ipn/localapi: stop logging "broken pipe" errors (#18487)

The Tailscale CLI has some methods to watch the IPN bus for
messages, say, the current netmap (`tailscale debug netmap`).
The Tailscale daemon supports this using a streaming HTTP
response. Sometimes, the client can close its connection
abruptly -- due to an interruption, or in the case of `debug netmap`,
intentionally after consuming one message.

If the server daemon is writing a response as the client closes
its end of the socket, the daemon typically encounters a "broken pipe"
error. The "Watch IPN Bus" handler currently logs such errors after
they're propagated by a JSON encoding/writer helper.

Since the Tailscale CLI nominally closes its socket with the daemon
in this slightly ungraceful way (viz. `debug netmap`), stop logging
these broken pipe errors as far as possible. This will help avoid
confounding users when they scan backend logs.

Updates #18477

Signed-off-by: Amal Bansode <amal@tailscale.com>

ipn/localapi/localapi.go

@@ -35,6 +35,7 @@ import (
 	"tailscale.com/ipn/ipnlocal"
 	"tailscale.com/ipn/ipnstate"
 	"tailscale.com/logtail"
+	"tailscale.com/net/neterror"
 	"tailscale.com/net/netns"
 	"tailscale.com/net/netutil"
 	"tailscale.com/tailcfg"
@@ -913,7 +914,9 @@ func (h *Handler) serveWatchIPNBus(w http.ResponseWriter, r *http.Request) {
 	h.b.WatchNotificationsAs(ctx, h.Actor, mask, f.Flush, func(roNotify *ipn.Notify) (keepGoing bool) {
 		err := enc.Encode(roNotify)
 		if err != nil {
-			h.logf("json.Encode: %v", err)
+			if !neterror.IsClosedPipeError(err) {
+				h.logf("json.Encode: %v", err)
+			}
 			return false
 		}
 		f.Flush()