device: reduce peer lock critical section in UAPI

The deferred RUnlock calls weren't executing until all peers
had been processed. Add an anonymous function so that each
peer may be unlocked as soon as it is completed.

Signed-off-by: Josh Bleecher Snyder <josh@tailscale.com>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

device/uapi.go

@@ -99,9 +99,10 @@ func (device *Device) IpcGetOperation(w io.Writer) error {
 			sendf("fwmark=%d", device.net.fwmark)
 		}
 
-		// serialize each peer state
-
 		for _, peer := range device.peers.keyMap {
+			// Serialize peer state.
+			// Do the work in an anonymous function so that we can use defer.
+			func() {
 				peer.RLock()
 				defer peer.RUnlock()
 
@@ -126,6 +127,7 @@ func (device *Device) IpcGetOperation(w io.Writer) error {
 					sendf("allowed_ip=%s", prefix.String())
 					return true
 				})
+			}()
 		}
 	}()