CTRL variables are reported incorrectly

[ralphlange]

Originally filed as https://bugs.launchpad.net/epics-base/+bug/1510955

This problem was originally reported here - pyepics/pyepics#40
There are some test results at the above link as well.

DBR_CTRL variables (units, etc) are reported correctly only on the initial connection for a channel subscription when the channel is hosted by either a Gateway or via pcaspy. This hints at a problem in libcas or libgdd.

Test code may be found here - https://github.com/dchabot/ca_client_test

This is simply a version of the caClient example code, generated via makeBaseApp, and modified to print the units of a channel.

To test, point the caMonitor executable at a channel hosted by a Gateway or hosted by pcaspy.

[ralphlange]

I have come to the conclusion that the problem should not be fixed in CAS or GDD, but in the server applications.

CAS itself does no caching of data that is posted by the server app. It keeps a prototype GDD of the structure that the client requested, and whatever GDD the server app posts is smart-copied into a clone of the prototype and pushed to the client.
read() operations give the prototype clone (e.g. a ctrl container) to the server app to fill in. That's why the first update on setting up the connection is correct: in that case CAS does a read(), and the server app fills the complete structure. If the server app then posts e.g. value/time/status GDDs, the smart-copy to the prototype clone leaves all other elements empty - the DBR_CTRL structure only gets the posted elements, the remaining are zero or empty.
The server app has no hint if a CAS issued read() is originating from a client-side read or a subscription request.

Adding a merging cache inside CAS would add a lot of complexity and threaten the stability of CAS.
It might be easier to have the server app do whatever is needed, and have it always post complete containers (i.e., a superset of DBR types), so that CAS's smart-copy in any prototype clone GDD will always fill the requested structure completely.