[ThomasWaldmann]

Description

An archive can contain the same hard link item twice. Two ways to get one:

• borg create ::a input input/file3, where input/file3 is a hard link and the first member of its hard link group. The root input/file3 lies inside the earlier root input.
• printf 'input/file1\ninput/file1\n' | borg create --paths-from-stdin ::a with a hard-linked input/file1. This is deterministic.

The duplicate is an h item whose source is its own path:

-rw-r--r-- input/file1 ->
hrw-r--r-- input/file1 -> input/file1
hrw-r--r-- input/file2 -> input/file1

borg extract first extracts input/file1. For the duplicate, the "remove existing files" step then unlinks that file, and hard linking the path to itself fails with ENOENT. Extract returns rc 1, and input/file1 is gone, along with its other hard links (input/file2 fails to link the same way), although the archive data is intact:

input/file1: link: [Errno 2] No such file or directory: '.../input/file1' -> '.../input/file1'
input/file2: link: [Errno 2] No such file or directory: '.../input/file1' -> '.../input/file2'

Fix: extract now skips a hard link item whose link target is its own path. That path was extracted already. The check runs before anything touches the filesystem. It uses the same link target that extract_helper would use (hardlink_masters and --strip-components taken into account). This is the extract-side part of #10394 (master), ported to 1.4's source / hardlink_masters hard link model. Because it works on the items as they are stored, it also makes existing archives that contain such duplicates restore correctly.

export-tar and mount get the same small guard:

• export-tar wrote a LNKTYPE entry that links to itself. GNU tar and Python's tarfile ignore it, but bsdtar (libarchive) says "Skipping hardlink pointing to itself" and exits with rc 1.
• mount counted the duplicate as another link, so st_nlink was one too high (3 instead of 2). Contents were fine.

The create side (not archiving such duplicates in the first place) is not part of this PR.

refs #10393

Checklist

• PR is against 1.4-maint
• New tests: test_extract_duplicate_hardlink_item (also with --strip-components), test_export_tar_duplicate_hardlink_item, test_fuse_mount_duplicate_hardlink_item. The extract and export-tar tests fail without the fix.
• archiver.py and archive.py test modules pass locally (macOS). The FUSE tests, including the new one, pass on Linux with pyfuse3.

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