OSSM-1430: CNI: Watch for modified files with a prefix (maistra#510)

Because our CNI pod contains more than one container, and they write to
the same directory, and they watch for changes on those directories,
changes made by one container trigger the watch on the other, which will
responde by copying the files to the directory, which will in turn
trigger the watcher of the other container in an endless loop.

This leads to high CPU usage on the node.

This PR changes the logic to only monitor for files that have the
desired prefix. Thus, for example, the 2.2 container will only react to
changes to files whose names  start with "v2-2". This avoid this race
condition and achieve the same end result.

cni/pkg/install/cniconfig.go

@@ -180,7 +180,7 @@ func getCNIConfigFilepath(ctx context.Context, cfg pluginConfig) (string, error)
 		return filepath.Join(cfg.mountedCNINetDir, filename), nil
 	}
 
-	watcher, fileModified, errChan, err := util.CreateFileWatcher(cfg.mountedCNINetDir)
+	watcher, fileModified, errChan, err := util.CreateFileWatcher("", []string{cfg.mountedCNINetDir})
 	if err != nil {
 		return "", err
 	}

cni/pkg/install/install.go

@@ -184,7 +184,7 @@ func readServiceAccountToken() (string, error) {
 func sleepCheckInstall(ctx context.Context, cfg *config.InstallConfig, cniConfigFilepath string, isReady *atomic.Value) error {
 	// Create file watcher before checking for installation
 	// so that no file modifications are missed while and after checking
-	watcher, fileModified, errChan, err := util.CreateFileWatcher(append(cfg.CNIBinTargetDirs, cfg.MountedCNINetDir)...)
+	watcher, fileModified, errChan, err := util.CreateFileWatcher(cfg.CNIBinariesPrefix, append(cfg.CNIBinTargetDirs, cfg.MountedCNINetDir))
 	if err != nil {
 		return err
 	}

cni/pkg/util/util.go

@@ -19,21 +19,30 @@ import (
 	"encoding/json"
 	"fmt"
 	"os"
+	"path/filepath"
+	"strings"
 
 	"github.com/fsnotify/fsnotify"
 
 	"istio.io/istio/pkg/file"
 )
 
+type watchPrefix struct {
+	watcher *fsnotify.Watcher
+	prefix  string
+}
+
 // Creates a file watcher that watches for any changes to the directory
-func CreateFileWatcher(dirs ...string) (watcher *fsnotify.Watcher, fileModified chan bool, errChan chan error, err error) {
+// If `prefix` is non-empty, it will only notify of changes for files whose filename starts with `prefix`
+func CreateFileWatcher(prefix string, dirs []string) (watcher *fsnotify.Watcher, fileModified chan bool, errChan chan error, err error) {
 	watcher, err = fsnotify.NewWatcher()
 	if err != nil {
 		return
 	}
 
 	fileModified, errChan = make(chan bool), make(chan error)
-	go watchFiles(watcher, fileModified, errChan)
+	wp := &watchPrefix{watcher: watcher, prefix: prefix}
+	go watchFiles(wp, fileModified, errChan)
 
 	for _, dir := range dirs {
 		if file.IsDirWriteable(dir) != nil {
@@ -50,17 +59,17 @@ func CreateFileWatcher(dirs ...string) (watcher *fsnotify.Watcher, fileModified
 	return
 }
 
-func watchFiles(watcher *fsnotify.Watcher, fileModified chan bool, errChan chan error) {
+func watchFiles(wp *watchPrefix, fileModified chan bool, errChan chan error) {
 	for {
 		select {
-		case event, ok := <-watcher.Events:
+		case event, ok := <-wp.watcher.Events:
 			if !ok {
 				return
 			}
-			if event.Op&(fsnotify.Create|fsnotify.Write|fsnotify.Remove) != 0 {
+			if event.Op&(fsnotify.Create|fsnotify.Write|fsnotify.Remove) != 0 && strings.HasPrefix(filepath.Base(event.Name), wp.prefix) {
 				fileModified <- true
 			}
-		case err, ok := <-watcher.Errors:
+		case err, ok := <-wp.watcher.Errors:
 			if !ok {
 				return
 			}