SCAMP: SDP messages are dropped silently if no resources available

[lplana]

When an SDP message arrives, scamp needs an 'sdp_msg_t' mailbox to hold it and an 'event_t' event holder to schedule the message for processing. There is a limited number of mailboxes and event holders, therefore a new SDP message may not find one or the other available.

Currently, scamp simply drops the SDP message silently:

• unavailable mailboxes are dealt with in 'scamp-3.c' lines [322-328].
• unavailable event holders are dealt with in 'scamp-3.c' lines [236-239].

It would be better to return an error code, such as 'RC_BUF'. The problem, obviously, is that there is no mailbox or event holder to use for the return message.

[alan-stokes]

how about a separate counter for this specific issue? and we could read it back as provenance at the end of a sim?