Improve SNS live listener behaviour when asking for start of run when we are not in a run

[mantid-builder]

Original Reporter: Russell Taylor

This ticket is blocked by :

• TRAC7843

In http://trac.mantidproject.org/mantid/ticket/6661, we decided that if you ask for data from the start of the run when the instrument is not in a run, then the listener would just throw an exception, causing StartLiveData to fail. This is actually pretty unpleasant, especially when using the listener behind some interface other than the StartLiveData one. What would be better is if the listener just waited until a run did start, and then start passing back the events. The reason we didn't do this before is that we didn't want things to just lock up, potentially indefinitely, for the start of a run that never came - since the listener isn't able to check for a cancellation signal.

It has occurred to me that we could do this by creating a specific exception type and having the listener throw that in this situation (instead of throwing a general exception type, which just stops the algorithm). LoadLiveData could catch this exception, log it, check for cancellation and then call extractData() once again a few seconds later.

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Keywords: LiveData

[mantid-builder]

This issue was originally trac ticket 7785