[Bug 1982847] Re: cryptic error message "parse error: premature EOF"

[Dirk Zimoch via Core-talk <core-talk at aps.anl.gov>]

I have added some context to the messages:

dbConvertJSON: String provided, numeric value(s) expected
...while parsing link x.INP ["x"]

dbConvertJSON: Embedded arrays not supported
...while parsing link x.INP [[]]

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https://bugs.launchpad.net/bugs/1982847

Title:
  cryptic error message "parse error: premature EOF"

Status in EPICS Base:
  New

Bug description:
  In EPICS 7, if an array input link (e.g. waveform.INP) is an explicit
  empty string, the parser prints a cryptic message without any helpful
  context.

  Example:
  record(waveform, "x")
  {
      field(INP, "")
  }

  
  dbConvertJSON: parse error: premature EOF
                                         
                       (right here) ------^

  Not very enlightening.

  It does not happen if INP is not set at all and it does not happen at least up to R3.16.1 but at least since 7.0.4.1 (I have not tested anything in between) and up to commit d82ab81.
  It also does not happen with scalar input links (e.g. ai.INP).

  First of all, any parser error should print a context (I prefer record.field) similar to this:
  dbConvertJSON: parse error: premature EOF in x.INP

  Unfortunately, dbPutConvertJSON (the function printing the
  dbConvertJSON message) does not have the context. All it has is the
  string. So maybe it should not print the message at all but leave it
  to the caller. The only callers are in dbConstLink.c. The interface
  should be modified to pass either the context (plink instead of
  plink->value.constantStr) or the error message (which is complicated)
  or an additional message should be printed by the outer context
  (difficult to find out when dbPutConvertJSON actually printed an error
  message).

  Second, passing "" should not result in an error at all.

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