Subject: 27.0.50; [PATCH] password-cache.el: confuses key absence with nil password
From: Óscar Fuentes <[email-masked]>
To: [email-masked]
Date: Mon, 29 Jul 2019 07:12:03 +0200
List-Id: <debbugs-submit.debbugs.gnu.org>

Recently observed that Gnus was creating lots of timers for
password-cache-remove, about 6 every time that it fetched new news/mail.

Upon inspection the cause was found in a change to password-cache.el:

commit d66dcde46a87ee8a9064db3d9b05da9b17036f5b
Author: Stefan Monnier <[email-masked]>
Date:   Fri Jul 28 12:27:00 2017 -0400

    * lisp/password-cache.el (password-data): Use a hash-table
    
    * lisp/auth-source.el (auth-source-magic): Remove.
    (auth-source-forget+, auth-source-forget-all-cached): Adjust to new
    format of password-data.
    (auth-source-format-cache-entry): Just use a cons.
    
    (password-cache-remove, password-cache-add, password-reset)
    (password-read-from-cache, password-in-cache-p): Adjust accordingly.
    
    Fixes: bug#26699

The points of interest of that change are:

 (defun password-in-cache-p (key)
   "Check if KEY is in the cache."
   (and password-cache
        key
-       (intern-soft key password-data)))
+       (gethash key password-data)))

and

 (defun password-cache-add (key password)
   "Add password to cache.
 The password is removed by a timer after `password-cache-expiry' seconds."
-  (when (and password-cache-expiry (null (intern-soft key password-data)))
+  (when (and password-cache-expiry (null (gethash key password-data)))

The change uses gethash instead of intern-soft, but those functions act
differently when the password (the value associated with the key) was
nil. The effect is that every call to password-cache-add with nil as
password creates a new timer, and password-in-cache-p returns nil if
there exists a (key nil) entry on password-data, when previously it
would return non-nil.

So I propose this patch:

diff --git a/lisp/password-cache.el b/lisp/password-cache.el
index 5a09ae4859..6009fb491e 100644
--- a/lisp/password-cache.el
+++ b/lisp/password-cache.el
@@ -81,7 +81,8 @@ password-in-cache-p
   "Check if KEY is in the cache."
   (and password-cache
        key
-       (gethash key password-data)))
+       (not (eq (gethash key password-data 'password-cache-no-data)
+                'password-cache-no-data))))
 
 (defun password-read (prompt &optional key)
   "Read password, for use with KEY, from user, or from cache if wanted.
@@ -125,7 +126,9 @@ password-cache-remove
 (defun password-cache-add (key password)
   "Add password to cache.
 The password is removed by a timer after `password-cache-expiry' seconds."
-  (when (and password-cache-expiry (null (gethash key password-data)))
+  (when (and password-cache-expiry
+             (eq (gethash key password-data 'password-cache-no-data)
+                 'password-cache-no-data))
     (run-at-time password-cache-expiry nil
 		 #'password-cache-remove
 		 key))

Okay to commit? To emacs-26 or master?

On another topic, before a cache entry is removed we try to overwrite
the stored password (see password-cache-remove). However, the same
change did this:

 (defun password-reset ()
   "Clear the password cache."
   (interactive)
-  (fillarray password-data 0))
+  (clrhash password-data))

I don't know if clrhash overwrites the data before releasing it.