avoid overwriting another user's database

Should fix issue #39

autojump

@@ -49,22 +49,25 @@ def dicadd(dic, key, increment=1):
 def save(path_dict, dic_file):
     """Save the database in an atomic way, and preserve
        a backup file."""
-    temp = NamedTemporaryFile(dir=CONFIG_DIR, delete=False)
-    pickle.dump(path_dict, temp, -1)
-    temp.flush()
-    os.fsync(temp)
-    temp.close()
-    #cf. http://thunk.org/tytso/blog/2009/03/15/dont-fear-the-fsync/
-    os.rename(temp.name, dic_file) 
-    try: #backup file
-        import time 
-        if (not os.path.exists(dic_file+".bak") or
-                time.time()-os.path.getmtime(dic_file+".bak")>86400):
-            import shutil
-            shutil.copy(dic_file, dic_file+".bak")
-    except OSError as ex:
-        print("Error while creating backup autojump file. (%s)" %
-                ex, file=stderr)
+    # If the dic_file exists, check that it belongs to us
+    # Otherwise, fail quietly
+    if (not os.path.exists(dic_file)) or os.getuid() == os.stat(dic_file)[4]:
+        temp = NamedTemporaryFile(dir=CONFIG_DIR, delete=False)
+        pickle.dump(path_dict, temp, -1)
+        temp.flush()
+        os.fsync(temp)
+        temp.close()
+        #cf. http://thunk.org/tytso/blog/2009/03/15/dont-fear-the-fsync/
+        os.rename(temp.name, dic_file) 
+        try: #backup file
+            import time 
+            if (not os.path.exists(dic_file+".bak") or
+                    time.time()-os.path.getmtime(dic_file+".bak")>86400):
+                import shutil
+                shutil.copy(dic_file, dic_file+".bak")
+        except OSError as ex:
+            print("Error while creating backup autojump file. (%s)" %
+                    ex, file=stderr)
 
 def forget(path_dict, dic_file):
     """Gradually forget about directories. Only call