wgengine/magicsock,ipn/ipnlocal: store and load homeDERP from cache (#19491)

With netmap caching, the home DERP of the self node was neither saved to
the cache or loaded from it, making nodes not stick to a DERP when
starting without a connection to control.

Instead, make sure that when a cache is available, load that cache,
before looking for DERP servers. This is implemented by allowing a skip
of ReSTUN in setting the DERP map (we must have a DERP map before
setting the home DERP), so the DERP from cache will set itself and be
sticky until a connection to control is established.

Making DERP only change when connected to control is handled by existing
code from f072d017bd.

Updates #19490

Signed-off-by: Claus Lensbøl <claus@tailscale.com>

ipn/ipnlocal/diskcache.go

@@ -35,7 +35,19 @@ func (b *LocalBackend) writeNetmapToDiskLocked(nm *netmap.NetworkMap) error {
 		b.diskCache.cache = netmapcache.NewCache(netmapcache.FileStore(dir))
 		b.diskCache.dir = dir
 	}
-	return b.diskCache.cache.Store(b.currentNode().Context(), nm)
+
+	// Set the homeDERP on the self node before saving. The self node homeDERP is
+	// generally not used since the homeDERP for self is stored in magicsock, but
+	// to be able to load it during loading the cache, we use the existing field
+	// to save it.
+
+	// Make a shallow copy and mutate a copy of the selfNode.
+	nmCopy := *nm
+	selfNode := nm.SelfNode.AsStruct()
+	selfNode.HomeDERP = int(b.currentNode().homeDERP.Load())
+	nmCopy.SelfNode = selfNode.View()
+
+	return b.diskCache.cache.Store(b.currentNode().Context(), &nmCopy)
 }
 
 func (b *LocalBackend) loadDiskCacheLocked() (om *netmap.NetworkMap, ok bool) {