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nonfree-firmware: when creating on-disc firmware links, use the same link creation logic as archive-area links

James Addison requested to merge (removed):bug-1031696/firmware-nonfree-linking-adjustment into master

This changeset switches the creation of firmware file links from using the Perl symlink function (that creates symbolic links) to the more configurable debian-cd good_link function that will create links based on the demands of the environment it is configured in.

Context:

Debian bug #1031696 describes an issue where firmware files on alpha-release debian-cd ISO images cannot easily be copied to create another installation medium by users of some operating systems.

Research:

When creating links to firmware files on the installation media in the /firmware directory, we want to retain the same content de-duplication and corresponding space-saving efficiencies as symlinks currently provide.

We'd also like the entire content of the ISO 9660 filesystem to be copyable to as many filesystems as possible, by as many operating systems as possible.

Write-side considerations:

Filesystem links are not supported everywhere, and where unavailable a reliable substitute for them is to create independent copies of the link target file for each link that is encountered in the input.

The ISO9660 specifications, ECMA-119[1], section 6.5.1, allows[2] a single file content block ("File Section") to be referenced from multiple filenames ("records"). The ISO9660 generation tools genisoimage and xorriso can use this to de-duplicate multiple hard linked files from their input, saving space in the output filesystem image.

Read-side considerations:

Operating systems that read shared content blocks usually present the files to the user as if no links exist. In the context of Debian bug #1031696, that's exactly what we want: copying of the files from the ISO filesystem produces independent data files without any filesystem-level links.

[1] - https://www.ecma-international.org/publications-and-standards/standards/ecma-119/

[2] - https://archive.org/details/ecma-119-1998/page/n13/mode/1up

Edit: description has been edited to include feedback received on the bug thread

Edited by James Addison

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