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In FTPS, CCC command can be used to switch back to a clear-text control connection.
This can be useful to take advantage of firewalls that know how to handle NAT with non-secure FTP without opening fixed ports.
Patch in attachment includes lib, test and doc changes.

> This can be useful to take advantage of firewalls that know how to
> handle NAT with non-secure FTP without opening fixed ports.
Can you explain this a little more?
+ resp = self.voidcmd('CCC')
+ self.sock = self.sock.unwrap()
+ return resp
Should the response be checked before calling unwrap()?

voidcmd() expects a response code starting with "2" and automatically raises an exception otherwise.
A server succesfully accepting the CCC command is supposed to first reply with 220 and then shutdown() the SSL layer.
Client is supposed to do the same: first receive the 2xx response and then make the shutdown().

New changeset d2eacbbdaf57 by Giampaolo Rodola' in branch 'default':
Issue 12139: add CCC command support to FTP_TLS class to revert the SSL connection back to clear-text.
http://hg.python.org/cpython/rev/d2eacbbdaf57 

http://www.python.org/dev/buildbot/all/builders/x86%20Ubuntu%20Shared%203.x/builds/4043/steps/test/logs/stdio
======================================================================
ERROR: test_ccc (test.test_ftplib.TestTLS_FTPClass)
----------------------------------------------------------------------
Traceback (most recent call last):
 File "/srv/buildbot/buildarea/3.x.bolen-ubuntu/build/Lib/test/test_ftplib.py", line 890, in test_ccc
 self.client.sendcmd('noop')
 File "/srv/buildbot/buildarea/3.x.bolen-ubuntu/build/Lib/ftplib.py", line 261, in sendcmd
 return self.getresp()
 File "/srv/buildbot/buildarea/3.x.bolen-ubuntu/build/Lib/ftplib.py", line 236, in getresp
 raise error_proto(resp)
ftplib.error_proto: 
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„Ö®o ̧qhёøcÞÅ39円úӅ#ï•å200 noop ok

http://www.python.org/dev/buildbot/all/builders/x86%20Tiger%203.x/builds/2792/steps/test/logs/stdio
======================================================================
ERROR: test_ccc (test.test_ftplib.TestTLS_FTPClass)
----------------------------------------------------------------------
Traceback (most recent call last):
 File "/Users/db3l/buildarea/3.x.bolen-tiger/build/Lib/test/test_ftplib.py", line 890, in test_ccc
 self.client.sendcmd('noop')
 File "/Users/db3l/buildarea/3.x.bolen-tiger/build/Lib/ftplib.py", line 261, in sendcmd
 return self.getresp()
 File "/Users/db3l/buildarea/3.x.bolen-tiger/build/Lib/ftplib.py", line 226, in getresp
 resp = self.getmultiline()
 File "/Users/db3l/buildarea/3.x.bolen-tiger/build/Lib/ftplib.py", line 212, in getmultiline
 line = self.getline()
 File "/Users/db3l/buildarea/3.x.bolen-tiger/build/Lib/ftplib.py", line 199, in getline
 line = self.file.readline()
 File "/Users/db3l/buildarea/3.x.bolen-tiger/build/Lib/socket.py", line 279, in readinto
 return self._sock.recv_into(b)
 File "/Users/db3l/buildarea/3.x.bolen-tiger/build/Lib/ssl.py", line 392, in recv_into
 return socket.recv_into(self, buffer, nbytes, flags)
socket.timeout: timed out

Hmm... Reopening. I'll look into this later.

The test is still failing on many buildbots.
x86 Ubuntu Shared 3.x:
======================================================================
ERROR: test_ccc (test.test_ftplib.TestTLS_FTPClass)
----------------------------------------------------------------------
Traceback (most recent call last):
 File "/srv/buildbot/buildarea/3.x.bolen-ubuntu/build/Lib/test/test_ftplib.py", line 890, in test_ccc
 self.client.sendcmd('noop')
 File "/srv/buildbot/buildarea/3.x.bolen-ubuntu/build/Lib/ftplib.py", line 261, in sendcmd
 return self.getresp()
 File "/srv/buildbot/buildarea/3.x.bolen-ubuntu/build/Lib/ftplib.py", line 236, in getresp
 raise error_proto(resp)
ftplib.error_proto: 
 žeÊc ̄ŸöZÖ8N ́†ýùô
"PPC Tiger 3.x", "AMD64 Leopard 3.x", "x86 FreeBSD 6.4 3.x" and "x86 Tiger 3.x":
======================================================================
ERROR: test_ccc (test.test_ftplib.TestTLS_FTPClass)
----------------------------------------------------------------------
Traceback (most recent call last):
 File "/Users/pythonbuildbot/buildarea/3.x.hansen-osx-x86-2/build/Lib/test/test_ftplib.py", line 890, in test_ccc
 self.client.sendcmd('noop')
 File "/Users/pythonbuildbot/buildarea/3.x.hansen-osx-x86-2/build/Lib/ftplib.py", line 261, in sendcmd
 return self.getresp()
 File "/Users/pythonbuildbot/buildarea/3.x.hansen-osx-x86-2/build/Lib/ftplib.py", line 226, in getresp
 resp = self.getmultiline()
 File "/Users/pythonbuildbot/buildarea/3.x.hansen-osx-x86-2/build/Lib/ftplib.py", line 212, in getmultiline
 line = self.getline()
 File "/Users/pythonbuildbot/buildarea/3.x.hansen-osx-x86-2/build/Lib/ftplib.py", line 199, in getline
 line = self.file.readline()
 File "/Users/pythonbuildbot/buildarea/3.x.hansen-osx-x86-2/build/Lib/socket.py", line 279, in readinto
 return self._sock.recv_into(b)
 File "/Users/pythonbuildbot/buildarea/3.x.hansen-osx-x86-2/build/Lib/ssl.py", line 392, in recv_into
 return socket.recv_into(self, buffer, nbytes, flags)
socket.timeout: timed out

The culprit here is the dummy FTP test server which does not complete the SSL shutdown procedure successfully.
I believe it's better if we remove the self.client.sendcmd('noop') line.
That's there in order to check that client and server can still "talk" after reverting the connection back to clear-text, but making the server doing this right is quite complicated, see for example:
http://code.google.com/p/pyftpdlib/source/browse/trunk/pyftpdlib/contrib/handlers.py?spec=svn868&r=861#196
I don't want to include and maintain that monster into test_ftplib.py so I think I'll just remove that line from the tests.

New changeset 0cef8cac71c8 by Giampaolo Rodola' in branch 'default':
Issue 12139: ftplib - remove 'post CCC' test to fix various buildot failures due to dummy test server not properly handling SSL shutdown(), see http://bugs.python.org/msg139499
http://hg.python.org/cpython/rev/0cef8cac71c8