gh-160561: Address review comments on the fromfd() leak fix

Remove the code comment and use a bare except, as requested. Rename
the test to testFromFdError, flatten it and use os_helper.fd_count()
so the descriptor count works on all platforms.

Author: scrieve

Lib/socket.py

@@ -574,9 +574,7 @@ def fromfd(fd, family, type, proto=0):
     nfd = dup(fd)
     try:
         return socket(family, type, proto, nfd)
-    except BaseException:
-        # socket() rejected the arguments; the dup()'d descriptor would
-        # otherwise leak, since nothing holds a reference to it.
+    except:
         close(nfd)
         raise
 

Lib/test/test_socket.py

@@ -3195,25 +3195,22 @@ def testFromFd(self):
     def _testFromFd(self):
         self.serv_conn.send(MSG)
 
-    def testFromFdInvalidArgsDoesNotLeak(self):
-        # fromfd() dup()s the descriptor before socket() validates its
-        # arguments; a rejected call must not leave the duplicate open.
-        fddir = '/proc/self/fd'
-        if not os.path.isdir(fddir):
-            self.skipTest('test needs /proc/self/fd')
+    def testFromFdError(self):
+        # Make sure that the fromfd() duplicated socket is closed on error.
+        # In debug mode, Python test suite checks for file descriptor leaks.
         fd = self.cli_conn.fileno()
-        base = dict(family=socket.AF_INET, type=socket.SOCK_STREAM, proto=0)
-        # family, type and proto are all converted to a C int, so each of them
-        # can fail after the dup() has already happened.
-        for name in base:
-            kwargs = dict(base, **{name: 1 << 100})
-            with self.subTest(name=name):
-                before = len(os.listdir(fddir))
-                with self.assertRaises(OverflowError):
-                    socket.fromfd(fd, **kwargs)
-                self.assertEqual(len(os.listdir(fddir)), before)
-
-    def _testFromFdInvalidArgsDoesNotLeak(self):
+        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_UNIX, "not-a-valid-type")
+        with self.assertRaises(OverflowError):
+            socket.fromfd(fd, socket.AF_UNIX, 1 << 100)
+
+        self.assertEqual(os_helper.fd_count(), fd_count_before)
+
+    def _testFromFdError(self):
         pass
 
     def testDup(self):