backport_branch.sh: keep packages that are sources of .link files in the target package

[mrodm]

Problem

When creating a backport branch, the script removes all packages except the target package and any packages it declares under requires.input / requires.content in its manifest.yml (handled by get_required_package_names in backport_branch_lib.sh).

However, packages can contain .link files — plain-text files with a .link extension managed by elastic-package. Each .link file contains a path (relative to the link file itself) pointing to a source file whose content it mirrors, plus a SHA-256 checksum. That relative path can traverse ../ boundaries and reference a file that lives inside a different package.

When a linked file's source package is not the target package and is not listed under requires.*, remove_other_packages will delete it, leaving the .link file pointing at a non-existent file and breaking the backport branch.

This was raised during review of elastic/package-spec#1209.

Expected behaviour

backport_branch.sh should detect all *.link files inside the target package, resolve their source paths, identify which package each source file belongs to (by finding the nearest manifest.yml ancestor), and add those packages to packages_to_keep — the same list that already preserves requires.* dependencies.

.github/CODEOWNERS entries for those extra packages must also be retained, which is already handled generically by remove_other_packages for packages in packages_to_keep, so no extra logic is needed there once the package list is correct.

Implementation hints

backport_branch_lib.sh already has the pattern to follow in get_required_package_names. A new function — e.g. get_linked_source_package_names — should:

1. Walk all *.link files under the given package path (find "$package_path" -name "*.link").
2. Read the first whitespace-delimited field from each file (the relative path to the source).
3. Resolve the absolute path: dirname(<link_file>)/<relative_path>.
4. Walk up from that resolved path to find the nearest directory containing a manifest.yml — that directory is the source package root.
5. If the source package root differs from the target package root, emit its path so the caller can add it to packages_to_keep.

For reference, elastic-package implements the equivalent logic in Go at internal/files/linkedfiles.go — specifically listLinkedFiles + newLinkedFile (which populates IncludedPackageName).

Files to change

• .buildkite/scripts/backport_branch_lib.sh — add get_linked_source_package_names
• .buildkite/scripts/backport_branch.sh — call it alongside get_required_package_names when building packages_to_keep

[infra-vault-gh-plugin-prod]

Pinging @elastic/ecosystem (Team:Ecosystem)