local: continue listing files/folders when a circular symlink is detected

Before this change a circular symlink would cause rclone to error out from the listings.

After this change rclone will skip a circular symlink and carry on the listing,
producing an error at the end.

Fixes #4743

backend/local/local.go

@@ -456,8 +456,8 @@ func (f *Fs) List(ctx context.Context, dir string) (entries fs.DirEntries, err e
 			if f.opt.FollowSymlinks && (mode&os.ModeSymlink) != 0 {
 				localPath := filepath.Join(fsDirPath, name)
 				fi, err = os.Stat(localPath)
-				if os.IsNotExist(err) {
+				if os.IsNotExist(err) || isCircularSymlinkError(err) {
-					// Skip bad symlinks
+					// Skip bad symlinks and circular symlinks
 					err = fserrors.NoRetryError(errors.Wrap(err, "symlink"))
 					fs.Errorf(newRemote, "Listing error: %v", err)
 					err = accounting.Stats(ctx).Error(err)

backend/local/symlink.go

@@ -0,0 +1,22 @@
+// +build !windows,!plan9,!js
+
+package local
+
+import (
+	"os"
+	"syscall"
+)
+
+// isCircularSymlinkError checks if the current error code is because of a circular symlink
+func isCircularSymlinkError(err error) bool {
+	if err != nil {
+		if newerr, ok := err.(*os.PathError); ok {
+			if errcode, ok := newerr.Err.(syscall.Errno); ok {
+				if errcode == syscall.ELOOP {
+					return true
+				}
+			}
+		}
+	}
+	return false
+}

backend/local/symlink_other.go

@@ -0,0 +1,17 @@
+// +build windows plan9 js
+
+package local
+
+import (
+	"strings"
+)
+
+// isCircularSymlinkError checks if the current error code is because of a circular symlink
+func isCircularSymlinkError(err error) bool {
+	if err != nil {
+		if strings.Contains(err.Error(), "The name of the file cannot be resolved by the system") {
+			return true
+		}
+	}
+	return false
+}