ipn: remove unused Backend interface

Only the macOS/iOS clients care about it still, so we'll move it
to their repo.

But keep a test that makes sure that LocalBackend continues to
implement it so we get an early warning sign before we break
macOS/iOS.

Change-Id: I56392b740fe55b4d28468b77124c821b5c46c22b
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/backend.go

@@ -210,35 +210,3 @@ type Options struct {
 	// new node key without user interaction.
 	AuthKey string
 }
-
-// Backend is the interface between Tailscale frontends
-// (e.g. cmd/tailscale, iOS/MacOS/Windows GUIs) and the tailscale
-// backend (e.g. cmd/tailscaled) running on the same machine.
-// (It has nothing to do with the interface between the backends
-// and the cloud control plane.)
-type Backend interface {
-	// SetNotifyCallback sets the callback to be called on updates
-	// from the backend to the client.
-	SetNotifyCallback(func(Notify))
-	// Start starts or restarts the backend, typically when a
-	// frontend client connects.
-	Start(Options) error
-	// StartLoginInteractive requests to start a new interactive login
-	// flow. This should trigger a new BrowseToURL notification
-	// eventually.
-	StartLoginInteractive()
-	// Login logs in with an OAuth2 token.
-	Login(token *tailcfg.Oauth2Token)
-	// Logout terminates the current login session and stops the
-	// wireguard engine.
-	Logout()
-	// SetPrefs installs a new set of user preferences, including
-	// WantRunning. This may cause the wireguard engine to
-	// reconfigure or stop.
-	SetPrefs(*Prefs)
-	// RequestEngineStatus polls for an update from the wireguard
-	// engine. Only needed if you want to display byte
-	// counts. Connection events are emitted automatically without
-	// polling.
-	RequestEngineStatus()
-}

ipn/ipnlocal/local_test.go

@@ -743,3 +743,39 @@ func TestPacketFilterPermitsUnlockedNodes(t *testing.T) {
 	}
 
 }
+
+// legacyBackend was the interface between Tailscale frontends
+// (e.g. cmd/tailscale, iOS/MacOS/Windows GUIs) and the tailscale
+// backend (e.g. cmd/tailscaled) running on the same machine.
+// (It has nothing to do with the interface between the backends
+// and the cloud control plane.)
+type legacyBackend interface {
+	// SetNotifyCallback sets the callback to be called on updates
+	// from the backend to the client.
+	SetNotifyCallback(func(ipn.Notify))
+	// Start starts or restarts the backend, typically when a
+	// frontend client connects.
+	Start(ipn.Options) error
+	// StartLoginInteractive requests to start a new interactive login
+	// flow. This should trigger a new BrowseToURL notification
+	// eventually.
+	StartLoginInteractive()
+	// Login logs in with an OAuth2 token.
+	Login(token *tailcfg.Oauth2Token)
+	// Logout terminates the current login session and stops the
+	// wireguard engine.
+	Logout()
+	// SetPrefs installs a new set of user preferences, including
+	// WantRunning. This may cause the wireguard engine to
+	// reconfigure or stop.
+	SetPrefs(*ipn.Prefs)
+	// RequestEngineStatus polls for an update from the wireguard
+	// engine. Only needed if you want to display byte
+	// counts. Connection events are emitted automatically without
+	// polling.
+	RequestEngineStatus()
+}
+
+// Verify that LocalBackend still implements the legacyBackend interface
+// for now, at least until the macOS and iOS clients move off of it.
+var _ legacyBackend = (*LocalBackend)(nil)