Implement CLASH_DETECTION for MS-Windows.

 src/filelock.c [WINDOWSNT]: Include w32.h.
 (MAKE_LOCK_NAME): Don't use 'lock', it clashes with MS runtime
 function of that name.  Up-case the macro arguments.
 (IS_LOCK_FILE): New macro.
 (fill_in_lock_file_name): Use IS_LOCK_FILE instead of S_ISLNK.
 (create_lock_file): New function, with body extracted from
 lock_file_1.
 [WINDOWSNT]: Implement lock files by writing a regular file with
 the lock information as its contents.
 (read_lock_data): New function, on Posix platforms just calls
 emacs_readlinkat.
 [WINDOWSNT]: Read the lock info from the file.
 (current_lock_owner): Call read_lock_data instead of calling
 emacs_readlinkat directly.
 (lock_file) [WINDOWSNT]: Run the file name through
 dostounix_filename.
 src/w32proc.c (sys_kill): Support the case of SIG = 0, in which case
 just check if the process by that PID exists.
 src/w32.c (sys_open): Don't reset the _O_CREAT flag if _O_EXCL is
 also present, as doing so will fail to error out if the file
 already exists.
 src/makefile.w32-in ($(BLD)/filelock.$(O)): Depend on src/w32.h.

 nt/inc/ms-w32.h (BOOT_TIME_FILE): Define.
 nt/config.nt (CLASH_DETECTION): Define to 1.

 lisp/emacs-lisp/bytecomp.el (byte-recompile-directory): Reject files
 that match "\`\.#", to avoid compiling lock files, even if they
 are readable (as they are on MS-Windows).

 doc/emacs/files.texi (Interlocking): Don't refer to symlinks as the
 exclusive means of locking files.

 etc/NEWS: Mention support for lock files on MS-Windows.

1 files changed, 31 insertions, 1 deletions

diff --git a/src/w32proc.c b/src/w32proc.c
index 961791a40ed..84589388cd7 100644
--- a/src/w32proc.c
+++ b/src/w32proc.c
@@ -2263,12 +2263,42 @@ sys_kill (pid_t pid, int sig)
     pid = -pid;
 
   /* Only handle signals that will result in the process dying */
-  if (sig != SIGINT && sig != SIGKILL && sig != SIGQUIT && sig != SIGHUP)
+  if (sig != 0
+      && sig != SIGINT && sig != SIGKILL && sig != SIGQUIT && sig != SIGHUP)
     {
       errno = EINVAL;
       return -1;
     }
 
+  if (sig == 0)
+    {
+      /* It will take _some_ time before PID 4 or less on Windows will
+         be Emacs...  */
+      if (pid <= 4)
+        {
+          errno = EPERM;
+          return -1;
+        }
+      proc_hand = OpenProcess (PROCESS_QUERY_INFORMATION, 0, pid);
+      if (proc_hand == NULL)
+        {
+          DWORD err = GetLastError ();
+
+          switch (err)
+            {
+            case ERROR_ACCESS_DENIED: /* existing process, but access denied */
+              errno = EPERM;
+              return -1;
+            case ERROR_INVALID_PARAMETER: /* process PID does not exist */
+              errno = ESRCH;
+              return -1;
+            }
+        }
+      else
+        CloseHandle (proc_hand);
+      return 0;
+    }
+
   cp = find_child_pid (pid);
   if (cp == NULL)
     {