Add Features::set_{required|optional}_custom_bit

Custom message handlers may need to set feature bits that are unknown to
LDK. Provide Features::set_required_custom_bit and
Features::set_optional_custom_bit to allow for this.

diff --git a/lightning/src/ln/features.rs b/lightning/src/ln/features.rs
index 4b49a56eacf73d5f4fe39c033a2206affbd889db..71bb66bacf7bc84380fe49316d9acec5ea293dff 100644 (file)
--- a/lightning/src/ln/features.rs
+++ b/lightning/src/ln/features.rs
@@ -746,6 +746,50 @@ impl<T: sealed::Context> Features<T> {
                }
                true
        }
+
+       /// Sets a required custom feature bit. Errors if `bit` is outside the custom range as defined
+       /// by [bLIP 2] or if it is a known `T` feature.
+       ///
+       /// Note: Required bits are even. If an odd bit is given, then the corresponding even bit will
+       /// be set instead (i.e., `bit - 1`).
+       ///
+       /// [bLIP 2]: https://github.com/lightning/blips/blob/master/blip-0002.md#feature-bits
+       pub fn set_required_custom_bit(&mut self, bit: usize) -> Result<(), ()> {
+               self.set_custom_bit(bit - (bit % 2))
+       }
+
+       /// Sets an optional custom feature bit. Errors if `bit` is outside the custom range as defined
+       /// by [bLIP 2] or if it is a known `T` feature.
+       ///
+       /// Note: Optional bits are odd. If an even bit is given, then the corresponding odd bit will be
+       /// set instead (i.e., `bit + 1`).
+       ///
+       /// [bLIP 2]: https://github.com/lightning/blips/blob/master/blip-0002.md#feature-bits
+       pub fn set_optional_custom_bit(&mut self, bit: usize) -> Result<(), ()> {
+               self.set_custom_bit(bit + (1 - (bit % 2)))
+       }
+
+       fn set_custom_bit(&mut self, bit: usize) -> Result<(), ()> {
+               if bit < 256 {
+                       return Err(());
+               }
+
+               let byte_offset = bit / 8;
+               let mask = 1 << (bit - 8 * byte_offset);
+               if byte_offset < T::KNOWN_FEATURE_MASK.len() {
+                       if (T::KNOWN_FEATURE_MASK[byte_offset] & mask) != 0 {
+                               return Err(());
+                       }
+               }
+
+               if self.flags.len() <= byte_offset {
+                       self.flags.resize(byte_offset + 1, 0u8);
+               }
+
+               self.flags[byte_offset] |= mask;
+
+               Ok(())
+       }
 }
 
 impl<T: sealed::UpfrontShutdownScript> Features<T> {
@@ -984,6 +1028,36 @@ mod tests {
                assert!(features.supports_payment_secret());
        }
 
+       #[test]
+       fn set_custom_bits() {
+               let mut features = InvoiceFeatures::empty();
+               features.set_variable_length_onion_optional();
+               assert_eq!(features.flags[1], 0b00000010);
+
+               assert!(features.set_optional_custom_bit(255).is_err());
+               assert!(features.set_required_custom_bit(256).is_ok());
+               assert!(features.set_required_custom_bit(258).is_ok());
+               assert_eq!(features.flags[31], 0b00000000);
+               assert_eq!(features.flags[32], 0b00000101);
+
+               let known_bit = <sealed::InvoiceContext as sealed::PaymentSecret>::EVEN_BIT;
+               let byte_offset = <sealed::InvoiceContext as sealed::PaymentSecret>::BYTE_OFFSET;
+               assert_eq!(byte_offset, 1);
+               assert_eq!(features.flags[byte_offset], 0b00000010);
+               assert!(features.set_required_custom_bit(known_bit).is_err());
+               assert_eq!(features.flags[byte_offset], 0b00000010);
+
+               let mut features = InvoiceFeatures::empty();
+               assert!(features.set_optional_custom_bit(256).is_ok());
+               assert!(features.set_optional_custom_bit(259).is_ok());
+               assert_eq!(features.flags[32], 0b00001010);
+
+               let mut features = InvoiceFeatures::empty();
+               assert!(features.set_required_custom_bit(257).is_ok());
+               assert!(features.set_required_custom_bit(258).is_ok());
+               assert_eq!(features.flags[32], 0b00000101);
+       }
+
        #[test]
        fn encodes_features_without_length() {
                let features = OfferFeatures::from_le_bytes(vec![1, 2, 3, 4, 5, 42, 100, 101]);