DCOP: Deadlock protection and reentrancy

[Waldo Bastian]

Hi,

I've added the following documentation blurb on DCOP's deadlock prevention.
Please let me know if you notice any inaccuracies.

Cheers,
Waldo

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Deadlock protection and reentrancy

When a DCOP call is made, the dcop client will be monitoring the
dcop connection for the reply on the call. When an incoming call is
received in this period, it will normally not be processed but queued
until the outgoing call has been fully handled.

However, the above scenario would cause deadlock if the incoming call
was directly or indirectly a result of the outgoing call and the reply
on the outgoing call is waiting for the result of the incoming call.
(E.g. a circular call such as client A calling client B, with client B
calling client A)

To prevent deadlock in this case, DCOP has a call tracing mechanism that
detects circular calls. When it detects an incoming circular call that
would otherwise be queued and as a result cause deadlock, it will handle
the incoming call immediately instead of queueing it. This means that the
incoming call may be processed at a point in the code where an outgoing
DCOP call is made. An application should be aware of this kind of
reentrancy. A special case of this is when a DCOP client makes a call
to itself, such calls are always handled directly.

Call tracing works by appending a key to each outgoing call. When a client
receives an incoming call while waiting for a response on an outgoing call,
it will check if the key of the incoming call is equal to the key used for
the last outgoing call. If the keys are equal a circular call has been 
detected.

The key used by clients is 0 if they have not yet received any key. In this
case the server will send them back a unique key that they should use in
further calls. If a client makes an outgoing call in response to an incoming
call it will use the key of the incoming call for the outgoing call instead
of the key that was received from the server.

A key value of 1 has a special meaning and is used for non-call messages
such as DCOPSend, DCOPReplyFailed and DCOP signals.

A key value of 2 has a special meaning and is used for priority calls.
When a dcop client is in priority call mode, it will only handle incoming
calls that have a key value of 2.

NOTE: If client A and client B would call each other simultaneously there
is still a risk of deadlock because both calls would have unique keys and
both clients would decide to queue the incoming call until they receive
a response on their outgoing call.
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