loopd: fix crash in instant if no --experimental

[starius]

Previously the daemon crashed with a panic. Now the CLI returns the following message:

[loop] rpc error: code = Unimplemented desc = Restart loop with --experimental

Pull Request Checklist

• Update release_notes.md if your PR contains major features, breaking changes or bugfixes

[gemini-code-assist]

Summary of Changes

Hello @starius, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses a critical stability issue in the loopd daemon. Previously, attempting to use instant features without enabling them via the --experimental flag would lead to a daemon panic. The changes introduce robust error handling to gracefully manage this scenario, providing a user-friendly Unimplemented RPC error message to the CLI, thereby enhancing the daemon's reliability and user experience.

Highlights

• Crash Fix: The daemon previously crashed with a panic when instant features were invoked without the --experimental flag. This pull request resolves the issue by preventing the crash.
• Improved Error Handling: Instead of crashing, the CLI now receives a clear Unimplemented RPC error message: "Restart loop with --experimental", guiding the user on how to properly enable the feature.

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[gemini-code-assist]

Code Review

This pull request addresses a crash that occurs when instant out features are used without the --experimental flag. The fix involves adding nil checks for instantOutManager in InstantOut, InstantOutQuote, and ListInstantOuts, which correctly prevents a panic and instead returns an informative error. My review includes a suggestion to refactor the duplicated check into a helper function to improve code maintainability.

[gemini-code-assist]

This check is duplicated in InstantOutQuote (line 1537) and ListInstantOuts (line 1561). To improve maintainability and reduce code duplication, consider extracting this logic into a private helper method on swapClientServer.

For example:

func (s *swapClientServer) requireInstantOutManager() error {
	if s.instantOutManager == nil {
		return status.Error(codes.Unimplemented, "Restart loop with --experimental")
	}
	return nil
}

You could then call this helper at the beginning of InstantOut, InstantOutQuote, and ListInstantOuts:

if err := s.requireInstantOutManager(); err != nil {
	return nil, err
}

This would also make the code more consistent, as ListReservations has a similar check for reservationManager.