[Bug 1815908] Re: casExpandRecvBuffer modifies the contents of the buffer

[Andrew Johnson via Core-talk <core-talk@aps.anl.gov>]

** Changed in: epics-base
       Status: Fix Committed => Fix Released

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https://bugs.launchpad.net/bugs/1815908

Title:
  casExpandRecvBuffer modifies the contents of the buffer

Status in EPICS Base:
  Fix Released

Bug description:
  For the following data base run in a SoftIOC (3.16.2):

  record(waveform, "PXD:H9999:D9:memoffsetdata_array:A:set"){
      field(FTVL, "UCHAR")
      field(NELM, "262144")
  }

  record(waveform, "PXD:H9999:D9:memswdata_array:A:set"){
      field(FTVL, "UCHAR")
      field(NELM, "262144")
  }

  the sequence of PV accesses created by this minimal sequence:

  // vi: ft=cpp
  program seqtest("")

  option +c; /* dont wait for db connections */
  option +r; /* make functions reentrant, multiple instances */
  option +d; /* toggle runtime debugging messages */
  option +W; /* extra warnings. */

  %%int i;

  unsigned char memswdata_array_set[16384];
  assign memswdata_array_set to "PXD:H9999:D9:memswdata_array:A:set";

  unsigned char memoffsetdata_array_set_small[32768];
  assign memoffsetdata_array_set_small to "PXD:H9999:D9:memoffsetdata_array:A:set";

  ss dhhseq_small {
    state init {
      entry {
        for( i = 0; i < sizeof memswdata_array_set; ++i ) memswdata_array_set[i] = i % 256;
        for( i = 0; i < sizeof memoffsetdata_array_set_small; ++i ) memoffsetdata_array_set_small[i] = 255 - i % 256;
        pvPut(memswdata_array_set, SYNC);
        pvPut(memoffsetdata_array_set_small, SYNC);
      }
      when() {} state done
    }

    state done {
          when () {} state done
    }
  }

  gives this error message on the softIOC side:

  CAS: request from 192.168.0.1:44512 => CAS: Missaligned protocol rejected
  CAS: Request from 192.168.0.1:44512 => cmmd=30720 cid=0x7800002c type=17023 count=0 postsize=44
  CAS: Request from 192.168.0.1:44512 =>   available=0x427f0000   N=0 paddr=(nil)
  CAS: forcing disconnect from 192.168.0.1:44512

  and a segfault on the sequence side.

  This report is (for now) about the softIOC.

  I can reliably produce this behavior on two different systems (SL7 and
  Debian) when both the softIOC and the sequence run on the same host.
  Communication is via the lo interface, so the data packets are large.

  During the second pvPut, the receive buffer is to be expanded a second
  time, and during this call, the contents change:

  before casExpandRecvBuffer
    start of buffer=00 13 ffffff80
  after casExpandRecvBuffer
    start of buffer=78 00 00

  I will try to follow this even further, but for now I'm creating this
  report to have a place to track my findings.

  Might there be a relation to https://bugs.launchpad.net/epics-
  base/+bug/1706703 ?

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