dd: implement iseek + oseek flags

[chordtoll]

These are the first half of changes needed to pass the dd/bytes.sh tests:

• Add iseek and oseek options (additive with skip and seek options)
• Implement tests for the new flags, matching those from dd/bytes.sh

[chordtoll]

--iseek and --oseek are fairly new dd features, added in coreutils commit 4439ef3ec4 this February.

[jfinkels]

The code itself is fine, I'm requesting some changes to the tests and eliminating some duplicate code.

[chordtoll]

I'm a bit confused by the failing GnuTests CI run- tests/misc/seq-long-double test passes both in a local build, and in GitHub CI in my fork. Is it possible the failure is an intermittent issue?

[sylvestre]

yeah, likely not your fault :)

[jfinkels]

I'm a bit confused by the failing GnuTests CI run- tests/misc/seq-long-double test passes both in a local build, and in GitHub CI in my fork. Is it possible the failure is an intermittent issue?

Yes, it is an intermittent issue #2924

[tertsdiepraam]

Hi! I'm doing a refactor of the argument handling of dd and ran into something I didn't quite understand. In this PR, you added tests to check that iseek and oseek are added on skip and seek, but I cannot reproduce that behaviour. Could you explain in what case they should be added?

This is with dd from GNU coreutils 9.1 and the current main branch:

❯ echo "0123456789abcdefghijklm" | dd iseek=4 skip=4 iflag=skip_bytes bs=2
456789abcdefghijklm
10+0 records in
10+0 records out
20 bytes copied, 0,000209508 s, 95,5 kB/s
❯ echo "0123456789abcdefghijklm" | cargo run --quiet -- dd iseek=4 skip=4 iflag=skip_bytes bs=2
89abcdefghijklm
8+0 records in
8+0 records out
16 bytes copied, 0.0 s, 16.0 KB/s

[chordtoll]

I likewise haven't been able to reproduce the additive behavior- I probably misinterpreted the results of my tests. I've checked out the pull request, and the behavior looks to be correct now.