More fine grained locking in discovery cache (fixes #2667)

We only need to protect the integrity of the "finders" and "caches"
slices, and for that we only need an RLock except while actually
appending to them. The actual finders and caches are concurrency safe on
their own.

lib/discover/cache.go

@@ -26,7 +26,7 @@ type CachingMux struct {
 	*suture.Supervisor
 	finders []cachedFinder
 	caches  []*cache
-	mut     sync.Mutex
+	mut     sync.RWMutex
 }
 
 // A cachedFinder is a Finder with associated cache timeouts.
@@ -54,7 +54,7 @@ type cachedError interface {
 func NewCachingMux() *CachingMux {
 	return &CachingMux{
 		Supervisor: suture.NewSimple("discover.cachingMux"),
-		mut:        sync.NewMutex(),
+		mut:        sync.NewRWMutex(),
 	}
 }
 
@@ -75,7 +75,7 @@ func (m *CachingMux) Add(finder Finder, cacheTime, negCacheTime time.Duration, p
 func (m *CachingMux) Lookup(deviceID protocol.DeviceID) (direct []string, relays []Relay, err error) {
 	var pdirect []prioritizedAddress
 
-	m.mut.Lock()
+	m.mut.RLock()
 	for i, finder := range m.finders {
 		if cacheEntry, ok := m.caches[i].Get(deviceID); ok {
 			// We have a cache entry. Lets see what it says.
@@ -129,7 +129,7 @@ func (m *CachingMux) Lookup(deviceID protocol.DeviceID) (direct []string, relays
 			m.caches[i].Set(deviceID, entry)
 		}
 	}
-	m.mut.Unlock()
+	m.mut.RUnlock()
 
 	direct = uniqueSortedAddrs(pdirect)
 	relays = uniqueSortedRelays(relays)
@@ -149,12 +149,12 @@ func (m *CachingMux) Error() error {
 }
 
 func (m *CachingMux) ChildErrors() map[string]error {
-	m.mut.Lock()
 	children := make(map[string]error, len(m.finders))
+	m.mut.RLock()
 	for _, f := range m.finders {
 		children[f.String()] = f.Error()
 	}
-	m.mut.Unlock()
+	m.mut.RUnlock()
 	return children
 }
 
@@ -163,7 +163,7 @@ func (m *CachingMux) Cache() map[protocol.DeviceID]CacheEntry {
 	// children's caches.
 	res := make(map[protocol.DeviceID]CacheEntry)
 
-	m.mut.Lock()
+	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.
@@ -183,7 +183,7 @@ func (m *CachingMux) Cache() map[protocol.DeviceID]CacheEntry {
 			}
 		}
 	}
-	m.mut.Unlock()
+	m.mut.RUnlock()
 
 	return res
 }