Fix TCP keepalive setup on OpenBSD

[dolonet]

Fixes #457.

Problem

OpenBSD has no user-settable per-socket TCP keepalive options. TCP_KEEPIDLE, TCP_KEEPINTVL and TCP_KEEPCNT do not exist on OpenBSD; keepalive timing is controlled system-wide via the sysctls net.inet.tcp.keepidle and net.inet.tcp.keepintvl. The Go runtime reflects this in src/net/tcpsockopt_openbsd.go: setKeepAliveIdle / setKeepAliveInterval / setKeepAliveCount return ENOPROTOOPT for any non-negative value, and only short-circuit to nil for negative values (the explicit "leave alone" sentinel).

mtg builds a net.KeepAliveConfig with zero-valued Idle / Interval / Count whenever the user does not override them, which is the default and the documented expectation. It then hands that config to (*TCPConn).SetKeepAliveConfig in two places:

• network/sockopts.go — applied to every connection accepted by internal/utils.Listener.Accept and to every server-side dial that goes through the v1 default network.
• network/v2/sockopts.go — applied to every connection produced by the v2 network's DialContext.

Both calls fail on OpenBSD with set tcp ...: protocol not available. The user-visible effects:

• mtg doctor reports the error for every Telegram DC.
• mtg run accepts incoming TCP connections at the kernel level but Listener.Accept closes each one before the proxy server ever sees it, so the client appears to hang on a half-open socket and nothing is written to the log. This is exactly what @babut85 reports in openbsd #457.
• There is no configuration workaround. Setting [network] keep-alive.disabled = true only zeroes Enable; Go's SetKeepAliveConfig still calls all four setters, and three of them still fail.

Fix

Extract the keepalive setup behind an applyKeepAlive(conn, cfg) helper that has a per-platform implementation, following the same build-tag pattern already used for sockopts_lowat, sockopts_congestion, sockopts_reuseaddr and sockopts_usertimeout.

• All platforms except OpenBSD (//go:build !openbsd): applyKeepAlive calls conn.SetKeepAliveConfig(cfg). Behaviour is unchanged.
• OpenBSD (//go:build openbsd): applyKeepAlive calls conn.SetKeepAlive(cfg.Enable), which only flips SO_KEEPALIVE on or off and never touches the missing per-socket options. OpenBSD users get the system-wide sysctl-controlled keepalive timing, which is the only thing the kernel exposes.

The same fix is applied symmetrically in network/ (v1) and network/v2/.

Verified

• GOOS=openbsd GOARCH=amd64 go build ./... — clean.
• GOOS=openbsd GOARCH=arm64 go build ./... — clean.
• go vet ./... clean for both linux and openbsd.
• go test -count=1 ./network/... passes on linux.

Cannot run the test suite under OpenBSD locally; happy to iterate if anything turns up on a real OpenBSD host.

[9seconds]

Понял, спасибо большое!