Abort if insufficient liquidity in pay_for_offer

This commit introduces a check to abort early if
there's insufficient liquidity when fulfilling
the payment in `pay_for_offer`

The logic gets the available liquidity by summing up the 
`next_outbound_htlc_limit_msat` from all usable channels.
Then, compares the total liquidity amount with the total
payment amount. If the payment amount is > the available
liquidity, the function logs an error and returns a 
Bolt12SemanticError::InsufficientLiquidity error.

lightning/src/ln/channelmanager.rs

@@ -64,7 +64,7 @@ use crate::ln::wire::Encode;
 use crate::offers::invoice::{BlindedPayInfo, Bolt12Invoice, DEFAULT_RELATIVE_EXPIRY, DerivedSigningPubkey, ExplicitSigningPubkey, InvoiceBuilder, UnsignedBolt12Invoice};
 use crate::offers::invoice_error::InvoiceError;
 use crate::offers::invoice_request::{DerivedPayerId, InvoiceRequestBuilder};
-use crate::offers::offer::{Offer, OfferBuilder};
+use crate::offers::offer::{Offer, OfferBuilder, Amount};
 use crate::offers::parse::Bolt12SemanticError;
 use crate::offers::refund::{Refund, RefundBuilder};
 use crate::onion_message::async_payments::{AsyncPaymentsMessage, HeldHtlcAvailable, ReleaseHeldHtlc, AsyncPaymentsMessageHandler};
@@ -8816,6 +8816,26 @@ where
 			None => builder,
 			Some(payer_note) => builder.payer_note(payer_note),
 		};
+
+		let total_liquidity: u64 = self.list_usable_channels().iter().map(|channel| channel.next_outbound_htlc_limit_msat).sum();
+		let total_amount_msats = match amount_msats {
+			Some(amount_msats) => Some(amount_msats),
+			None => match offer.amount() {
+				Some(Amount::Bitcoin {amount_msats}) => Some(amount_msats),
+				_ => None,
+			},
+		};
+
+		if let Some(amount) = total_amount_msats {
+			return if amount > total_liquidity {
+				log_error!(self.logger, "Insufficient liquidity for payment with payment id: {}", payment_id);
+				Err(Bolt12SemanticError::InsufficientLiquidity)
+			} else {
+				log_error!(self.logger, "Offer amount is missing for payment with id: {}", payment_id);
+				Err(Bolt12SemanticError::InvalidAmount)
+			}
+		}
+
 		let invoice_request = builder.build_and_sign()?;
 
 		let context = OffersContext::OutboundPayment { payment_id };

lightning/src/offers/parse.rs

@@ -193,6 +193,8 @@ pub enum Bolt12SemanticError {
 	UnexpectedPaymentHash,
 	/// A signature was expected but was missing.
 	MissingSignature,
+	/// There is insufficient liquidity to complete the payment.
+	InsufficientLiquidity,
 }
 
 impl From<bech32::Error> for Bolt12ParseError {