ipn/ipnlocal: rename setAutoExitNodeIDLockedOnEntry to pickNewAutoExitNode; drop old function

Currently, (*LocalBackend).pickNewAutoExitNode() is just a wrapper around
setAutoExitNodeIDLockedOnEntry that sends a prefs-change notification at the end.
It doesn't need to do that, since setPrefsLockedOnEntry already sends the notification
(setAutoExitNodeIDLockedOnEntry calls it via editPrefsLockedOnEntry).

This PR removes the old pickNewAutoExitNode function and renames
setAutoExitNodeIDLockedOnEntry to pickNewAutoExitNode for clarity.

Updates tailscale/corp#29969

Signed-off-by: Nick Khyl <nickk@tailscale.com>

ipn/ipnlocal/local.go

@@ -2001,20 +2001,6 @@ func mutationsAreWorthyOfTellingIPNBus(muts []netmap.NodeMutation) bool {
 	return false
 }
 
-// pickNewAutoExitNode picks a new automatic exit node if needed.
-func (b *LocalBackend) pickNewAutoExitNode() {
-	unlock := b.lockAndGetUnlock()
-	defer unlock()
-
-	newPrefs := b.setAutoExitNodeIDLockedOnEntry(unlock)
-	if !newPrefs.Valid() {
-		// Unchanged.
-		return
-	}
-
-	b.send(ipn.Notify{Prefs: &newPrefs})
-}
-
 // setExitNodeID updates prefs to reference an exit node by ID, rather
 // than by IP. It returns whether prefs was mutated.
 func setExitNodeID(prefs *ipn.Prefs, nm *netmap.NetworkMap) (prefsChanged bool) {
@@ -5840,40 +5826,37 @@ func (b *LocalBackend) setNetInfo(ni *tailcfg.NetInfo) {
 	}
 	cc.SetNetInfo(ni)
 	if refresh {
-		unlock := b.lockAndGetUnlock()
-		defer unlock()
-		b.setAutoExitNodeIDLockedOnEntry(unlock)
+		b.pickNewAutoExitNode()
 	}
 }
 
-func (b *LocalBackend) setAutoExitNodeIDLockedOnEntry(unlock unlockOnce) (newPrefs ipn.PrefsView) {
-	var zero ipn.PrefsView
+// pickNewAutoExitNode picks a new automatic exit node if needed.
+func (b *LocalBackend) pickNewAutoExitNode() {
+	unlock := b.lockAndGetUnlock()
 	defer unlock()
 
 	prefs := b.pm.CurrentPrefs()
 	if !prefs.Valid() {
 		b.logf("[unexpected]: received tailnet exit node ID pref change callback but current prefs are nil")
-		return zero
+		return
 	}
 	prefsClone := prefs.AsStruct()
 	newSuggestion, err := b.suggestExitNodeLocked(nil)
 	if err != nil {
 		b.logf("setAutoExitNodeID: %v", err)
-		return zero
+		return
 	}
 	if prefsClone.ExitNodeID == newSuggestion.ID {
-		return zero
+		return
 	}
 	prefsClone.ExitNodeID = newSuggestion.ID
-	newPrefs, err = b.editPrefsLockedOnEntry(&ipn.MaskedPrefs{
+	_, err = b.editPrefsLockedOnEntry(&ipn.MaskedPrefs{
 		Prefs:         *prefsClone,
 		ExitNodeIDSet: true,
 	}, unlock)
 	if err != nil {
 		b.logf("setAutoExitNodeID: failed to apply exit node ID preference: %v", err)
-		return zero
 	}
-	return newPrefs
 }
 
 // setNetMapLocked updates the LocalBackend state to reflect the newly