On 12/06/2015 07:57 PM, Matthew Jordan wrote:
Hello all -
One of the efforts that a number of developers in the community here
at Digium have been at work at are cleaning up test failures exposed
by Jenkins [1]. One of these, in particular, has been rather difficult
to resolve - namely, fax/pjsip/directmedia_reinvite_t38 [2]. This
e-mail goes over what has been accomplished, and asks some questions
on how we might try and fix Asterisk under this scenario.
The directmedia_reinvite_t38 test attempts to do the following:
(1) UAC1 calls UAC2 through Asterisk, with audio as the media. The
dial is performed using the 'g' flag, such that UAC2 will continue on
if UAC1 hangs up.
(2) UAC1 and UAC2 are configured for direct media. Asterisk sends a
re-INVITE to UAC1 and UAC2 to initiate direct media.
(3) After responding with a 200 OK to the direct media requests, UAC1
sends a re-INVITE offering T.38.
(4) Asterisk sends an INVITE with T.38 to UAC2
(5) UAC2 sends back a 200 OK for T.38; Asterisk sends that to UAC1.
Asterisk switches out of a direct media bridge to a core bridge.
(6) UAC1 hangs up. Asterisk sends a re-INVITE to UAC2 for audio back
to Asterisk. UAC2 responds with a 200 OK for the audio.
(7) Asterisk ejects UAC2 back to the dialplan.
It's important to note that this test never should have passed - an
update to the test suite "fixed" the test erroneously passing, which
led to us investigating why the scenario was failing. This test was
copied over from an identical chan_sip test, which passes.
The PJSIP stack has two issues which make life difficult for it in
this scenario:
(1) The T.38 logic is implemented in res_pjsip_t38. While that is
_mostly_ a very good thing - as it keeps all the fax state logic
outside of the channel driver - we are also a layer removed from
interactions that occur in the channel driver. That makes it
challenging to influence direct media checks and other
Asterisk/channel interactions.
(2) Being very asynchronous, requests may be serviced that influence
T.38 state while other interactions are occurring in the core.
Informing the core of what has occurred can have more race conditions
than what occurs in chan_sip, which is single threaded.
The first bug discovered when the test was investigated was an issue
in step (2). We never actually initiated a direct media re-INVITE.
This was due to res_pjsip_t38 using a frame hook, and not implementing
the .consume_cb callback. That callback allows a framehook to inform
the core (and also the bridging framework) of the types of frames that
a framehook wants to consume. If a framehook needs audio, a direct
media bridge will be explicitly denied, and - by default - the
bridging framework assumes that framehooks will want all frames.
Another bug that was discovered occurred in step (6). When UAC1 sends
a BYE request, nothing informed UAC2 that the fax had ended - instead,
it was merely ejected from the bridge. This meant that it kept its
T.38 session going, and Asterisk never sent a re-INVITE to UAC2. Both
of these bugs were fixed by 726ee873a6.
Except, unfortunately, the second bug wasn't really fixed.
726ee873a6 did the "right" thing by intercepting the BYE request sent
by UAC1, and queueing up a control frame of type
AST_CONTROL_T38_PARAMETERS with a new state of AST_T38_TERMINATED.
This is supposed to be passed on to UAC2, informing it that the T.38
fax has ended, and that it should have its media re-negotiated back to
the last known state (audio) but also back to Asterisk (since we
aren't going to be in a bridge any longer). Unfortunately, this code
was insufficient.
A race condition exists in this case. On the one hand, we've just
queued up a frame on UAC1's channel to be passed into the bridge,
which should get tossed onto UAC2's channel. On the other hand, we've
just told the bridging framework to kill UAC1's channel with extreme
prejudice, thereby also terminating the bridge and ejecting UAC2 off
into the dialplan. In the first case, this is an asynchronous, message
passing mechanism; in the second case, the bridging framework inspects
the channel to see if it should be hung up on *every frame* and
*immediately* starts the hangup/shutdown procedure if it knows the
channel should die. This is not asynchronous in any way. As a result,
UAC1 may be hung up and the bridge dissolved before UAC2 ever gets its
control frame from UAC1.
There were a couple of solutions to this problem that were tried:
(1) First, I tried to make sure that enqueued control frames were
flushed out of a channel and passed over the bridge when a hangup was
detected. In practice, this was incredibly cumbersome - some control
frames should get tossed, others need to be preserved. What was worse
was the sheer number of places the bridge dissolution can be
triggered. While it wasn't hard to make sure we flushed frames off an
ejected channel into a bridge, it was nigh impossible to ensure that
this occurred every single time before the other channels were
ejected. Again, the bridging framework is ridiculously - perhaps
ludicrously - aggressive in tossing channels out of a bridge once it
has decided the bridge should be dissolved.
(2) Second, I tried to make the bridge ejection process asynchronous.
This was done by enqueuing another control frame onto the channel
being ejected; when it leaves, it flushes its control frames into the
bridge. When the 'ejection' control frame gets passed into the
bridging core, that causes the bridge to dissolve. This worked well in
some scenarios, and it also guaranteed that the T.38 control frame
would be delivered. Unfortunately, in other cases, it caused all of
the channels to hang out in the bridge ... permanently. Again, there's
a lot of edge cases in the bridging code that deal with channels being
kicked out of a bridge, and the bridge dissolving... and it was more
than I could chew on.
The long and short of it is: while Asterisk 12+ has a nice bridging
framework that hides or eliminates a lot of the horrendous
masquerade/transfer code, as well as the 'triple infinite loop' in
features/channel that existed in Asterisk 11-, it is still
ridiculously complex and prone to breaking spectacularly in subtle
ways. Not to mention both (1) and (2) end up being massive changes to
the design that are risky in an LTS (no one likes it when a channel
can't be hung up.)
So those ideas were scratched.
The next solution was to try a bridge mixing technology that
specifically managed the T.38 state. This worked ... really well.
Incredibly well, in fact. It avoided all of the previous problems
because, unlike external modules or even certain places in the
bridging core, a bridge technology is guaranteed by the core to be
called in a synchronized fashion when any of the following occurs:
(1) When a bridge technology is chosen
(2) When that technology is started
(3) When that bridge has a channel added
(4) When that bridge has a channel removed
(5) When that technology is stopped
All of which covers the necessary places to know when a channel has
hung up, and gives us a place where we can safely inform the other
channels before the bridging framework starts doing mean things.
bridge_t38 was the result [3]. It managed a bit of T.38 state for the
two channels in a core bridge that were in a T.38 fax, and, when one
of them leaves, it informed the other channel that it should end its
T.38 fax.
Problem solved.
\o/
Not quite.
After merging [3] in f42d22d3a1, we noticed that the masquerade test
[4] started to fail. That's a really, really bad sign. The masquerade
'super test' was originally tested to stress test masquerades in
Asterisk 1.8 and 11. It constructs a chain of 300 Local channels, then
optimizes them all down to a single pair of 'real' channels. In
Asterisk 12+, masquerades were eliminated in this scenario, but we
instead have a series of incredibly complex Local channel
optimization-caused bridge/swaps/merges that kick off as the Local
channels collapse and merge their bridges down to one. It's a great
"canary in the coal mine" test, as when it fails, it almost certainly
means you've introduced a dead lock into one of the more complex
operations in Asterisk - regardless of the versions.
And lo and behold, we had.
Local channels are weird. One of the 'fun things' they do is 'help'
T.38 along by passing along a channel query option for T.38 state.
This lets us do ridiculous things like make sure a T.38 fax works
across a Local channel chain (and is covered by the
fax/sip/local_channel_t38_queryoption test). Unfortunately, the
bridge_t38 module had to query for T.38 state in its compatible
callback - this allowed it to determine the current state of T.38 on
the channels in the bridge to see if it needed to be activated.
Unfortunately, in a 300 Local channel chain, that means reaching
across 300 bridges - simultaneously - locking bridges,
bridge_channels, channels, PVTs, and the entire world in the process.
Since the bridge lock was already held in the compatible callback,
this caused a locking inversion (no surprise there), deadlocking the
whole thing.
This is not a trivial locking situation to resolve. Even if we unlock
the bridge, we're still liable to deadlock merely by trying to lock
300 bridges simultaneously. (There may even be another bug in here,
but it is hardly worth trying to find or fix at this point.) And we
can't remove the query option code in chan_local, as T.38 faxes will
no longer work across Local channels.
As an aside, if there's a lesson in all this, it is that synchronous
code in a heavily multi-threaded environment is bad. Message passing
may be harder to write, but it is far easier to maintain.
Anyway, as a result, I've reverted the bridge_t38 module in 75c800eb28.
So what do we do now?
The crux of this problem is that the bridging framework does not have
a standard way of informing a channel when it has joined or - more
importantly - left a bridge. Direct media has its own mechanism
managed by the RTP engine - so it works around this. However, we have
a number of scenarios where "things happen" in a bridge that involves
state on a channel and - right now - we don't have a unified way of
handling it. In addition to T.38, we also have channels being put on
hold, DTMF traversing a channel, and more. Often, the channel driver
has this state - but instead, we have a lot of 'clean up' logic being
added to the bridging core to handle these situations.
As I see it, we really only have two options here:
(1) Add code to the bridging framework to clean up T.38 on a channel
when it leaves. This is kind of annoying, as it will happen on every
channel when it leaves, regardless of whether or not the channel even
supports T.38.
(2) Add a new channel technology callback that a bridge can use to
inform a channel driver that it is being ejected from a bridge. This
would give us a single place to put cleanup logic that has to happen
in a channel driver when it is no longer bridged.
I'm not sure those two options will work, exactly, but it's the best
options that I can think of after exhausting lots of other code
changes in the bridging core. If someone has other suggestions, I'd be
more than happy to entertain them.
Matt
[1] https://jenkins.asterisk.org/
[2]
https://jenkins.asterisk.org/jenkins/job/periodic-asterisk-master/75/testReport/junit/%28root%29/AsteriskTestSuite/tests_fax_pjsip_directmedia_reinvite_t38/
[3] https://gerrit.asterisk.org/#/c/1761/
[4]
https://jenkins.asterisk.org/jenkins/job/periodic-asterisk-master/80/testReport/junit/%28root%29/AsteriskTestSuite/tests_masquerade/
--
Matthew Jordan
Digium, Inc. | Director of Technology
445 Jan Davis Drive NW - Huntsville, AL 35806 - USA
Check us out at: http://digium.com & http://asterisk.org
Hi Matt,
Of the two ideas you propose, the channel technology callback sounds
like the better option. That way, only relevant channel drivers will
need to bother implementing the callback, limiting the scope of the work.
Something else to consider is that the T.38 bridge technology worked
really well except for when chains of local channels were involved. The
main issue was the method by which the local channels proxied
information. Would there be some non-earth-shattering change that could
be made to make it so that local channels store the proxied information
locally when the proxied information is first set? That way, when the
bridge technology queries the local channel, there is no complex
reaching across bridges to get the information; the local channel
already has that knowledge stored on it. I'm not 100% sure of the
mechanism for getting this state stored on the local channel, but I was
curious more if the idea had crossed your mind to explore that avenue.
--
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