Support for put-logging

[anjohnson]

I doubt if QSRV makes the necessary calls to support put-logging yet (see the various asTrapWrite* routines in asLib.h for the server API and Benjamin's caPutLog website for the main listener module) but APS would like to be able to use a version of this to log puts that come into an IOC through pvAccess. Not urgent and we'd be willing to help, I'm just filing this as I remember it.

The existing server API was designed for CA and uses the db_access.h version of dbrType though, so may need modifying somewhat.

Tim Mooney's CA Put Recorder synapps module also uses the listener API.

[mdavidsaver]

Usage of asLib in general is missing. Although this isn't strictly a blocker, I haven't done this yet because there is no way for the PDBProvider to see username (or any credentials).

[mdavidsaver]

As I think about it, supporting read access permission change will be quite complicated in the case of group monitor. So I'm considering only implementing the write access checks and ignoring attempts to make unreadable PVs. I've only ever cared about limiting/logging writes. Thoughts?

[anjohnson]

The ability to do put-logging and write protection should cover our use cases. I doubt if many client developers have considered the case of unreadable PVs anyway.

Thanks. As I said before, it's still a long time from when we'll actually need this.

[mdavidsaver]

Also relevant is that PVA doesn't (I think) have a mechanism to notify of permissions changes.

[anjohnson]

PVA doesn't (I think) have a mechanism to notify of permissions changes

That seems like it might be more of a problem that we should discuss in the group. The caProvider has one call to each of ca_write_access(channelID) and ca_read_access(channelID) in its getAccessRights() method but nothing dynamic, so I think you're right.