sftp: follow symlinks correctly - fixes #2145

The sftp library delivers the attributes of the symlink rather than
the object pointed to in directory listings, however when we use Stat
from the library it points to the objects.

Previous to this fix this caused items pointed to by symlinks to be
unusable.

After the fix both symlinked files and directories work as expected.

backend/sftp/sftp.go

@@ -481,6 +481,14 @@ func (f *Fs) List(dir string) (entries fs.DirEntries, err error) {
 	}
 	for _, info := range infos {
 		remote := path.Join(dir, info.Name())
+		// If file is a symlink (not a regular file is the best cross platform test we can do), do a stat to
+		// pick up the size and type of the destination, instead of the size and type of the symlink.
+		if !info.Mode().IsRegular() {
+			info, err = f.stat(remote)
+			if err != nil {
+				return nil, errors.Wrap(err, "stat of non-regular file/dir failed")
+			}
+		}
 		if info.IsDir() {
 			d := fs.NewDir(remote, info.ModTime())
 			entries = append(entries, d)
@@ -805,14 +813,21 @@ func (o *Object) setMetadata(info os.FileInfo) {
 	o.mode = info.Mode()
 }
 
+// statRemote stats the file or directory at the remote given
+func (f *Fs) stat(remote string) (info os.FileInfo, err error) {
+	c, err := f.getSftpConnection()
+	if err != nil {
+		return nil, errors.Wrap(err, "stat")
+	}
+	absPath := path.Join(f.root, remote)
+	info, err = c.sftpClient.Stat(absPath)
+	f.putSftpConnection(&c, err)
+	return info, err
+}
+
 // stat updates the info in the Object
 func (o *Object) stat() error {
-	c, err := o.fs.getSftpConnection()
-	if err != nil {
-		return errors.Wrap(err, "stat")
-	}
-	info, err := c.sftpClient.Stat(o.path())
-	o.fs.putSftpConnection(&c, err)
+	info, err := o.fs.stat(o.remote)
 	if err != nil {
 		if os.IsNotExist(err) {
 			return fs.ErrorObjectNotFound