ipn/ipnlocal: update netmap cache after peer deltas are applied (#20111)

Add an UpdatePeers method to the cache. This allows us to support netmap peer deltas,
by allowing just the peers to be updated in an existing cache. As a safety check, reject
an update if there was no base netmap data to apply a change to.

Then, when processing peer mutations in the backend, capture any changes that should
be applied to the cache and update it, if one is enabled.

Updates #12542

Change-Id: I2f8790a8fdc5e85fce6700ba4821a8cb10dddffa
Signed-off-by: M. J. Fromberger <fromberger@tailscale.com>

ipn/ipnlocal/diskcache.go

@@ -10,6 +10,7 @@ import (
 
 	"tailscale.com/feature/buildfeatures"
 	"tailscale.com/ipn/ipnlocal/netmapcache"
+	"tailscale.com/tailcfg"
 	"tailscale.com/types/netmap"
 )
 
@@ -21,6 +22,20 @@ type diskCache struct {
 	cache *netmapcache.Cache
 }
 
+// writePeerDeltaToDiskLocked applies the specified peer delta to the cache,
+// leaving other existing cached fields (self, profiles, other peers not
+// mentioned in the arguments) unchanged.  It does nothing (without error) if
+// both slices are empty.
+func (b *LocalBackend) writePeerDeltaToDiskLocked(update []tailcfg.NodeView, remove []tailcfg.StableNodeID) error {
+	if !buildfeatures.HasCacheNetMap || (len(update) == 0 && len(remove) == 0) {
+		return nil
+	} else if err := b.ensureDiskCacheLocked(); err != nil {
+		return err
+	}
+	b.logf("updating netmap peers in disk cache (%d to update, %d to remove)", len(update), len(remove))
+	return b.diskCache.cache.UpdatePeers(b.currentNode().Context(), update, remove)
+}
+
 // writeNetmapToDiskLockedWithoutPeers updates nm in the cache, excluding peers and profiles.
 func (b *LocalBackend) writeNetmapToDiskLockedWithoutPeers(nm *netmap.NetworkMap) error {
 	if !buildfeatures.HasCacheNetMap || nm == nil || nm.Cached {