log.Fatalf in HandleSignInvoiceRequest terminates process instead of returning error

[Ggwpnobe-dev]

Summary

HandleSignInvoiceRequest in remotesigning/remote_signing.go:302 calls log.Fatalf() when SignInvoiceHash returns an error. Since log.Fatalf calls os.Exit(1), this immediately terminates the entire
process — the return nil, err on line 303 is dead code that never executes.

This is in the remotesigning library package, so any Go server that imports it and handles SIGN_INVOICE webhooks inherits an unrecoverable crash on signing errors.

Affected Code

https://github.com/lightsparkdev/go-sdk/blob/main/remotesigning/remote_signing.go#L302

signedInvoice, err := lightspark_crypto.SignInvoiceHash(seedBytes, bitcoinNetwork, hash)
if err != nil {
    log.Fatalf("Error signing invoice: %v", err)  // ← calls os.Exit(1)
    return nil, err                                 // ← dead code
}

Why This Is a Problem

1. Library code should not call os.Exit. Package remotesigning is an importable library, not a main package. Callers cannot prevent or recover from os.Exit(1) — Go's recover() doesn't catch it and deferred
   cleanup functions don't run.
2. Every other handler in the same file returns errors properly:

Function	Error Handling
HandleEcdhRequest	return nil, err
HandleGetPerCommitmentPointRequest	return nil, err
HandleReleasePerCommitmentSecretRequest	return nil, err
HandleDeriveKeyAndSignRequest	return nil, err
HandleInvoicePaymentHashRequest	return nil, err
HandleReleaseInvoicePreimageRequest	return nil, err
HandleSignInvoiceRequest	log.Fatalf → os.Exit(1)

3. The Kotlin SDK handles the same case correctly with a recoverable exception:
   throw RemoteSigningException("Error signing invoice", cause = e)
4. The reference server in examples/remote-signing-server/server.go calls this function via HandleRemoteSigningWebhook. A signing error crashes the entire server instead of returning a 500 response.

Suggested Fix

Replace log.Fatalf with a normal error return, matching all other handlers:

  signedInvoice, err := lightspark_crypto.SignInvoiceHash(seedBytes, bitcoinNetwork, hash)
  if err != nil {
      return nil, fmt.Errorf("error signing invoice: %w", err)
  }