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This is an automatically generated e-mail. To reply, visit:
https://reviewboard.asterisk.org/r/3930/
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(Updated Aug. 24, 2014, 11:37 p.m.)
Review request for Asterisk Developers.
Changes
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Added doxygen for the two callbacks in res_pjsip_session.c
Bugs: ASTERISK-24212
https://issues.asterisk.org/jira/browse/ASTERISK-24212
Repository: Asterisk
Description
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In /r/3927, I introduced a changeset that fixes a scheduler race condition. I
noted on that issue that even though I was no longer seeing the same crashes
when running tests, I was still seeing occasional test failures due to some
media-related race condition.
I have determined the cause of this race condition. PJSIP's inv_session API
provides two callbacks, on_state_changed() and on_tsx_state_changed(). Both of
these are called into when the state of a transaction changes, and the
documentation is not particularly clear about what the difference is between
these. Through some investigation, I've found that what happens is:
1) Transaction layer detects a state change (such as a request/response being
sent/received, or a timeout)
2) Transaction layer informs the inv_session layer.
3) inv_session layer sets the new state and calls the on_state_changed()
callback.
4) inv_session layer performs its processing of the transaction state change
(including potential calls to the on_media_update() callback).
5) inv_session layer calls the on_tsx_state_changed() callback.
res_pjsip_session is hooked into step (3) such that when a new SIP request or
response is sent or received, we call into session supplements to let them
react to the new message. One of these session supplements is in chan_pjsip and
queues an AST_CONTROL_ANSWER frame when a 200 OK response is received.
In the test where I am seeing the race condition occur, a call is originated to
a PJSIP channel (Alice). Once Alice answers, her channel is placed into the
dialplan to execute the TalkDetect() application. The problem here is that at
step (3), the AST_CONTROL_ANSWER frame is queued onto Alice's channel, and the
PBX thread moves the channel into the TalkDetect() application before step (4)
is executed. Step (4) is important for setting formats on Alice's channel and
setting appropriate translation paths as well. Since step (4) has not been
completed yet, the attempt to play a file as part of TalkDetect() fails, and
therefore the test fails. In most runs, however, step (4) does get completed
before the PBX thread moves Alice's channel into TalkDetect().
The fix presented here is pretty simple. For transaction state changes, we are
now hooked into step (5) instead of step (3) to call session supplements. This
means that we do not call into session supplements until media has been
negotiated on the channel. This eliminates the race condition entirely.
Diffs (updated)
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/branches/12/res/res_pjsip_session.c 421883
Diff: https://reviewboard.asterisk.org/r/3930/diff/
Testing
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I ran the
tests/channels/pjsip/basic_calls/two_parties/nominal/alice_initiated/alice_hangs_up
test in a loop on my console both before and after applying this patch. Before
applying the patch, I would usually encounter a test failure within an hour or
two. After applying this patch, I left the test running in a loop for over 24
hours and never had a test failure.
Thanks,
Mark Michelson
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