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

From: Mohammad Akhlaghi
Subject: [task #16072] Fits should update CHECKSUM when keywords are changed
Date: Thu, 11 Nov 2021 17:46:39 -0500 (EST)
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:94.0) Gecko/20100101 Firefox/94.0


                 Summary: Fits should update CHECKSUM when keywords are
                 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



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
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

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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