Avoid 'Connection refused' when using multithreaded gthread worker wi…

…th keep-alive

Fixes benoitc#1698

gunicorn/workers/gthread.py

@@ -55,6 +55,7 @@ def __init__(self, cfg, sock, client, server):
 
         # set the socket to non blocking
         self.sock.setblocking(False)
+        self.is_active = True
 
     def init(self):
         self.sock.setblocking(True)
@@ -148,8 +149,9 @@ def reuse_connection(self, conn, client):
             try:
                 self._keep.remove(conn)
             except ValueError:
-                # race condition
-                return
+                # race condition (other thread executed murder_keepalived)
+                pass
+            conn.is_active = True
 
         # submit the connection to a worker
         self.enqueue_req(conn)
@@ -168,7 +170,8 @@ def murder_keepalived(self):
             if delta > 0:
                 # add the connection back to the queue
                 with self._lock:
-                    self._keep.appendleft(conn)
+                    if not conn.is_active:
+                        self._keep.appendleft(conn)
                 break
             else:
                 self.nr_conns -= 1
@@ -259,6 +262,7 @@ def finish_request(self, fs):
                 conn.set_timeout()
                 with self._lock:
                     self._keep.append(conn)
+                    conn.is_active = False
 
                     # add the socket to the event loop
                     self.poller.register(conn.sock, selectors.EVENT_READ,