GNU bug report logs - #13860
24.2; dir-locals-directory-cache unreliable in one rare case

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Message #5 received at submit <at> debbugs.gnu.org (full text, mbox):

From: Johan Claesson <johanclaesson <at> bredband.net>
To: bug-gnu-emacs <at> gnu.org
Subject: 24.2; dir-locals-directory-cache unreliable in one rare case
Date: Sun, 03 Mar 2013 12:01:14 +0100

[Message part 1 (text/plain, inline)]

Hi,

Maybe i have misunderstood something but i find the cache
mechanism for directory local variables unreliable in one rare
case.  If first .dir-locals.el is read and a binding say foo = 1 is
inserted in the dir-locals-directory-cache.  And then foo = 2 is
written in the same second.  Now next time the dir locals the old
binding foo = 1 will be returned.  The cache entry is considered
valid when it should not.  

Of course it is unlikely that a user is typing so fast that they
trigger this :).  But it can be triggered from Lisp.  I found out
when playing with a ert test case that was tampering with a dir
local.  

Here is a recipe for this:

(defvar foo nil)
(put 'foo 'safe-local-variable 'numberp)
(let ((dir (make-temp-file "cache-test" t)))
  ;; Create .dir-locals.el with foo = 1.
  (with-temp-buffer
    (cd dir)
    (add-dir-local-variable nil 'foo 1)
    (save-buffer)
    (kill-buffer))
  ;; Read it into dir-locals-directory-cache
  ;; and save .dir-locals.el with foo = 2.
  (with-temp-buffer
    (cd dir)
    (make-local-variable 'foo)
    (hack-dir-local-variables-non-file-buffer)
    (add-dir-local-variable nil 'foo 2)
    (save-buffer)
    (kill-buffer))
  ;; Read it back again.
  ;; It should be 2 at this point but is 1 most of the times. 
  ;; (At the rare occasion that seconds increase
  ;; between the two add-dir-local-variable it returns 1).
  (with-temp-buffer
    (cd dir)
    (make-local-variable 'foo)
    (hack-dir-local-variables-non-file-buffer)
    (delete-file dir-locals-file)
    (delete-directory dir) 
    foo))



The current mechanism: The bindings found when reading a
.dir-locals.el file is put into dir-locals-directory-cache.  The
mtime of the file is stored in the cache entry.  Later a cache
entry is considered valid if it's mtime equals that of the
file-attribute mtime of corresponding .dir-locals.el file.

Suggestion: The mtime stored in the cache entry should be the
time when .dir-local.el was read into memory.  A cache entry
should be considered valid if mtime of cache entry is greater
than the file-attribute mtime of corresponding .dir-locals.el
file.  This will invalidate a couple of entries that should
ideally have been considered valid but it will not let through
any invalid ones.

Or it should not let through any invalid entries, but in fact it
seem to do so sometimes.  If the above test is looped it will
return the incorrect value one time in about 500 on my standard
Ubuntu GNU/Linux system.  I don't know why, maybe reading the mtime
from a file is not that reliable.  Anyway, to be on the safe side i
subtract one second from the mtime when storing it to the cache.
Now the test ran 50000 times with no trouble.

Attached is a patch for files.el with this modification.  If this
makes sense and is accepted i will update the patch with the
appropriate documentation changes as well.

Regards,

/Johan

[files.patch (text/x-diff, inline)]

--- a/files.el	2013-03-03 10:15:44.498835000 +0000
+++ b/files.el	2013-03-03 10:28:02.662835000 +0000
@@ -3610,14 +3610,14 @@ (defun dir-locals-find-file (file)
 		(let ((cached-file (expand-file-name dir-locals-file-name
 						     (car dir-elt))))
 		  (and (file-readable-p cached-file)
-		       (equal (nth 2 dir-elt)
-			      (nth 5 (file-attributes cached-file))))))
+		       (time-less-p (nth 5 (file-attributes cached-file))
+				    (nth 2 dir-elt)))))
 	    ;; This cache entry is OK.
-	    dir-elt
+	      dir-elt
 	  ;; This cache entry is invalid; clear it.
 	  (setq dir-locals-directory-cache
 		(delq dir-elt dir-locals-directory-cache))
-	  ;; Return the first existing dir-locals file.  Might be the same
+	  ;; Return the first existing dir-locals file.	 Might be the same
 	  ;; as dir-elt's, might not (eg latter might have been deleted).
 	  locals-file)
       ;; No cache entry.
@@ -3638,10 +3638,13 @@ (defun dir-locals-read-from-file (file)
 	    (let* ((dir-name (file-name-directory file))
 		   (class-name (intern dir-name))
 		   (variables (let ((read-circle nil))
-				(read (current-buffer)))))
+				(read (current-buffer))))
+		   (just-now (time-subtract (current-time)
+					    (list 0 1 0)))
+		   (mtime (with-decoded-time-value ((high low micro just-now))
+			    (encode-time-value high low micro 1))))
 	      (dir-locals-set-class-variables class-name variables)
-	      (dir-locals-set-directory-class dir-name class-name
-					      (nth 5 (file-attributes file)))
+	      (dir-locals-set-directory-class dir-name class-name mtime)
 	      class-name))
 	(error (message "Error reading dir-locals: %S" err) nil)))))
 

[Message part 3 (text/plain, inline)]

In GNU Emacs 24.2.2 (i686-pc-linux-gnu, GTK+ Version 2.24.10)
 of 2013-01-12 on goblin
Windowing system distributor `The X.Org Foundation', version 11.0.11203000
Configured using:
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Major mode: Fundamental

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Recent input:
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Recent messages:
Saving file /home/jcl/.newsrc...
Wrote /home/jcl/.newsrc
Saving /home/jcl/.newsrc.eld...
Saving file /home/jcl/.newsrc.eld...
Wrote /home/jcl/.newsrc.eld
Saving /home/jcl/.newsrc.eld...done
Error during redisplay: (wrong-type-argument arrayp nil) [50 times]
Contacting host: debbugs.gnu.org:80
Error during redisplay: (wrong-type-argument arrayp nil) [9 times]
Reporting new bug!

Load-path shadows:
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Message #8 received at 13860 <at> debbugs.gnu.org (full text, mbox):

From: Lars Ingebrigtsen <larsi <at> gnus.org>
To: Johan Claesson <johanclaesson <at> bredband.net>
Cc: 13860 <at> debbugs.gnu.org
Subject: Re: bug#13860: 24.2; dir-locals-directory-cache unreliable in one
 rare case
Date: Wed, 14 Aug 2019 23:18:44 -0700

Johan Claesson <johanclaesson <at> bredband.net> writes:

> Maybe i have misunderstood something but i find the cache
> mechanism for directory local variables unreliable in one rare
> case.  If first .dir-locals.el is read and a binding say foo = 1 is
> inserted in the dir-locals-directory-cache.  And then foo = 2 is
> written in the same second.  Now next time the dir locals the old
> binding foo = 1 will be returned.  The cache entry is considered
> valid when it should not.  
>
> Of course it is unlikely that a user is typing so fast that they
> trigger this :).  But it can be triggered from Lisp.  I found out
> when playing with a ert test case that was tampering with a dir
> local.  

(I'm going through old bug reports that have unfortunately gotten no
responses yet.)

> Here is a recipe for this:
>
> (defvar foo nil)
> (put 'foo 'safe-local-variable 'numberp)
> (let ((dir (make-temp-file "cache-test" t)))
>   ;; Create .dir-locals.el with foo = 1.
>   (with-temp-buffer
>     (cd dir)
>     (add-dir-local-variable nil 'foo 1)
>     (save-buffer)
>     (kill-buffer))
>   ;; Read it into dir-locals-directory-cache
>   ;; and save .dir-locals.el with foo = 2.
>   (with-temp-buffer
>     (cd dir)
>     (make-local-variable 'foo)
>     (hack-dir-local-variables-non-file-buffer)
>     (add-dir-local-variable nil 'foo 2)
>     (save-buffer)
>     (kill-buffer))
>   ;; Read it back again.
>   ;; It should be 2 at this point but is 1 most of the times. 
>   ;; (At the rare occasion that seconds increase
>   ;; between the two add-dir-local-variable it returns 1).
>   (with-temp-buffer
>     (cd dir)
>     (make-local-variable 'foo)
>     (hack-dir-local-variables-non-file-buffer)
>     (delete-file dir-locals-file)
>     (delete-directory dir) 
>     foo))

I'm unable to reproduce this problem in Emacs 27, but perhaps this could
be a problem if the file system doesn't have sub-second resolution?  Are
there such file systems still in use (I mean, for something where'd you
store file with file-local variables)?

-- 
(domestic pets only, the antidote for overdose, milk.)
   bloggy blog: http://lars.ingebrigtsen.no

Message #13 received at 13860 <at> debbugs.gnu.org (full text, mbox):

From: Johan Claesson <johanclaesson <at> bredband.net>
To: Lars Ingebrigtsen <larsi <at> gnus.org>
Cc: 13860 <at> debbugs.gnu.org
Subject: Re: bug#13860: 24.2;
 dir-locals-directory-cache unreliable in one rare case
Date: Sun, 18 Aug 2019 21:19:01 +0200

Hi Lars,

I can reproduce with emacs -Q on Emacs 27 (ee1c638cff).  I get the wrong
value about 1 time in 3 tries.

(I cannot reproduce without -Q.  In particular when magit-autorevert is
loaded that will "fix" the issue.  Probably by slowing things down.)

My original description is a bit incorrect/misleading.  The race
condition is not about if it occurs in the same second or not.  But it
is about when two file modification timestamps are equal.

What happens seem to be that sometimes the second time the
.dir-locals.el file is written (now with value foo = 2) the file will
get the same timestamp as it already have from the first write.  At this
point the dir-locals-class-alist cache is not updated (it still have foo
= 1).

The cache is supposed to be updated when reading the .dir-locals.el. 
But that is only done if .dir-locals.el modification timestamp is newer
than the timestamp in the cache.  Sometimes they are the same just because 
the clock have not ticked forward.

I think one way to remedy this would be to explicitly invalidate the
directory cache every time a .dir-local.el file is modified.  Something
like the following seem to do the trick:

diff --git a/lisp/files-x.el b/lisp/files-x.el
index b71e9204f3..6b04518fe4 100644
--- a/lisp/files-x.el
+++ b/lisp/files-x.el
@@ -491,6 +491,13 @@ modify-dir-local-variable
                (cons `(,mode . ((,variable . ,value)))
                      variables))))
 
+      ;; Invalidate cache (may be needed if this .dir-locals.el file
+      ;; will be written with the same timestamp as is already present
+      ;; in the cache, see bug#13860).
+      (setq dir-locals-directory-cache
+            (assoc-delete-all (file-name-directory variables-file)
+                              dir-locals-directory-cache))
+
       ;; Insert modified alist of directory-local variables.
       (insert ";;; Directory Local Variables\n")
       (insert ";;; For more information see (info \"(emacs) Directory Variables\")\n\n")


Regards,

/Johan





On Wed, Aug 14 2019, Lars Ingebrigtsen wrote:
> Johan Claesson <johanclaesson <at> bredband.net> writes:
>
>> Maybe i have misunderstood something but i find the cache
>> mechanism for directory local variables unreliable in one rare
>> case.  If first .dir-locals.el is read and a binding say foo = 1 is
>> inserted in the dir-locals-directory-cache.  And then foo = 2 is
>> written in the same second.  Now next time the dir locals the old
>> binding foo = 1 will be returned.  The cache entry is considered
>> valid when it should not.  
>>
>> Of course it is unlikely that a user is typing so fast that they
>> trigger this :).  But it can be triggered from Lisp.  I found out
>> when playing with a ert test case that was tampering with a dir
>> local.  
>
> (I'm going through old bug reports that have unfortunately gotten no
> responses yet.)
>
>> Here is a recipe for this:
>>
>> (defvar foo nil)
>> (put 'foo 'safe-local-variable 'numberp)
>> (let ((dir (make-temp-file "cache-test" t)))
>>   ;; Create .dir-locals.el with foo = 1.
>>   (with-temp-buffer
>>     (cd dir)
>>     (add-dir-local-variable nil 'foo 1)
>>     (save-buffer)
>>     (kill-buffer))
>>   ;; Read it into dir-locals-directory-cache
>>   ;; and save .dir-locals.el with foo = 2.
>>   (with-temp-buffer
>>     (cd dir)
>>     (make-local-variable 'foo)
>>     (hack-dir-local-variables-non-file-buffer)
>>     (add-dir-local-variable nil 'foo 2)
>>     (save-buffer)
>>     (kill-buffer))
>>   ;; Read it back again.
>>   ;; It should be 2 at this point but is 1 most of the times. 
>>   ;; (At the rare occasion that seconds increase
>>   ;; between the two add-dir-local-variable it returns 1).
>>   (with-temp-buffer
>>     (cd dir)
>>     (make-local-variable 'foo)
>>     (hack-dir-local-variables-non-file-buffer)
>>     (delete-file dir-locals-file)
>>     (delete-directory dir) 
>>     foo))
>
> I'm unable to reproduce this problem in Emacs 27, but perhaps this could
> be a problem if the file system doesn't have sub-second resolution?  Are
> there such file systems still in use (I mean, for something where'd you
> store file with file-local variables)?

Message #16 received at 13860 <at> debbugs.gnu.org (full text, mbox):

From: Lars Ingebrigtsen <larsi <at> gnus.org>
To: Johan Claesson <johanclaesson <at> bredband.net>
Cc: 13860 <at> debbugs.gnu.org
Subject: Re: bug#13860: 24.2; dir-locals-directory-cache unreliable in one
 rare case
Date: Sun, 18 Aug 2019 15:49:49 -0700

Johan Claesson <johanclaesson <at> bredband.net> writes:

> I think one way to remedy this would be to explicitly invalidate the
> directory cache every time a .dir-local.el file is modified.  Something
> like the following seem to do the trick:
>
> diff --git a/lisp/files-x.el b/lisp/files-x.el
> index b71e9204f3..6b04518fe4 100644
> --- a/lisp/files-x.el
> +++ b/lisp/files-x.el
> @@ -491,6 +491,13 @@ modify-dir-local-variable
>                 (cons `(,mode . ((,variable . ,value)))
>                       variables))))
>
> +      ;; Invalidate cache (may be needed if this .dir-locals.el file
> +      ;; will be written with the same timestamp as is already present
> +      ;; in the cache, see bug#13860).
> +      (setq dir-locals-directory-cache
> +            (assoc-delete-all (file-name-directory variables-file)
> +                              dir-locals-directory-cache))
> +

Thanks; I think this makes sense, so I've applied it to Emacs 27.

-- 
(domestic pets only, the antidote for overdose, milk.)
   bloggy blog: http://lars.ingebrigtsen.no

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