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RE: [Question] Signature calculation ignoring parts of binary files




On September 12, 2018 4:54 PM, I wrote:
> On September 12, 2018 4:48 PM, Johannes Sixt wrote:
> > Am 12.09.18 um 21:16 schrieb Randall S. Becker:
> > > I feel really bad asking this, and I should know the answer, and yet.
> > >
> > > I have a binary file that needs to go into a repo intact (unchanged).
> > > I also have a program that interprets the contents, like a textconv,
> > > that can output the relevant portions of the file in whatever format
> > > I like - used for diff typically, dumps in 1K chunks by file section.
> > > What I'm looking for is to have the SHA1 signature calculated with
> > > just the relevant portions of the file so that two actually
> > > different files will be considered the same by git during a commit
> > > or status. In real terms, I'm trying to ignore the Creator metadata
> > > of a JPG because it is mutable and irrelevant to my repo contents.
> > >
> > > I'm sorry to ask, but I thought this was in .gitattributes but I
> > > can't confirm the SHA1 behaviour.
> >
> > You are looking for a clean filter. See the 'filter' attribute in gitattributes(5).
> > Your clean filter program or script should strip the unwanted metadata
> > or set it to a constant known-good value.
> >
> > (You shouldn't need a smudge filter.)
> >
> > -- Hannes
> 
> Thanks Hannes. I thought about the clean filter, but I don't actually want to
> modify the file when going into git, just for SHA calculation. I need to be able
> to keep some origin metadata that might change with subsequent copies, so
> just cleaning the origin is not going to work - actually knowing the original
> author is important to our process. My objective is to keep the original file
> 100% exact as supplied and then ignore any changes to the metadata that I
> don't care about (like Creator) if the remainder of the file is the same.

I had a thought that might be workable, opinions are welcome on this.

The commit of my rather weird project is done by a script so I have flexibility in my approach. What I could do is set up a diff textconv configuration so that the text diff of the two JPG files will show no differences if the immutable fields and the image are the same. I can then trigger a git add and git commit for only those files where git diff reports no differences. That way the actual original file is stored in git with 100% fidelity (no cleaning). It's not as elegant as I'd like, but it does solve what I'm trying to do. Does this sound reasonable and/or is there a better way?

Cheers,
Randall

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