gui: Add addresses for disconnected devices (fixes #3340)

Also fixes an issue where the discovery cache call would only return the
newest cache entry for a given device instead of the merged addresses
from all cache entries (which is more useful).

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3344

lib/discover/cache.go

@@ -13,6 +13,7 @@ import (
 
 	"github.com/syncthing/syncthing/lib/protocol"
 	"github.com/syncthing/syncthing/lib/sync"
+	"github.com/syncthing/syncthing/lib/util"
 	"github.com/thejerf/suture"
 )
 
@@ -165,12 +166,17 @@ func (m *cachingMux) Cache() map[protocol.DeviceID]CacheEntry {
 
 	m.mut.RLock()
 	for i := range m.finders {
-		// Each finder[i] has a corresponding cache at cache[i]. Go through it
-		// and populate the total, if it's newer than what's already in there.
-		// We skip any negative cache entries.
+		// Each finder[i] has a corresponding cache at cache[i]. Go through
+		// it and populate the total, appending any addresses and keeping
+		// the newest "when" time. We skip any negative cache entries.
 		for k, v := range m.caches[i].Cache() {
-			if v.found && v.when.After(res[k].when) {
-				res[k] = v
+			if v.found {
+				cur := res[k]
+				if v.when.After(cur.when) {
+					cur.when = v.when
+				}
+				cur.Addresses = append(cur.Addresses, v.Addresses...)
+				res[k] = cur
 			}
 		}
 
@@ -178,13 +184,23 @@ func (m *cachingMux) Cache() map[protocol.DeviceID]CacheEntry {
 		// finder is a global discovery client, it will have no cache. If it's
 		// a local discovery client, this will be it's current state.
 		for k, v := range m.finders[i].Cache() {
-			if v.found && v.when.After(res[k].when) {
-				res[k] = v
+			if v.found {
+				cur := res[k]
+				if v.when.After(cur.when) {
+					cur.when = v.when
+				}
+				cur.Addresses = append(cur.Addresses, v.Addresses...)
+				res[k] = cur
 			}
 		}
 	}
 	m.mut.RUnlock()
 
+	for k, v := range res {
+		v.Addresses = util.UniqueStrings(v.Addresses)
+		res[k] = v
+	}
+
 	return res
 }