lib/model: Reduce fmut locking in ClusterConfig (#6913)

The FileSet.Drop operation in there needs to potentially update a whole lot of global lists, which can take a while (longer than the deadlock interval apparently)

lib/model/model.go

@@ -1022,7 +1022,6 @@ func (m *model) ClusterConfig(deviceID protocol.DeviceID, cm protocol.ClusterCon
 		}
 	}
 
-	m.fmut.RLock()
 	var paused []string
 	for _, folder := range cm.Folders {
 		cfg, ok := m.cfg.Folder(folder.ID)
@@ -1048,7 +1047,9 @@ func (m *model) ClusterConfig(deviceID protocol.DeviceID, cm protocol.ClusterCon
 		if cfg.Paused {
 			continue
 		}
+		m.fmut.RLock()
 		fs, ok := m.folderFiles[folder.ID]
+		m.fmut.RUnlock()
 		if !ok {
 			// Shouldn't happen because !cfg.Paused, but might happen
 			// if the folder is about to be unpaused, but not yet.
@@ -1122,9 +1123,11 @@ func (m *model) ClusterConfig(deviceID protocol.DeviceID, cm protocol.ClusterCon
 					// likely use delta indexes. We might already have files
 					// that we need to pull so let the folder runner know
 					// that it should recheck the index data.
+					m.fmut.RLock()
 					if runner := m.folderRunners[folder.ID]; runner != nil {
 						defer runner.SchedulePull()
 					}
+					m.fmut.RUnlock()
 				}
 			}
 		}
@@ -1143,7 +1146,6 @@ func (m *model) ClusterConfig(deviceID protocol.DeviceID, cm protocol.ClusterCon
 		// implementing suture.IsCompletable).
 		m.Add(is)
 	}
-	m.fmut.RUnlock()
 
 	m.pmut.Lock()
 	m.remotePausedFolders[deviceID] = paused