control/controlclient: refactor in prep for optimized delta handling

See issue. This is a baby step towards passing through deltas
end-to-end from node to control back to node and down to the various
engine subsystems, not computing diffs from two full netmaps at
various levels. This will then let us support larger netmaps without
burning CPU.

But this change itself changes no behavior. It just changes a func
type to an interface with one method. That paves the way for future
changes to then add new NetmapUpdater methods that do more
fine-grained work than updating the whole world.

Updates #1909

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

control/controlclient/auto.go

@@ -480,11 +480,42 @@ func (c *Auto) unpausedChanLocked() <-chan struct{} {
 	return unpaused
 }
 
+// mapRoutineState is the state of Auto.mapRoutine while it's running.
+type mapRoutineState struct {
+	c  *Auto
+	bo *backoff.Backoff
+}
+
+func (mrs mapRoutineState) UpdateFullNetmap(nm *netmap.NetworkMap) {
+	c := mrs.c
+	health.SetInPollNetMap(true)
+
+	c.mu.Lock()
+	ctx := c.mapCtx
+	c.synced = true
+	if c.loggedIn {
+		c.state = StateSynchronized
+	}
+	c.expiry = ptr.To(nm.Expiry)
+	stillAuthed := c.loggedIn
+	c.logf("[v1] mapRoutine: netmap received: %s", c.state)
+	c.mu.Unlock()
+
+	if stillAuthed {
+		c.sendStatus("mapRoutine-got-netmap", nil, "", nm)
+	}
+	// Reset the backoff timer if we got a netmap.
+	mrs.bo.BackOff(ctx, nil)
+}
+
 // mapRoutine is responsible for keeping a read-only streaming connection to the
 // control server, and keeping the netmap up to date.
 func (c *Auto) mapRoutine() {
 	defer close(c.mapDone)
-	bo := backoff.NewBackoff("mapRoutine", c.logf, 30*time.Second)
+	mrs := &mapRoutineState{
+		c:  c,
+		bo: backoff.NewBackoff("mapRoutine", c.logf, 30*time.Second),
+	}
 
 	for {
 		if err := c.waitUnpause("mapRoutine"); err != nil {
@@ -531,25 +562,7 @@ func (c *Auto) mapRoutine() {
 		} else {
 			health.SetInPollNetMap(false)
 
-			err := c.direct.PollNetMap(ctx, func(nm *netmap.NetworkMap) {
-				health.SetInPollNetMap(true)
-
-				c.mu.Lock()
-				c.synced = true
-				if c.loggedIn {
-					c.state = StateSynchronized
-				}
-				c.expiry = ptr.To(nm.Expiry)
-				stillAuthed := c.loggedIn
-				c.logf("[v1] mapRoutine: netmap received: %s", c.state)
-				c.mu.Unlock()
-
-				if stillAuthed {
-					c.sendStatus("mapRoutine-got-netmap", nil, "", nm)
-				}
-				// Reset the backoff timer if we got a netmap.
-				bo.BackOff(ctx, nil)
-			})
+			err := c.direct.PollNetMap(ctx, mrs)
 
 			health.SetInPollNetMap(false)
 			c.mu.Lock()
@@ -561,16 +574,14 @@ func (c *Auto) mapRoutine() {
 			c.mu.Unlock()
 
 			if paused {
+				mrs.bo.BackOff(ctx, nil)
 				c.logf("mapRoutine: paused")
 				continue
 			}
 
-			if err != nil {
-				report(err, "PollNetMap")
-				bo.BackOff(ctx, err)
-				continue
-			}
-			bo.BackOff(ctx, nil)
+			report(err, "PollNetMap")
+			mrs.bo.BackOff(ctx, err)
+			continue
 		}
 	}
 }