safesocket: add ConnectionStrategy, provide control over fallbacks

fee2d9fad added support for cmd/tailscale to connect to IPNExtension.
It came in two parts: If no socket was provided, dial IPNExtension first,
and also, if dialing the socket failed, fall back to IPNExtension.

The second half of that support caused the integration tests to fail
when run on a machine that was also running IPNExtension.
The integration tests want to wait until the tailscaled instances
that they spun up are listening. They do that by dialing the new
instance. But when that dial failed, it was falling back to IPNExtension,
so it appeared (incorrectly) that tailscaled was running.
Hilarity predictably ensued.

If a user (or a test) explicitly provides a socket to dial,
it is a reasonable assumption that they have a specific tailscaled
in mind and don't want to fall back to IPNExtension.
It is certainly true of the integration tests.

Instead of adding a bool to Connect, split out the notion of a
connection strategy. For now, the implementation remains the same,
but with the details hidden a bit. Later, we can improve that.

Signed-off-by: Josh Bleecher Snyder <josh@tailscale.com>

client/tailscale/tailscale.go

@@ -38,6 +38,9 @@ var (
 	// TailscaledSocket is the tailscaled Unix socket. It's used by the TailscaledDialer.
 	TailscaledSocket = paths.DefaultTailscaledSocket()
 
+	// TailscaledSocketSetExplicitly reports whether the user explicitly set TailscaledSocket.
+	TailscaledSocketSetExplicitly bool
+
 	// TailscaledDialer is the DialContext func that connects to the local machine's
 	// tailscaled or equivalent.
 	TailscaledDialer = defaultDialer
@@ -47,7 +50,8 @@ func defaultDialer(ctx context.Context, network, addr string) (net.Conn, error)
 	if addr != "local-tailscaled.sock:80" {
 		return nil, fmt.Errorf("unexpected URL address %q", addr)
 	}
-	if TailscaledSocket == paths.DefaultTailscaledSocket() {
+	// TODO: make this part of a safesocket.ConnectionStrategy
+	if !TailscaledSocketSetExplicitly {
 		// On macOS, when dialing from non-sandboxed program to sandboxed GUI running
 		// a TCP server on a random port, find the random port. For HTTP connections,
 		// we don't send the token. It gets added in an HTTP Basic-Auth header.
@@ -56,7 +60,11 @@ func defaultDialer(ctx context.Context, network, addr string) (net.Conn, error)
 			return d.DialContext(ctx, "tcp", "localhost:"+strconv.Itoa(port))
 		}
 	}
-	return safesocket.Connect(TailscaledSocket, safesocket.WindowsLocalPort)
+	s := safesocket.DefaultConnectionStrategy(TailscaledSocket)
+	// The user provided a non-default tailscaled socket address.
+	// Connect only to exactly what they provided.
+	s.UseFallback(false)
+	return safesocket.Connect(s)
 }
 
 var (