Fix race conditions with MS-Windows lock files by using _sopen.

 src/filelock.c (create_lock_file) [WINDOWSNT]: Use _sopen with
 _SH_DENYRW flag, instead of emacs_open, to deny any other process
 access to the lock file until it is written and closed.

Fixes: debbugs:13807

1 files changed, 10 insertions, 4 deletions

diff --git a/src/filelock.c b/src/filelock.c
index 4d556de2454..78cd60a12e1 100644
--- a/src/filelock.c
+++ b/src/filelock.c
@@ -44,6 +44,7 @@ along with GNU Emacs.  If not, see <http://www.gnu.org/licenses/>.  */
 #include "coding.h"
 #include "systime.h"
 #ifdef WINDOWSNT
+#include <share.h>
 #include "w32.h"        /* for dostounix_filename */
 #endif
 
@@ -353,12 +354,17 @@ create_lock_file (char *lfname, char *lock_info_str, bool force)
      create a regular file with the lock info written as its
      contents.  */
   {
-    int fd = emacs_open (lfname, O_WRONLY | O_BINARY | O_CREAT | O_EXCL,
-                         S_IREAD | S_IWRITE);
+    /* Deny everybody else any kind of access to the file until we are
+       done writing it and close the handle.  This makes the entire
+       open/write/close operation atomic, as far as other processes
+       are concerned.  */
+    int fd = _sopen (lfname,
+                     _O_WRONLY | _O_BINARY | _O_CREAT | _O_EXCL | _O_NOINHERIT,
+                     _SH_DENYRW, S_IREAD | S_IWRITE);
 
     if (fd < 0 && errno == EEXIST && force)
-      fd = emacs_open (lfname, O_WRONLY | O_BINARY | O_TRUNC,
-                       S_IREAD | S_IWRITE);
+      fd = _sopen (lfname, _O_WRONLY | _O_BINARY | _O_TRUNC |_O_NOINHERIT,
+                   _SH_DENYRW, S_IREAD | S_IWRITE);
     if (fd >= 0)
       {
         ssize_t lock_info_len = strlen (lock_info_str);