nfd-worker: use fsnotify for watching for config file changes

Add support for detecting configuration file changes via file system
notifications (fsnotify). Watches are added for the whole directory
chain (up to root directory) so that all changes (even directory
renames) affecting the given configuration file path are captured.

Previously dynamic (re-)configuration of nfd-worker was implemented by
(re-)reading the configuration file on every labeling pass. This was
simple and effective, even if a bit wasteful. However, it didn't provide
asynchronous configuration updates that will be required for e.g.
controlling the "sleep-interval" parameter dynamically which will be
implemented by later patches.

go.mod

@@ -4,6 +4,7 @@ go 1.15
 
 require (
 	github.com/docopt/docopt-go v0.0.0-20180111231733-ee0de3bc6815
+	github.com/fsnotify/fsnotify v1.4.9
 	github.com/golang/protobuf v1.4.3
 	github.com/klauspost/cpuid/v2 v2.0.2
 	github.com/onsi/ginkgo v1.11.0

pkg/nfd-worker/nfd-worker.go

@@ -24,10 +24,12 @@ import (
 	"io/ioutil"
 	"log"
 	"os"
+	"path/filepath"
 	"regexp"
 	"strings"
 	"time"
 
+	"github.com/fsnotify/fsnotify"
 	"golang.org/x/net/context"
 	"google.golang.org/grpc"
 	"google.golang.org/grpc/credentials"
@@ -178,6 +180,40 @@ func NewNfdWorker(args Args) (NfdWorker, error) {
 	return nfd, nil
 }
 
+func addConfigWatch(path string) (*fsnotify.Watcher, map[string]struct{}, error) {
+	paths := make(map[string]struct{})
+
+	// Create watcher
+	w, err := fsnotify.NewWatcher()
+	if err != nil {
+		return w, paths, fmt.Errorf("failed to create fsnotify watcher: %v", err)
+	}
+
+	// Add watches for all directory components so that we catch e.g. renames
+	// upper in the tree
+	added := false
+	for p := path; ; p = filepath.Dir(p) {
+
+		if err := w.Add(p); err != nil {
+			stdoutLogger.Printf("failed to add fsnotify watch for %q: %v", p, err)
+		} else {
+			stdoutLogger.Printf("added fsnotify watch %q", p)
+			added = true
+		}
+
+		paths[p] = struct{}{}
+		if filepath.Dir(p) == p {
+			break
+		}
+	}
+
+	if !added {
+		// Want to be sure that we watch something
+		return w, paths, fmt.Errorf("failed to add any watch")
+	}
+	return w, paths, nil
+}
+
 // Run NfdWorker client. Returns if a fatal error is encountered, or, after
 // one request if OneShot is set to 'true' in the worker args.
 func (w *nfdWorker) Run() error {
@@ -191,13 +227,19 @@ func (w *nfdWorker) Run() error {
 	}
 	defer w.disconnect()
 
-	trigger := time.After(0)
+	// Create watcher for config file and read initial configuration
+	configFilePath := filepath.Clean(w.args.ConfigFile)
+	configWatch, paths, err := addConfigWatch(configFilePath)
+	if err != nil {
+		return (err)
+	}
+	w.configure(configFilePath, w.args.Options)
+
+	labelTrigger := time.After(0)
+	var configTrigger <-chan time.Time
 	for {
 		select {
-		case <-trigger:
-			// Parse and apply configuration
-			w.configure(w.args.ConfigFile, w.args.Options)
-
+		case <-labelTrigger:
 			// Get the set of feature labels.
 			labels := createFeatureLabels(w.sources, w.labelWhiteList)
 
@@ -214,11 +256,38 @@ func (w *nfdWorker) Run() error {
 			}
 
 			if w.args.SleepInterval > 0 {
-				trigger = time.After(w.args.SleepInterval)
+				labelTrigger = time.After(w.args.SleepInterval)
 			}
+
+		case e := <-configWatch.Events:
+			name := filepath.Clean(e.Name)
+
+			// If any of our paths (directories or the file itself) change
+			if _, ok := paths[name]; ok {
+				stdoutLogger.Printf("fsnotify event in %q detected, reconfiguring fsnotify and reloading configuration", name)
+
+				// Blindly remove existing watch and add a new one
+				if err := configWatch.Close(); err != nil {
+					stderrLogger.Printf("WARNING: failed to close fsnotify watcher: %v", err)
+				}
+				configWatch, paths, err = addConfigWatch(configFilePath)
+				if err != nil {
+					return err
+				}
+
+				// Rate limiter. In certain filesystem operations we get
+				// numerous events in quick succession and we only want one
+				// config re-load
+				configTrigger = time.After(time.Second)
+			}
+
+		case e := <-configWatch.Errors:
+			stderrLogger.Printf("ERROR: config file watcher error: %v", e)
+
+		case <-configTrigger:
+			w.configure(configFilePath, w.args.Options)
 		}
 	}
-	return nil
 }
 
 // connect creates a client connection to the NFD master