build(nix): Wrap vtysh so it loads the gateway's commands

[daniel-noland]

Addresses the goal of issue #1674, by a different route than the one the issue proposes.

Our FRR fork teaches vtysh a -X <path> flag that dlopen()s an extension, and the commands the gateway adds live in one such extension, libvtysh_hedgehog.so. Nobody passes the flag today, so those commands are absent unless whoever is typing knows to ask.

Why a wrapper rather than an alias

An alias only reaches callers that read a shell rc, and the two that matter most do not: kubectl exec and docker exec start no login shell. The shortcut would be missing in exactly the session someone opened to work out what a running gateway is doing.

[coderabbitai]

📝 Walkthrough

Walkthrough

Changes

FRR packaging now accepts optional vtysh extension libraries. The dataplane overlay enables extension loading, while the host build uses upstream FRR without extensions. Enabled builds wrap vtysh with -X arguments.

FRR vtysh extension support

Layer / File(s)	Summary
FRR package extension wrapping nix/pkgs/frr/default.nix	The package conditionally adds makeWrapper, accepts extension library paths, removes the extension patch, and wraps vtysh with -X arguments when enabled.
Overlay build wiring nix/overlays/frr.nix	frr-build forwards the extension option. The dataplane build enables it, and the host build disables it.

Suggested reviewers: copilot, frostman

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Title check	✅ Passed	The title clearly and concisely describes wrapping vtysh to load the gateway commands.
Description check	✅ Passed	The description explains the vtysh extension wrapper and its purpose for kubectl exec and docker exec sessions.

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_{Comment @coderabbitai help to get the list of available commands.}

[Copilot]

Pull request overview

This PR updates the Nix packaging for FRR to ensure vtysh automatically loads the gateway’s custom command extension (libvtysh_hedgehog.so) when building the dataplane FRR fork, avoiding reliance on shell aliases that don’t apply to kubectl exec / docker exec.

Changes:

• Add optional makeWrapper support and a postFixup wrapper that injects -X <extension> flags into vtysh when vtysh-extensions = true.
• Introduce configurable vtysh-extension-libs (defaulting to /lib/libvtysh_hedgehog.so) as image-path targets to avoid Nix-store dependency cycles.
• Adjust the FRR overlay to enable the wrapper only for frr.dataplane (fork), leaving frr.host (upstream) unwrapped.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
nix/pkgs/frr/default.nix	Adds optional wrapping of vtysh in postFixup to auto-load configured extension libraries.
nix/overlays/frr.nix	Enables the wrapper only for the dataplane FRR fork and keeps upstream host FRR unchanged.

[mvachhar]

Do we care about the AI nitpick about the install vs. post-install in the comment? If we care fix and then merge, otherwise just merge.