bug#22311: 25.1.50; package.el misused (read-from-string) will potentially cause "elpa/archives/xxx/archive-contents" file malformed

[Tao Fang]

Hi, all
  There is a misused function read-from-string in package.el L1485:

  1472  (defun package--download-one-archive (archive file &optional async)
  1473    "Retrieve an archive file FILE from ARCHIVE, and cache it.
  1474  ARCHIVE should be a cons cell of the form (NAME . LOCATION),
  1475  similar to an entry in `package-alist'.  Save the cached copy to
  1476  \"archives/NAME/FILE\" in `package-user-dir'."
  1477    (package--with-response-buffer (cdr archive) :file file
  1478      :async async
  1479      :error-form (package--update-downloads-in-progress archive)
  1480      (let* ((location (cdr archive))
  1481             (name (car archive))
  1482             (content (buffer-string))
  1483             (dir (expand-file-name (format "archives/%s" name) 
package-user-dir))
  1484             (local-file (expand-file-name file dir)))
  1485        (when (listp (read-from-string content))
  1486          (make-directory dir t)
  1487          (if (or (not package-check-signature)

listp checks return value of (read-from-string content) to make sure we
get file content with correct format, but as its doc says:
"
(read-from-string STRING &optional START END)

Read one Lisp expression which is represented as text by STRING.
Returns a cons: (OBJECT-READ . FINAL-STRING-INDEX).
"
(listp (read-from-string content)) will always return t, if archive-contents 
file download
finished with malformed content (e.g. error message return from proxy
server), it will be parsed and saved by mistake.

Simply replace (read-from-string) with (read) would resolve this, I think.