Issue #4473: Ensure the socket is shutdown cleanly in POP3.close().

Patch by Lorenzo Catucci.

Lib/poplib.py

@@ -259,7 +259,14 @@ class POP3:
         if self.file is not None:
             self.file.close()
         if self.sock is not None:
-            self.sock.close()
+            try:
+                self.sock.shutdown(socket.SHUT_RDWR)
+            except socket.error as e:
+                # The server might already have closed the connection
+                if e.errno != errno.ENOTCONN:
+                    raise
+            finally:
+                self.sock.close()
         self.file = self.sock = None
 
     #__del__ = quit

Misc/NEWS

@@ -138,6 +138,9 @@ Core and Builtins
 Library
 -------
 
+- Issue #4473: Ensure the socket is shutdown cleanly in POP3.close().
+  Patch by Lorenzo Catucci.
+
 - Issue #16522: added FAIL_FAST flag to doctest.
 
 - Issue #15627: Add the importlib.abc.SourceLoader.compile_source() method.