[Bug 1824277] Re: Regression in calcout setting constant links at runtime

[Ben Franksen via Core-talk <core-talk@aps.anl.gov>]

My immediate reaction was the same as Ralph's: if we agree that this is
desired behavior then it should apply to all input links in a generic
manner, independent of record support and not requiring special
processing. I am not familiar with the internal workings of how writing
to a link field is implemented, but given that redirection to another PV
works, I guess it shouldn't be too hard to make it work for numeric
constants?

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https://bugs.launchpad.net/bugs/1824277

Title:
  Regression in calcout setting constant links at runtime

Status in EPICS Base:
  In Progress
Status in EPICS Base 7.0 series:
  In Progress

Bug description:
  With the 3.15 branch, writing a literal number to a calcout.INP[A-L]
  field causes the corresponding [A-L] field to be set to that number.
  This works because the calcout record marks the INP fields as
  special(SPC_MOD) and the calcout::special() routine calls
  recGblInitConstantLink() to initialize the corresponding [A-L] field
  from the new link string. I suspect that a number of the synApps
  record types (transform, scalc, acalc etc.) probably use this same
  technique.

  Keenan Lang reported that this doesn't currently work on the 7.0
  branch; the value in the [A-L] field doesn't change, which I have
  confirmed.

  This use of special() to reinitialize constant links is unusual,
  normally putting a literal into a link field after record
  initialization has no effect, but someone who worked on the calcout
  record discovered this way to get it to work.

  I think I know what's going on and how to fix it (change
  dbPutFieldLink() to call dbAddLink() before the second call to
  dbPutSpecial()), but I need to check that this won't break other
  cases.

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