Refactor to be thread-safe

[Tom-Willemsen]

ISISComputingGroup/IBEX#7470

The error in the ticket looks to be caused by non thread-safe access.

Thread 1	Thread 2
var count = getChannelCount()	-
for(int i=0; i<count; ++i) {	-
-	channels = <new smaller list>
getChannel(i) (boom)	-

I've refactored it to use thread-safe iterators over CopyOnWriteArrayList in one case, and iterators over an explicit copy of an ArrayList in another similar case. Both of these approaches should be thread-safe against the scenario described above, which looks to be the only possible cause of the stack trace seen in the ticket.

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Unfortunately I wasn't able to find a way to reliably reproduce the original issue in order to write tests for it. My attempt to reproduce was:

• Set up config 1 with 250 blocks
• Set up config 2 with 150 blocks (to maximise amount of time spent in the for loop above)
• Switch from config 1 to config 2 while making a large number of requests to the archive engine webserver - in order to try to be in the for loop when the channels get changed over.

However this workflow didn't seem to be able to reproduce the exact error in the ticket.

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To test:

• Build this branch of CSS
• Stop ibex server
• Copy the .zips from isis_css_top\cs-studio\product\repository\target\products to EPICS\ICP_Binaries\css
• Run setup_css.bat in EPICS\css\master
• Restart IBEX server
• Verify archivers still work as before (http://localhost:4813/groups and http://localhost:4812/groups)
• Verify by code-review that the new code is thread-safe against the condition described above, and that you agree with my analysis that the stack trace in the ticket can only be caused by this type of threading condition.