all: use network less when running in v86 emulator

Updates #5794

Change-Id: I1d8b005a1696835c9062545f87b7bab643cfc44d
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

hostinfo/hostinfo_plan9.go

@@ -0,0 +1,39 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package hostinfo
+
+import (
+	"bytes"
+	"os"
+	"strings"
+
+	"tailscale.com/tailcfg"
+	"tailscale.com/types/lazy"
+)
+
+func init() {
+	RegisterHostinfoNewHook(func(hi *tailcfg.Hostinfo) {
+		if isPlan9V86() {
+			hi.DeviceModel = copyV86DeviceModel
+		}
+	})
+}
+
+var isPlan9V86Cache lazy.SyncValue[bool]
+
+// isPlan9V86 reports whether we're running in the wasm
+// environment (https://github.com/copy/v86/).
+func isPlan9V86() bool {
+	return isPlan9V86Cache.Get(func() bool {
+		v, _ := os.ReadFile("/dev/cputype")
+		s, _, _ := strings.Cut(string(v), " ")
+		if s != "PentiumIV/Xeon" {
+			return false
+		}
+
+		v, _ = os.ReadFile("/dev/config")
+		v, _, _ = bytes.Cut(v, []byte{'\n'})
+		return string(v) == "# pcvm - small kernel used to run in vm"
+	})
+}