`Event::PaymentSent::fee_paid_msat` can be `None` despite docs to the contrary

[phlip9]

Summary

The Event::PaymentSent::fee_paid_msat docs claim the field is never None (for events after v0.0.103), however abandon_payment -> HTLC(s) still in-flight -> successful claim_htlc -> Event::PaymentSent will have the field set to None.

It appears that PendingOutboundPayment::get_pending_fee_msat() only returns a value for PendingOutboundPayment::Retryable. In some sense, even though a payment might still succeed, PendingOutboundPayment::Abandoned "forgets" too much info, when it should instead preserve some data from PendingOutboundPayment::Retryable in case the payment actually goes through.

Ideally we don't forget the fees, so our accounting stays clean. If someone's relying on Event::PaymentSent::bolt12_invoice, they'll also lose that data unexpectedly.

Context

/// Indicates an outbound payment we made succeeded (i.e. it made it all the way to its target
/// and we got back the payment preimage for it).
/// ...
/// # Failure Behavior and Persistence
/// This event will eventually be replayed after failures-to-handle (i.e., the event handler
/// returning `Err(ReplayEvent ())`) and will be persisted across restarts.
PaymentSent {
	// ...
	/// The total fee which was spent at intermediate hops in this payment, across all paths.
	/// ...
	/// This is only `None` for payments initiated on LDK versions prior to 0.0.103.
	fee_paid_msat: Option<u64>,
	// ...
}

Following the docs, since we had no pending payments pre-0.0.103, we had an unwrap on this field:

match event {
  Event::PaymentSent { fee_paid_msat, .. } => {
    // ..
    let fee_paid_msat =
      fee_paid_msat.expect("No pending sends before 0.0.103");
    // ..
  }
}

See: https://github.com/lexe-app/lexe-public/blob/42b307e6e0adb2447381a7a98492b10407ff15b8/node/src/event_handler.rs#L467

And so a user node hit this edge case and panicked :'(

In the meantime, we'll just fee_paid_msat.unwrap_or(0) to stop panicking, but it would be real nice to not lose the payment fee.