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From Tobias Duckworth <tob...@duckworth.uk.com>
Subject Re: pn_delivery_finalize Assertion failure
Date Thu, 09 Mar 2017 09:06:35 GMT
I got to the bottom of this problem.

The problem is to to with the lifetime of the proton::delivery object.

In a connection_engine build, the IO is asynchronous to the proton::handler
interface, and therefore this problem may only occur in connection_engine
builds (not absolutely certain, as I've not tested on a 'normal' build).

When a message is received, it comes along with a delivery object. The
delivery object can be used to accept, reject, release the delivery with the
broker.

Internally to the proton-c C implementation there are a complicated set of
state transitions that happen which I have not totally got my head around.
However, what I do know is that the delivery object needs to stick around
until the operation is totally finished - Otherwise it's possible for the
observed behaviour to happen, which causes the assertion failures or a much
nastier crash.

The call to proton::delivery::accept (or release, reject) returns almost
straightaway, and then the actual work of settling the message with the
broker happens when the underlying IO operation is invoked with process(),
or process_nothrow(). If, by this time, the delivery object that was used
has been deleted or destroyed, then the problem will happen

Turning on tracing using PN_TRACE_FRM, one can see the following when the
engine has finished with the delivery:

[0x2b4af8030500]:1 -> @disposition(21) [role=true, first=124, last=124,
settled=true, state=@accepted(36) []] .

Unfortunately, there is no interface back to the C++ world to tell us when
the delivery is finished with, and so there appears to be no way to know
when it is safe to destroy the delivery object.

Other than keeping proton::delivery objects sticking around in memory
forever (which does solve the problem, but is not an option), does anyone
have any ideas for solving this?

It seems to me like the C++ proton::delivery object itself should know about
whether it has been settled with the broker or not, so unless anyone has a
better suggestion this is likely to be what I will add.

Thanks,
Toby








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