[FAB-13591] reduce scope of rlock around observer

Change-Id: I7ea50a583b0af311b02366a1e5124dd9faedf380
Signed-off-by: Matthew Sykes <sykesmat@us.ibm.com>

common/flogging/logging.go

@@ -157,6 +157,14 @@ func (s *Logging) SetWriter(w io.Writer) {
 	s.mutex.Unlock()
 }
 
+// SetObserver is used to provide a log observer that will be called as log
+// levels are checked or written.. Only a single observer is supported.
+func (s *Logging) SetObserver(observer Observer) {
+	s.mutex.Lock()
+	s.observer = observer
+	s.mutex.Unlock()
+}
+
 // Write satisfies the io.Write contract. It delegates to the writer argument
 // of SetWriter or the Writer field of Config. The Core uses this when encoding
 // log records.
@@ -218,24 +226,22 @@ func (s *Logging) ZapLogger(name string) *zap.Logger {
 
 func (s *Logging) Check(e zapcore.Entry, ce *zapcore.CheckedEntry) {
 	s.mutex.RLock()
-	if s.observer != nil {
-		s.observer.Check(e, ce)
-	}
+	observer := s.observer
 	s.mutex.RUnlock()
+
+	if observer != nil {
+		observer.Check(e, ce)
+	}
 }
 
 func (s *Logging) WriteEntry(e zapcore.Entry, fields []zapcore.Field) {
 	s.mutex.RLock()
-	if s.observer != nil {
-		s.observer.WriteEntry(e, fields)
-	}
+	observer := s.observer
 	s.mutex.RUnlock()
-}
 
-func (s *Logging) SetObserver(observer Observer) {
-	s.mutex.Lock()
-	s.observer = observer
-	s.mutex.Unlock()
+	if observer != nil {
+		observer.WriteEntry(e, fields)
+	}
 }
 
 // Logger instantiates a new FabricLogger with the specified name. The name is