Read fresh metadata for unmodified files

[fd0]

What is the purpose of this change? What does it change?

Restic took all metadata for files which were detected as unmodified, not
taking into account changed metadata (ownership, mode). This PR corrects it,
only the list of blobs (=the content) is taken from the previous file, the
metadata is read anew.

Was the change discussed in an issue or in the forum before?

Closes #2249

Checklist

• I have read the Contribution Guidelines
• I have added tests for all changes in this PR
• I have added documentation for the changes (in the manual)
• There's a new file in changelog/unreleased/ that describes the changes for our users (template here)
• I have run gofmt on the code in all commits
• All commit messages are formatted in the same style as the other commits in the repo
• I'm done, this Pull Request is ready for review

[codecov-io]

Codecov Report

Merging #2252 into master will increase coverage by 0.11%.
The diff coverage is 60%.

@@            Coverage Diff             @@
##           master    #2252      +/-   ##
==========================================
+ Coverage    50.8%   50.91%   +0.11%     
==========================================
  Files         178      178              
  Lines       14530    14533       +3     
==========================================
+ Hits         7382     7400      +18     
+ Misses       6080     6059      -21     
- Partials     1068     1074       +6

Impacted Files	Coverage Δ
internal/archiver/archiver.go	68.35% <60%> (-0.27%)	⬇️
internal/checker/checker.go	70.25% <0%> (+3.66%)	⬆️

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[cbane]

The test for this fails if the system has SELinux enabled:

--- FAIL: TestMetadataChanged (0.01s)
    testing.go:30: using low-security KDF parameters for test                                                          
    config.go:65: disabling check of the chunker polynomial
    archiver_nowin_test.go:27: chdir to /tmp/restic-test-978949359                                                     
    archiver_nowin_test.go:53: metadata does not match:
        {*restic.Node}.ExtendedAttributes:
                -: []restic.ExtendedAttribute(nil)
                +: []restic.ExtendedAttribute{{Name: "security.selinux", Value: []uint8{0x75, 0x6e, 0x63, 0x6f, 0x6e, 0x66, 0x69, 0x6e, 0x65, 0x64, 0x5f, 0x75, 0x3a, 0x6f, 0x62, 0x6a, 0x65, 0x63, 0x74, 0x5f, 0x72, 0x3a, 0x75, 0x73, 0x65, 0x72, 0x5f, 0x74, 0x6d, 0x70, 0x5f, 0x74, 0x3a, 0x73, 0x30, 0x00}}}
    asm_amd64.s:522: chdir back to /home/courtney/src/restic/internal/archiver
FAIL
FAIL    github.com/restic/restic/internal/archiver      4.323s

Having a default ACL on /tmp will also trigger the problem, with a system.posix_acl_access extended attribute. (You can use e.g. setfacl -d -m user:root:rwx /tmp to set one; and then setfacl -b /tmp to remove all ACLs.)

My idea for fixing this is to copy the security.selinux and system.posix_acl_access extended attributes from node2/node3 (if they exist) to want before checking if they match, but I haven't thought about this deeply.

[fd0]

Oh, thanks for the hint. I'll fix the Windows tests first, then we'll see about these.

[fd0]

FYI: I've fixed this (more or less accidentally) in #2266 by setting the field ExtendedAttributes to nil.