accepted sockets shouldn't inherit the SOCK_NONBLOCK flag (closes #25471)

Lib/socket.py

@@ -185,7 +185,11 @@ class socket(_socket.socket):
         For IP sockets, the address info is a pair (hostaddr, port).
         """
         fd, addr = self._accept()
-        sock = socket(self.family, self.type, self.proto, fileno=fd)
+        # If our type has the SOCK_NONBLOCK flag, we shouldn't pass it onto the
+        # new socket. We do not currently allow passing SOCK_NONBLOCK to
+        # accept4, so the returned socket is always blocking.
+        type = self.type & ~globals().get("SOCK_NONBLOCK", 0)
+        sock = socket(self.family, type, self.proto, fileno=fd)
         # Issue #7995: if no default timeout is set and the listening
         # socket had a (non-zero) timeout, force the new socket in blocking
         # mode to override platform-specific socket flags inheritance.

Lib/test/test_socket.py

@@ -3863,6 +3863,7 @@ class NonBlockingTCPTests(ThreadedTCPSocketTest):
         read, write, err = select.select([self.serv], [], [])
         if self.serv in read:
             conn, addr = self.serv.accept()
+            self.assertIsNone(conn.gettimeout())
             conn.close()
         else:
             self.fail("Error trying to do accept after select.")

Misc/NEWS

@@ -96,6 +96,9 @@ Core and Builtins
 Library
 -------
 
+- Issue #25471: Sockets returned from accept() shouldn't appear to be
+  nonblocking.
+
 - Issue #25441: asyncio: Raise error from drain() when socket is closed.
 
 - Issue #25411: Improved Unicode support in SMTPHandler through better use of