Filesystem.Monitor: don't auto monitor paths that don't exist.

This should mitigate a race condition with inotify causing short lived
directory monitors to remain indefinitely.

lib/modules/Filesystem.pmod/Monitor.pmod/basic.pike

@@ -293,6 +293,12 @@ protected class Monitor(string path,
 
   void create()
   {
+    if(flags & MF_AUTO || flags & MF_HARD) {
+      Stdio.Stat fstat = file_stat(path);
+      if(!fstat) throw(Error.Generic("Cannot create Monitor for non-existent path " + path + 
+                                       " when flags MF_AUTO or MF_HARD are present.\n"));
+    }
+
     MON_WERR("Creating monitor for %O.\n", path);
     Element::create(this);
     register_path(1);
@@ -1001,10 +1007,12 @@ protected void inotify_event(int wd, int event, int cookie, string(8bit) path)
 	//
 	// Create the submonitor.
 	MON_WARN("Monitor lost for path %O.\n", full_path);
-        m = monitor(full_path, m->flags | MF_AUTO | MF_HARD,
+        mixed err = catch(m = monitor(full_path, m->flags | MF_AUTO | MF_HARD,
             m->max_dir_check_interval,
             m->file_interval_factor,
-            m->stable_time);
+            m->stable_time));
+        if(err)
+          MON_WARN("Monitor for %O failed. Message: %s\n", full_path, Error.mkerror(err)->message());
         // monitor() will not register monitor for path if filter_file()
         // returns 0.
         if (m) {