Better error handling in fsnotify recursive monitor (elastic#6949)

When the recursive `file_monitor` is scanning a directory, it will ignore
paths that include an error. This is wrong, as sometimes those paths can
still be processed. For example, a directory might not be readable, but
the user wants to receive an event when it is created, even if its
contents can't be known.

This has been uncovered by recent changes in how directory errors are
reported by `filepath.Walk`, introduced in Go 1.10. Previously `walkFn`
would be called twice. Once without an error (when the directory has
been found during scan of the parent directory), and the second time
with an error (when access to the directory is attempted).

auditbeat/module/file_integrity/monitor/monitor_test.go

@@ -181,7 +181,7 @@ func TestRecursiveSubdirPermissions(t *testing.T) {
 
 	assertNoError(t, os.Chmod(filepath.Join(outDir, "b"), 0))
 
-	// Setup watched on watched dir
+	// Setup watches on watched dir
 
 	watcher, err := New(true)
 	assertNoError(t, err)

auditbeat/module/file_integrity/monitor/recursive.go

@@ -54,11 +54,16 @@ func (watcher *recursiveWatcher) ErrorChannel() <-chan error {
 
 func (watcher *recursiveWatcher) addRecursive(path string) error {
 	var errs multierror.Errors
-	err := filepath.Walk(path, func(path string, info os.FileInfo, err error) error {
-		if err != nil {
-			errs = append(errs, errors.Wrapf(watcher.inner.Add(path), "recursion into dir '%s' failed", path))
-			return nil
+	err := filepath.Walk(path, func(path string, info os.FileInfo, fnErr error) error {
+		if fnErr != nil {
+			errs = append(errs, errors.Wrapf(fnErr, "error walking path '%s'", path))
+			// If FileInfo is not nil, the directory entry can be processed
+			// even if there was some error
+			if info == nil {
+				return nil
+			}
 		}
+		var err error
 		if info.IsDir() {
 			if err = watcher.tree.AddDir(path); err == nil {
 				if err = watcher.inner.Add(path); err != nil {