Log changed files during restarts

This adds logging for file changes that trigger restarts. It should help
with debugging cases where Zeus appeared not to restart correctly.

go/processtree/processtree.go

@@ -70,13 +70,15 @@ var restartMutex sync.Mutex
 func (tree *ProcessTree) RestartNodesWithFeatures(files []string) {
 	restartMutex.Lock()
 	defer restartMutex.Unlock()
+	tree.Root.trace("%d files changed, beginning with %q", len(files), files[0])
 	tree.Root.restartNodesWithFeatures(tree, files)
 }
 
 // Serialized: restartMutex is always held when this is called.
 func (node *SlaveNode) restartNodesWithFeatures(tree *ProcessTree, files []string) {
 	for _, file := range files {
 		if node.HasFeature(file) {
+			node.trace("restarting for %q", file)
 			node.RequestRestart()
 			return
 		}

go/processtree/slavemonitor.go

@@ -61,7 +61,9 @@ func StartSlaveMonitor(tree *ProcessTree, fileChanges <-chan []string, done chan
 			case fd := <-registeringFds:
 				go monitor.slaveDidBeginRegistration(fd)
 			case files := <-fileChanges:
-				tree.RestartNodesWithFeatures(files)
+				if len(files) > 0 {
+					tree.RestartNodesWithFeatures(files)
+				}
 			}
 		}
 	}()