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hook_functions: Fix copy_file with source including a directory symlink

Ben Hutchingsrequested to merge
benh/initramfs-tools:copy-file-symlink-dir into master

Debian bug #1076539 (and its many duplicates) involved a firmware package with a directory structure like:

vendor/model1/
vendor/model1/firmware.bin
vendor/model2 -> model1
vendor/model3 -> model1
...

and the associated kernel module referred to firmware names like vendor/model1/firmware.bin, vendor/model2/firmware.bin, etc.

copy_file currently checks whether the source itself is a symlink, and all of the filenames resolve to a regular file, so copy_file would happily install multiple copies of that.

The particular problem with firmware was fixed by the switch to dracut-install, but the same problem could occur with other files. Also, that change is not suitable for backporting.

To handle directory symlinks as well, replace this check with a comparison of:

  • The canonicalised filename with symlinks dereferenced, as produced by readlink -f.
  • The canonicalised filename without symlinks dereferenced, as produced by realpath -s.

Signed-off-by: Ben Hutchings benh@debian.org

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