The lock for FILE is now .#FILE or .#-FILE.

The old approach, which fell back on DIR/.#FILE.0 through
DIR/.#FILE.9, had race conditions that could not be easily fixed.
If DIR/.#FILE is a non-symlink file, Emacs now does not create a
lock file for DIR/FILE; that is, DIR/FILE is no longer partly
protected by a lock if DIR/.#FILE is a non-symlink file ("partly"
because the locking mechanism was never reliable in that case).
This patch fixes this and other bugs discovered by a code
inspection that was prompted by
<http://lists.gnu.org/archive/html/emacs-devel/2013-02/msg00531.html>.
Also, this patch switches to .#-FILE (not .#FILE) on MS-Windows,
to avoid interoperability problems between the MS-Windows and
non-MS-Windows implementations.  MS-Windows and non-MS-Windows
instances of Emacs now ignore each others' locks.
* etc/NEWS: Document this.
* src/filelock.c (defined_WINDOWSNT): New constant.
(MAKE_LOCK_NAME, fill_in_lock_file_name):
Don't create DIR/.#FILE.0 through DIR/.#FILE.9.  Instead, create
DIR/.#FILE symlinks on non-MS-Windows hosts, and DIR/.#-FILE
regular files on MS-Windows hosts.
(MAKE_LOCK_NAME, unlock_file, Ffile_locked_p):
Use SAFE_ALLOCA to avoid problems with long file names.
(MAX_LFINFO): Now a local constant, not a global macro.
(IS_LOCK_FILE): Remove.
(lock_file_1): Don't inspect errno if symlink call succeeds;
that's not portable.
(lock_file): Document that this function can return if lock
creation fails.

Fixes: debbugs:13807

2 files changed, 16 insertions, 0 deletions

diff --git a/etc/ChangeLog b/etc/ChangeLog
index a13ab60be36..7ebb2a751ab 100644
--- a/etc/ChangeLog
+++ b/etc/ChangeLog
@@ -1,3 +1,7 @@
+2013-03-02  Paul Eggert  <eggert@cs.ucla.edu>
+
+        * NEWS: The lock for FILE is now .#FILE or .#-FILE (Bug#13807).
+
 2013-03-01  Michael Albinus  <michael.albinus@gmx.de>
 
         * NEWS: Fix Tramp "adb" entry.  Extend list of discontinued Tramp
diff --git a/etc/NEWS b/etc/NEWS
index b344f643069..73f7991823f 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -320,6 +320,18 @@ text-property on the first char.
 ** The `defalias-fset-function' property lets you catch calls to defalias
 and redirect them to your own function instead of `fset'.
 
+** The lock for 'DIR/FILE' is now 'DIR/.#FILE' or 'DIR/.#-FILE'.
+When you edit DIR/FILE, Emacs normally creates a symbolic link
+DIR/.#FILE as a lock that warns other instances of Emacs that DIR/FILE
+is being edited.  Formerly, if there was already a non-symlink file
+named DIR/.#FILE, Emacs fell back on the lock names DIR/.#FILE.0
+through DIR/.#FILE.9.  These fallbacks have been removed, so that
+Emacs now no longer locks DIR/FILE in that case.
+
+On MS-Windows the lock is a regular file DIR/.#-FILE, not a symlink.
+MS-Windows and non-MS-Windows implementations of Emacs ignore each
+other's locks.
+
 ** The 9th element returned by `file-attributes' is now unspecified.
 Formerly, it was t if the file's gid would change if file were deleted
 and recreated.  This value has been inaccurate for years on many