Fix crash closing already-closed socket. Refs #1596

```
error: uncaptured python exception, closing channel <supervisor.http.deferring_http_channel connected 127.0.0.1:58792 at 0x102d3a310 channel#: 0 requests:4> (<class 'OSError'>:[Errno 57] Socket is not connected [/Users/username/git/supervisor/supervisor/medusa/asynchat_25.py|handle_read|89] [/Users/username/git/supervisor/supervisor/medusa/http_server.py|recv|528] [/Users/username/git/supervisor/supervisor/medusa/asyncore_25.py|recv|354] [/Users/username/git/supervisor/supervisor/medusa/asynchat_25.py|handle_close|156] [/Users/username/git/supervisor/supervisor/medusa/asyncore_25.py|close|361])
error: uncaptured python exception, closing channel <supervisor.http.deferring_http_channel connected 127.0.0.1:58792 at 0x102d3a310 channel#: 0 requests:4> (<class 'OSError'>:[Errno 57] Socket is not connected [/Users/username/git/supervisor/supervisor/supervisord.py|runforever|218] [/Users/username/git/supervisor/supervisor/medusa/asyncore_25.py|handle_read_event|392] [/Users/username/git/supervisor/supervisor/medusa/asynchat_25.py|handle_read|91] [/Users/username/git/supervisor/supervisor/medusa/http_server.py|handle_error|546] [/Users/username/git/supervisor/supervisor/medusa/asyncore_25.py|handle_error|422] [/Users/username/git/supervisor/supervisor/medusa/asyncore_25.py|close|361])
Traceback (most recent call last):
  File "/Users/username/git/supervisor/supervisor/medusa/asyncore_25.py", line 347, in recv
    self.handle_close()
  File "/Users/username/git/supervisor/supervisor/medusa/asynchat_25.py", line 156, in handle_close
    self.close()
  File "/Users/username/git/supervisor/supervisor/medusa/asyncore_25.py", line 361, in close
    self.socket.shutdown(socket.SHUT_RDWR)
OSError: [Errno 57] Socket is not connected

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/Users/username/git/supervisor/supervisor/medusa/asynchat_25.py", line 89, in handle_read
    data = self.recv (self.ac_in_buffer_size)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/username/git/supervisor/supervisor/medusa/http_server.py", line 528, in recv
    result = asynchat.async_chat.recv (self, buffer_size)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/username/git/supervisor/supervisor/medusa/asyncore_25.py", line 354, in recv
    self.handle_close()
  File "/Users/username/git/supervisor/supervisor/medusa/asynchat_25.py", line 156, in handle_close
    self.close()
  File "/Users/username/git/supervisor/supervisor/medusa/asyncore_25.py", line 361, in close
    self.socket.shutdown(socket.SHUT_RDWR)
OSError: [Errno 57] Socket is not connected

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/Users/username/git/supervisor/supervisor/supervisord.py", line 218, in runforever
    dispatcher.handle_read_event()
  File "/Users/username/git/supervisor/supervisor/medusa/asyncore_25.py", line 392, in handle_read_event
    self.handle_read()
  File "/Users/username/git/supervisor/supervisor/medusa/asynchat_25.py", line 91, in handle_read
    self.handle_error()
  File "/Users/username/git/supervisor/supervisor/medusa/http_server.py", line 546, in handle_error
    asynchat.async_chat.handle_error (self)
  File "/Users/username/git/supervisor/supervisor/medusa/asyncore_25.py", line 422, in handle_error
    self.close()
  File "/Users/username/git/supervisor/supervisor/medusa/asyncore_25.py", line 361, in close
    self.socket.shutdown(socket.SHUT_RDWR)
OSError: [Errno 57] Socket is not connected

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/opt/homebrew/bin/supervisord", line 33, in <module>
    sys.exit(load_entry_point('supervisor', 'console_scripts', 'supervisord')())
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/username/git/supervisor/supervisor/supervisord.py", line 373, in main
    go(options)
  File "/Users/username/git/supervisor/supervisor/supervisord.py", line 383, in go
    d.main()
  File "/Users/username/git/supervisor/supervisor/supervisord.py", line 78, in main
    self.run()
  File "/Users/username/git/supervisor/supervisor/supervisord.py", line 94, in run
    self.runforever()
  File "/Users/username/git/supervisor/supervisor/supervisord.py", line 224, in runforever
    combined_map[fd].handle_error()
  File "/Users/username/git/supervisor/supervisor/medusa/http_server.py", line 546, in handle_error
    asynchat.async_chat.handle_error (self)
  File "/Users/username/git/supervisor/supervisor/medusa/asyncore_25.py", line 422, in handle_error
    self.close()
  File "/Users/username/git/supervisor/supervisor/medusa/asyncore_25.py", line 361, in close
    self.socket.shutdown(socket.SHUT_RDWR)
OSError: [Errno 57] Socket is not connected
```

supervisor/medusa/asyncore_25.py

@@ -358,8 +358,16 @@ def recv(self, buffer_size):
 
     def close(self):
         self.del_channel()
-        self.socket.shutdown(socket.SHUT_RDWR)
-        self.socket.close()
+
+        try:
+            self.socket.shutdown(socket.SHUT_RDWR)
+        except socket.error:
+            # must swallow exception from already-closed socket
+            # (at least with Python 3.11.7 on macOS 14.2.1)
+            pass
+
+        # does not raise if called on already-closed socket
+        self.socket.close()  
 
     # cheap inheritance, used to pass all other attribute
     # references to the underlying socket object.

supervisor/tests/fixtures/issue-1596.conf

@@ -0,0 +1,12 @@
+[supervisord]
+loglevel=info                ; log level; default info; others: debug,warn,trace
+logfile=/tmp/issue-1596.log  ; main log file; default $CWD/supervisord.log
+pidfile=/tmp/issue-1596.pid  ; supervisord pidfile; default supervisord.pid
+nodaemon=true                ; start in foreground if true; default false
+identifier=from_config_file
+
+[rpcinterface:supervisor]
+supervisor.rpcinterface_factory = supervisor.rpcinterface:make_main_rpcinterface
+
+[unix_http_server]
+file=/tmp/issue-1596.sock    ; the path to the socket file

supervisor/tests/test_end_to_end.py

@@ -426,3 +426,32 @@ def test_pull_request_1578_echo_supervisord_conf(self):
         echo_supervisord_conf = pexpect.spawn(sys.executable, args, encoding='utf-8')
         self.addCleanup(echo_supervisord_conf.kill, signal.SIGKILL)
         echo_supervisord_conf.expect_exact('Sample supervisor config file')
+
+    def test_issue_1596_asyncore_close_does_not_crash(self):
+        """If the socket is already closed when socket.shutdown(socket.SHUT_RDWR)
+        is called in the close() method of an asyncore dispatcher, an exception
+        will be raised (at least with Python 3.11.7 on macOS 14.2.1).  If it is
+        not caught in that method, supervisord will crash."""
+        filename = resource_filename(__package__, 'fixtures/issue-1596.conf')
+        args = ['-m', 'supervisor.supervisord', '-c', filename]
+        supervisord = pexpect.spawn(sys.executable, args, encoding='utf-8')
+        self.addCleanup(supervisord.kill, signal.SIGINT)
+        supervisord.expect_exact('supervisord started with pid')
+
+        from supervisor.compat import xmlrpclib
+        from supervisor.xmlrpc import SupervisorTransport
+
+        socket_url = 'unix:///tmp/issue-1596.sock'
+        dummy_url = 'http://transport.ignores.host/RPC2'
+
+        # supervisord will crash after close() if it has the bug
+        t1 = SupervisorTransport('', '', socket_url)
+        s1 = xmlrpclib.ServerProxy(dummy_url, t1)
+        s1.system.listMethods()
+        t1.close()
+
+        # this call will only succeed if supervisord did not crash
+        t2 = SupervisorTransport('', '', socket_url)
+        s2 = xmlrpclib.ServerProxy(dummy_url, t2)
+        s2.system.listMethods()
+        t2.close()