asyncio: race condition in SSLProtocol

[vstinner]

BPO	33674
Nosy	@vstinner, @ned-deily, @asvetlov, @1st1, @miss-islington
PRs	bpo-33674: asyncio: Fix SSLProtocol race #7175 [3.7] bpo-33674: asyncio: Fix SSLProtocol race (GH-7175) #7187 [3.6] bpo-33674: asyncio: Fix SSLProtocol race (GH-7175) #7188 bpo-33674: Pause the transport as early as possible #7192 [3.7] bpo-33674: Pause the transport as early as possible (GH-7192) #7193

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2018-05-29.15:17:16.102>
created_at = <Date 2018-05-28.20:37:18.279>
labels = ['3.8', 'expert-asyncio']
title = 'asyncio: race condition in SSLProtocol'
updated_at = <Date 2018-05-29.15:17:16.101>
user = 'https://github.com/vstinner'

bugs.python.org fields:

activity = <Date 2018-05-29.15:17:16.101>
actor = 'yselivanov'
assignee = 'none'
closed = True
closed_date = <Date 2018-05-29.15:17:16.102>
closer = 'yselivanov'
components = ['asyncio']
creation = <Date 2018-05-28.20:37:18.279>
creator = 'vstinner'
dependencies = []
files = []
hgrepos = []
issue_num = 33674
keywords = ['patch']
message_count = 15.0
messages = ['317916', '317917', '317919', '317922', '317923', '317924', '317960', '317974', '317981', '317983', '317984', '318003', '318004', '318025', '318044']
nosy_count = 5.0
nosy_names = ['vstinner', 'ned.deily', 'asvetlov', 'yselivanov', 'miss-islington']
pr_nums = ['7175', '7187', '7188', '7192', '7193']
priority = None
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue33674'
versions = ['Python 3.8']

[vstinner]

While debugging bpo-32458 (functional test on START TLS), I found a race condition in SSLProtocol of asyncio/sslproto.py.

Sometimes, _sslpipe.feed_ssldata() is called before _sslpipe.shutdown().

• SSLProtocol.connection_made() -> SSLProtocol._start_handshake(): self._loop.call_soon(self._process_write_backlog)
• SSLProtoco.data_received(): direct call to self._sslpipe.feed_ssldata(data)
• Later, self._process_write_backlog() calls self._sslpipe.do_handshake()

The first write is delayed by call_soon(), whereas the first read is a direct call to the SSL pipe.

[vstinner]

Workaround:

diff --git a/Lib/asyncio/sslproto.py b/Lib/asyncio/sslproto.py
index 2bfa45dd15..4a5dbb38a1 100644
--- a/Lib/asyncio/sslproto.py
+++ b/Lib/asyncio/sslproto.py
@@ -592,7 +592,7 @@ class SSLProtocol(protocols.Protocol):
         # (b'', 1) is a special value in _process_write_backlog() to do
         # the SSL handshake
         self._write_backlog.append((b'', 1))
-        self._loop.call_soon(self._process_write_backlog)
+        self._process_write_backlog()
         self._handshake_timeout_handle = \
             self._loop.call_later(self._ssl_handshake_timeout,
                                   self._check_handshake_timeout)

[vstinner]

loop.start_tls() method is new in Python 3.7. If possible, I would prefer to not see it with a builtin race condition, since such race condition is really hard to debug :-( So I raise the priority to release blocker. Sorry again Ned!

[vstinner]

Yury Selivanov told me that usually it's safer to add call_soon(), than to remove call_soon(). But adding many call_soon() can make asyncio SSL slower.

SSLProtocol doesn't seem to like call_soon(), it's only used at:

• connection_lost(): call_soon(self._app_protocol.connection_lost, exc)
• connection_made() ~~> call_soon(self._process_write_backlog)
• _on_handshake_complete(): call_soon(self._process_write_backlog)

That's all. All other operations are done immediately.

_on_handshake_complete() contains a long comment:

        # In case transport.write() was already called. Don't call
        # immediately _process_write_backlog(), but schedule it:
        # _on_handshake_complete() can be called indirectly from
        # _process_write_backlog(), and _process_write_backlog() is not
        # reentrant.
        self._loop.call_soon(self._process_write_backlog)

[1st1]

The fix is correct and the bug is now obvious: data_received() occur pretty much any time after connection_made() call; if call_soon() is used in connection_made(), data_received() may find the protocol in an incorrect state.

Kudos Victor for debugging this.

[1st1]

And I agree, this should make it to 3.7.0

[vstinner]

New changeset be00a55 by Victor Stinner in branch 'master':
bpo-33674: asyncio: Fix SSLProtocol race (GH-7175)
be00a55

[ned-deily]

New changeset 0dd8fd0 by Ned Deily (Miss Islington (bot)) in branch '3.7':
bpo-33674: asyncio: Fix SSLProtocol race (GH-7175) (bpo-7187)
0dd8fd0

[1st1]

New changeset 7593b8a by Yury Selivanov (Victor Stinner) in branch '3.6':
bpo-33674: asyncio: Fix SSLProtocol race (GH-7175) (GH-7188)
7593b8a

[1st1]

New changeset f295587 by Yury Selivanov in branch 'master':
bpo-33674: Pause the transport as early as possible (bpo-7192)
f295587

[miss-islington]

New changeset eca0859 by Miss Islington (bot) in branch '3.7':
bpo-33674: Pause the transport as early as possible (GH-7192)
eca0859

[vstinner]

I fixed the main issue, so I remove the "release blocker" priority.