channel filters should ignore unknown filter tags

Makes a lot of sense.

Would you suggest printing a warning message or just silently ignore?
(I think giving absolutely no hint would make it unnecessary hard to trace errors due to filters missing on specific IOCs. Filling up logs with useless expected warnings would also be bad, on the other hand.)

I would use a shell variable (e.g. var caFilterDebug) to switch error messages on/off.

Also we could extend the syntax to distinguish "important" from "unimportant" filters. E.g. prefixing important filters with + and/or prefixing unimportant filters with -:

caget 'channel.{"+important_filter":"value","-dont_care_much":42,"other_filter":{}}'

In that case + filters can cause errors that prevent connection, - filters can be silently ignored and filters without prefix can print a warning.

"filter", "-filter" and "+filter" would be the same filter, only error handling would be different.

Core Group review at ESS:

We don't like the case of the leading '+', but we would allow a leading '-' to mean "ignore my request for this filter if the filter is not found, and don't print an error on the IOC".

Implementation would probably require adding a dummy filter to dbChannel.c which accepts any filter arguments, but the result should not be added to the list of filters for the channel.