connection logged "backend -> client close connection: ...
socket is not connected". Keep only the CloseWrite calls, which are what
propagate the half-close.
The regression was introduced by 04d24cdbd4 (#16462), merged 2025-07-07.
Tested on Linux and on macOS 27 (arm64): the wgengine/netstack tests
pass on both. The same ENOTCONN failure was reproduced on macOS via the
identical pattern in net/socks5 (#21522).
Updates #21522
Updates #16462
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
Change-Id: I3e9b1d7f4a2c8e6b0f5d9a3c7e1b4f8d2a6c0e5b