limit filelock use

lib/iris/tests/__init__.py

@@ -804,10 +804,17 @@ def check_graphic(self):
 
         """
         fname = os.path.join(_RESULT_PATH, 'imagerepo.lock')
-        lock = filelock.FileLock(fname)
-        # The imagerepo.json file is a critical resource, so ensure thread
+        # The imagerepo.json file is a critical resource, during development. So,
+        # if the file is writeable, or it doesn't exist but could be written, ensure thread
         # safe read/write behaviour via platform independent file locking.
-        with lock.acquire(timeout=600):
+        # Otherwise, do not lock, as this prevents tests being run without
+        # write permissions to this _RESULT_PATH location.
+        if (os.access(fname, os.W_OK) or
+            (not os.access(fname, os.R_OK) and os.access(_RESULT_PATH, os.W_OK))):
+            lock = filelock.FileLock(fname)
+            with lock.acquire(timeout=600):
+                self._assert_graphic()
+        else:
             self._assert_graphic()
 
     def _remove_testcase_patches(self):