gh-160561: Close the duplicated descriptor when socket.fromfd() fails (#160579)

Co-authored-by: wlkr73 <86650480+wlkr73@users.noreply.github.com>
Co-authored-by: lumenyr <86650480+lumenyr@users.noreply.github.com>
Co-authored-by: Victor Stinner <vstinner@python.org>

Author: scrieve

Lib/socket.py

@@ -572,7 +572,11 @@ def fromfd(fd, family, type, proto=0):
     descriptor.  The remaining arguments are the same as for socket().
     """
     nfd = dup(fd)
-    return socket(family, type, proto, nfd)
+    try:
+        return socket(family, type, proto, nfd)
+    except:
+        close(nfd)
+        raise
 
 if hasattr(_socket.socket, "sendmsg"):
     def send_fds(sock, buffers, fds, flags=0, address=None):

Lib/test/test_socket.py

@@ -3195,6 +3195,23 @@ def testFromFd(self):
     def _testFromFd(self):
         self.serv_conn.send(MSG)
 
+    def testFromFdError(self):
+        # Make sure that the fromfd() duplicated socket is closed on error.
+        fd = self.cli_conn.fileno()
+        fd_count_before = os_helper.fd_count()
+
+        with self.assertRaises(TypeError):
+            socket.fromfd(fd, "not-a-valid-family", socket.SOCK_STREAM)
+        with self.assertRaises(TypeError):
+            socket.fromfd(fd, socket.AF_INET, "not-a-valid-type")
+        with self.assertRaises(OverflowError):
+            socket.fromfd(fd, socket.AF_INET, 1 << 100)
+
+        self.assertEqual(os_helper.fd_count(), fd_count_before)
+
+    def _testFromFdError(self):
+        pass
+
     def testDup(self):
         # Testing dup()
         sock = self.cli_conn.dup()

Misc/NEWS.d/next/Library/2026-10-24-07-58-00.gh-issue-160561.b3Kq1z.rst

@@ -0,0 +1 @@
+Fix a descriptor leak in :func:`socket.fromfd` when the socket constructor rejects its arguments. The duplicated descriptor was never closed.