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paulscherrerinstitute / pcaspy

Portable Channel Access Server in Python
BSD 3-Clause "New" or "Revised" License
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Variable array with more than 'count' elements #60

Open kasemir opened 5 years ago

kasemir commented 5 years ago

When creating an array PV with 'count' : 6, one can later call setParam with 0, 1, 2, .. 6 elements. Reasonably recent Channel Access clients support such changing array sizes.

Turns out that pcaspy even allows calling setParam with more elements than originally specified by count, and some CA clients handle it, but the CA gateway does not. Our PV Gateway Version 2.1.0 built with EPICS R3.14.12.6 crashed with symptoms of memory corruption each time we accessed such a PV that declared 'count': 1000 but then set the value to an array with 100000 elements. The last log message before each crash mentioned a value update with 100000 elements, but erroneously associated it with a different channel, again as a result of memory corruption.

While this is in part a bug in the gateway, the common agreement for the use of variable size arrays is that the 'count', which is for record-based IOCs configured via the waveform NELM field, sets the maximum array size, and value updates will then contain 0, 1, 2, .. elements up to that maximum element count. This allows clients to reserve a known amount of memory for storing received value updates.

Can pcaspy check the data passed to setParam, and in case of arrays at least warn, maybe even truncate or ignore array values with more than count elements?

anacso17 commented 1 year ago

Related issue: https://github.com/epics-extensions/ca-gateway/issues/49.

xiaoqiangwang commented 1 year ago

I did not react on this issue because it is a good "feature" to provide variable length array PV. It is upon the application developer to respect the array size limit.

kasemir commented 1 year ago

Well, yes, it's these days common to have 'array' classes that can simply change size, internally freeing/re-allocating memory as necessary. For control system devices, however, it's often better to reserve the maximum required array memory once and then keep using that same memory to prevent memory fragmentation or delays from re-allocations. So the array size may change, but must stay within an upper limit. In other words: "It is upon the application developer to configure the array size limit.", and pcaspy should then stay within that limit.