Solve race condition

Signed-off-by: Euclid Ye <yezhizhenjiakang@gmail.com>

Author: yezhizhen

tools/wptrunner/wptrunner/browsers/servodriver.py

@@ -1,6 +1,6 @@
 # mypy: allow-untyped-defs
 
-from http.client import HTTPConnection
+from http.client import HTTPConnection, ResponseNotReady
 import os
 import tempfile
 import time
@@ -147,11 +147,19 @@ def stop(self, force=False):
         conn = HTTPConnection(self.host, self.port)
         while self.is_alive():
             self.logger.info("Trying to shut down gracefully by extension command")
-            
-            conn.request("DELETE", "/session/dummy-session-id/servo/shutdown")
-            res = conn.getresponse()
-            self.logger.info(f"Got response status for shutdown command: {res.status}")
-            
+            while True:
+                try:
+                    conn.request("DELETE", "/session/dummy-session-id/servo/shutdown")
+                    res = conn.getresponse()
+                    self.logger.info(f"Got response status for shutdown command: {res.status}")
+                except ResponseNotReady:
+                    self.logger.info(f"Shutdown request not sent yet, retrying...")
+                    continue
+                except Exception as e:
+                    self.logger.info(f"Servo browser already shut down: {e}")
+                    return
+                # Successfully sent the shutdown command
+                break
             time.sleep(0.1)
 
     def find_wpt_prefs(self, logger):