PVA access permissions info #3506
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added 15 commits
August 15, 2025 09:49
`ChannelSearch.runSearches` used to re-use one `to_search` array list. By the time the submitted searches encode the channels from that list, it might get cleared and re-populated. Need to create a new one in each `runSearches` call. At same time removed some unnecessary casts.
Lower default EPICS_PVA_MAX_ARRAY_FORMATTING
removeClient already includes unregisterSubscription
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Compared to Channel Access, PV Access so far lacked access permissions info. A client like an operator display cannot show that a PV is read-only. Users would only notice that a PV is actually read-only when writing and then receiving an error.
A protocol update has been proposed and discussed with PVXS developers, epics-docs/epics-docs#140 It has not been implemented in PVXS, yet, but this implementation in java provides a preview of what it would look like. It is fully compatible with PVXS, the new PVA protocol message is simply ignored.
It allows https://github.com/controlSystemStudio/pvaify to mirror the write permissions from the CA client side to the PVA server side and then see the result in CS-Studio displays.
./serverdemocreates PVs "demo" (read-only) and "demo3" (writable).A display with
pva://demoandpva://demo3will reflect write access or lack thereof:With the corresponding pva_acl branch of pvaify, a demo database and access config can dynamically change write permissions and the display will then update its widgets to allow or block write access.