[task #16072] Fits should update CHECKSUM when keywords are changed

[Mohammad Akhlaghi]

URL:
  <https://savannah.gnu.org/task/?16072>

                 Summary: Fits should update CHECKSUM when keywords are
changed
                 Project: GNU Astronomy Utilities
            Submitted by: makhlaghi
            Submitted on: Thu 11 Nov 2021 10:46:37 PM UTC
         Should Start On: Thu 11 Nov 2021 12:00:00 AM UTC
   Should be Finished on: Thu 11 Nov 2021 12:00:00 AM UTC
                Category: Fits
                Priority: 5 - Normal
              Item Group: Enhancement
                  Status: Postponed
                 Privacy: Public
        Percent Complete: 0%
             Assigned to: makhlaghi
             Open/Closed: Open
         Discussion Lock: Any
                  Effort: 0.00

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Details:

Currently, when Gnuastro's FITS program ('astfits') changes the keywords
within a HDU, it does just that (changing the keywords!). However, when the
HDU has a CHECKSUM keyword, a change in the keywords will invalidate it! 

Currently the only way to avoid the invalide checksum, is that user also calls
'--write=checksum' along with their other modifications (as described under
'--write' in the manual
<https://www.gnu.org/software/gnuastro/manual/html_node/Keyword-inspection-and-manipulation.html>).
For example the command below to delete the 'GSL' keyword (which is available
in any FITS file created by Gnuastro, keeping the version of the GNU
Scientific Library).


astfits a.fits --delete=GSL --write=checksum


However, this is only useful for those keyword writing operators that have
less precedence than '--write' (precedence is also shown as a numbered list in
the manual
<https://www.gnu.org/software/gnuastro/manual/html_node/Keyword-inspection-and-manipulation.html>).

So it is necessary for Gnuastro's Fits program to check if the file has a
'CHECKSUM' keyword or not. If 'CHECKSUM' is present, then upon any change in
the keywords (Fits doesn't change the data!), the 'CHECKSUM' keyword should
also be updated in the end.




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