gh-81403: Fix for CacheFTPHandler in urllib (#13951)

bpo-37222: Fix for CacheFTPHandler in urllib

A call to FTP.ntransfercmd must be followed by FTP.voidresp to clear
the "end transfer" message. Without this, the client and server get
out of sync, which will result in an error if the FTP instance is
reused to open a second URL. This scenario occurs for even the most
basic usage of CacheFTPHandler.

Reverts the patch merged as a resolution to bpo-16270 and adds a test
case for the CacheFTPHandler in test_urllib2net.py.

Co-authored-by: Senthil Kumaran <senthil@python.org>

Lib/test/test_urllib2net.py

@@ -134,7 +134,9 @@ def setUp(self):
     # They do sometimes catch some major disasters, though.
 
     def test_ftp(self):
+        # Testing the same URL twice exercises the caching in CacheFTPHandler
         urls = [
+            'ftp://www.pythontest.net/README',
             'ftp://www.pythontest.net/README',
             ('ftp://www.pythontest.net/non-existent-file',
              None, urllib.error.URLError),

Lib/urllib/request.py

@@ -2475,7 +2475,13 @@ def retrfile(self, file, type):
         return (ftpobj, retrlen)
 
     def endtransfer(self):
+        if not self.busy:
+            return
         self.busy = 0
+        try:
+            self.ftp.voidresp()
+        except ftperrors():
+            pass
 
     def close(self):
         self.keepalive = False