X-Git-Url: http://git.bitcoin.ninja/index.cgi?a=blobdiff_plain;f=lightning%2Fsrc%2Fln%2Ffeatures.rs;h=ad8942326322f75207e42d0a9ec3f09ee86fc0a7;hb=22dded737b6771d9de0eb20abac0d20e31ff929a;hp=ac9af1434012be9abdca4ce6bad85d76b3464345;hpb=cd5a11fe0d56fc6520fe94bad2cb5ea461608442;p=rust-lightning

diff --git a/lightning/src/ln/features.rs b/lightning/src/ln/features.rs
index ac9af143..ad894232 100644
--- a/lightning/src/ln/features.rs
+++ b/lightning/src/ln/features.rs
@@ -1,5 +1,20 @@
-//! Lightning exposes sets of supported operations through "feature flags". This module includes
-//! types to store those feature flags and query for specific flags.
+//! Feature flag definitions for the Lightning protocol according to [BOLT #9].
+//!
+//! Lightning nodes advertise a supported set of operation through feature flags. Features are
+//! applicable for a specific context as indicated in some [messages]. [`Features`] encapsulates
+//! behavior for specifying and checking feature flags for a particular context. Each feature is
+//! defined internally by a trait specifying the corresponding flags (i.e., even and odd bits). A
+//! [`Context`] is used to parameterize [`Features`] and defines which features it can support.
+//!
+//! Whether a feature is considered "known" or "unknown" is relative to the implementation, whereas
+//! the term "supports" is used in reference to a particular set of [`Features`]. That is, a node
+//! supports a feature if it advertises the feature (as either required or optional) to its peers.
+//! And the implementation can interpret a feature if the feature is known to it.
+//!
+//! [BOLT #9]: https://github.com/lightningnetwork/lightning-rfc/blob/master/09-features.md
+//! [messages]: ../msgs/index.html
+//! [`Features`]: struct.Features.html
+//! [`Context`]: sealed/trait.Context.html
 
 use std::{cmp, fmt};
 use std::result::Result;
@@ -8,63 +23,260 @@ use std::marker::PhantomData;
 use ln::msgs::DecodeError;
 use util::ser::{Readable, Writeable, Writer};
 
-/// The context in which a Feature object appears determines which bits of features the node
-/// supports will be set. We use this when creating our own Feature objects to select which bits to
-/// set and when passing around Feature objects to ensure the bits we're checking for are
-/// available.
-///
-/// This Context represents when the Feature appears in the init message, sent between peers and not
-/// rumored around the P2P network.
-pub struct FeatureContextInit {}
-/// The context in which a Feature object appears determines which bits of features the node
-/// supports will be set. We use this when creating our own Feature objects to select which bits to
-/// set and when passing around Feature objects to ensure the bits we're checking for are
-/// available.
-///
-/// This Context represents when the Feature appears in the node_announcement message, as it is
-/// rumored around the P2P network.
-pub struct FeatureContextNode {}
-/// The context in which a Feature object appears determines which bits of features the node
-/// supports will be set. We use this when creating our own Feature objects to select which bits to
-/// set and when passing around Feature objects to ensure the bits we're checking for are
-/// available.
-///
-/// This Context represents when the Feature appears in the ChannelAnnouncement message, as it is
-/// rumored around the P2P network.
-pub struct FeatureContextChannel {}
-/// The context in which a Feature object appears determines which bits of features the node
-/// supports will be set. We use this when creating our own Feature objects to select which bits to
-/// set and when passing around Feature objects to ensure the bits we're checking for are
-/// available.
-///
-/// This Context represents when the Feature appears in an invoice, used to determine the different
-/// options available for routing a payment.
-///
-/// Note that this is currently unused as invoices come to us via a different crate and are not
-/// native to rust-lightning directly.
-pub struct FeatureContextInvoice {}
-
-/// An internal trait capturing the various future context types
-pub trait FeatureContext {}
-impl FeatureContext for FeatureContextInit {}
-impl FeatureContext for FeatureContextNode {}
-impl FeatureContext for FeatureContextChannel {}
-impl FeatureContext for FeatureContextInvoice {}
-
-/// An internal trait capturing FeatureContextInit and FeatureContextNode
-pub trait FeatureContextInitNode : FeatureContext {}
-impl FeatureContextInitNode for FeatureContextInit {}
-impl FeatureContextInitNode for FeatureContextNode {}
+mod sealed {
+	/// The context in which [`Features`] are applicable. Defines which features are required and
+	/// which are optional for the context.
+	///
+	/// [`Features`]: ../struct.Features.html
+	pub trait Context {
+		/// Features that are known to the implementation, where a required feature is indicated by
+		/// its even bit and an optional feature is indicated by its odd bit.
+		const KNOWN_FEATURE_FLAGS: &'static [u8];
+
+		/// Bitmask for selecting features that are known to the implementation, regardless of
+		/// whether each feature is required or optional.
+		const KNOWN_FEATURE_MASK: &'static [u8];
+	}
+
+	/// Defines a [`Context`] by stating which features it requires and which are optional. Features
+	/// are specified as a comma-separated list of bytes where each byte is a pipe-delimited list of
+	/// feature identifiers.
+	///
+	/// [`Context`]: trait.Context.html
+	macro_rules! define_context {
+		($context: ident {
+			required_features: [$( $( $required_feature: ident )|*, )*],
+			optional_features: [$( $( $optional_feature: ident )|*, )*],
+		}) => {
+			pub struct $context {}
+
+			impl Context for $context {
+				const KNOWN_FEATURE_FLAGS: &'static [u8] = &[
+					// For each byte, use bitwise-OR to compute the applicable flags for known
+					// required features `r_i` and optional features `o_j` for all `i` and `j` such
+					// that the following slice is formed:
+					//
+					// [
+					//  `r_0` | `r_1` | ... | `o_0` | `o_1` | ...,
+					//  ...,
+					// ]
+					$(
+						0b00_00_00_00 $(|
+							<Self as $required_feature>::REQUIRED_MASK)*
+						$(|
+							<Self as $optional_feature>::OPTIONAL_MASK)*,
+					)*
+				];
+
+				const KNOWN_FEATURE_MASK: &'static [u8] = &[
+					// Similar as above, but set both flags for each feature regardless of whether
+					// the feature is required or optional.
+					$(
+						0b00_00_00_00 $(|
+							<Self as $required_feature>::REQUIRED_MASK |
+							<Self as $required_feature>::OPTIONAL_MASK)*
+						$(|
+							<Self as $optional_feature>::REQUIRED_MASK |
+							<Self as $optional_feature>::OPTIONAL_MASK)*,
+					)*
+				];
+			}
+		};
+	}
+
+	define_context!(InitContext {
+		required_features: [
+			// Byte 0
+			,
+			// Byte 1
+			,
+			// Byte 2
+			,
+		],
+		optional_features: [
+			// Byte 0
+			DataLossProtect | InitialRoutingSync | UpfrontShutdownScript,
+			// Byte 1
+			VariableLengthOnion | PaymentSecret,
+			// Byte 2
+			BasicMPP,
+		],
+	});
+	define_context!(NodeContext {
+		required_features: [
+			// Byte 0
+			,
+			// Byte 1
+			,
+			// Byte 2
+			,
+		],
+		optional_features: [
+			// Byte 0
+			DataLossProtect | UpfrontShutdownScript,
+			// Byte 1
+			VariableLengthOnion | PaymentSecret,
+			// Byte 2
+			BasicMPP,
+		],
+	});
+	define_context!(ChannelContext {
+		required_features: [],
+		optional_features: [],
+	});
+
+	/// Defines a feature with the given bits for the specified [`Context`]s. The generated trait is
+	/// useful for manipulating feature flags.
+	///
+	/// [`Context`]: trait.Context.html
+	macro_rules! define_feature {
+		($odd_bit: expr, $feature: ident, [$($context: ty),+], $doc: expr) => {
+			#[doc = $doc]
+			///
+			/// See [BOLT #9] for details.
+			///
+			/// [BOLT #9]: https://github.com/lightningnetwork/lightning-rfc/blob/master/09-features.md
+			pub trait $feature: Context {
+				/// The bit used to signify that the feature is required.
+				const EVEN_BIT: usize = $odd_bit - 1;
+
+				/// The bit used to signify that the feature is optional.
+				const ODD_BIT: usize = $odd_bit;
+
+				/// Assertion that [`EVEN_BIT`] is actually even.
+				///
+				/// [`EVEN_BIT`]: #associatedconstant.EVEN_BIT
+				const ASSERT_EVEN_BIT_PARITY: usize;
+
+				/// Assertion that [`ODD_BIT`] is actually odd.
+				///
+				/// [`ODD_BIT`]: #associatedconstant.ODD_BIT
+				const ASSERT_ODD_BIT_PARITY: usize;
+
+				/// The byte where the feature is set.
+				const BYTE_OFFSET: usize = Self::EVEN_BIT / 8;
+
+				/// The bitmask for the feature's required flag relative to the [`BYTE_OFFSET`].
+				///
+				/// [`BYTE_OFFSET`]: #associatedconstant.BYTE_OFFSET
+				const REQUIRED_MASK: u8 = 1 << (Self::EVEN_BIT - 8 * Self::BYTE_OFFSET);
+
+				/// The bitmask for the feature's optional flag relative to the [`BYTE_OFFSET`].
+				///
+				/// [`BYTE_OFFSET`]: #associatedconstant.BYTE_OFFSET
+				const OPTIONAL_MASK: u8 = 1 << (Self::ODD_BIT - 8 * Self::BYTE_OFFSET);
+
+				/// Returns whether the feature is required by the given flags.
+				#[inline]
+				fn requires_feature(flags: &Vec<u8>) -> bool {
+					flags.len() > Self::BYTE_OFFSET &&
+						(flags[Self::BYTE_OFFSET] & Self::REQUIRED_MASK) != 0
+				}
+
+				/// Returns whether the feature is supported by the given flags.
+				#[inline]
+				fn supports_feature(flags: &Vec<u8>) -> bool {
+					flags.len() > Self::BYTE_OFFSET &&
+						(flags[Self::BYTE_OFFSET] & (Self::REQUIRED_MASK | Self::OPTIONAL_MASK)) != 0
+				}
+
+				/// Sets the feature's required (even) bit in the given flags.
+				#[inline]
+				fn set_required_bit(flags: &mut Vec<u8>) {
+					if flags.len() <= Self::BYTE_OFFSET {
+						flags.resize(Self::BYTE_OFFSET + 1, 0u8);
+					}
+
+					flags[Self::BYTE_OFFSET] |= Self::REQUIRED_MASK;
+				}
+
+				/// Sets the feature's optional (odd) bit in the given flags.
+				#[inline]
+				fn set_optional_bit(flags: &mut Vec<u8>) {
+					if flags.len() <= Self::BYTE_OFFSET {
+						flags.resize(Self::BYTE_OFFSET + 1, 0u8);
+					}
+
+					flags[Self::BYTE_OFFSET] |= Self::OPTIONAL_MASK;
+				}
+
+				/// Clears the feature's required (even) and optional (odd) bits from the given
+				/// flags.
+				#[inline]
+				fn clear_bits(flags: &mut Vec<u8>) {
+					if flags.len() > Self::BYTE_OFFSET {
+						flags[Self::BYTE_OFFSET] &= !Self::REQUIRED_MASK;
+						flags[Self::BYTE_OFFSET] &= !Self::OPTIONAL_MASK;
+					}
+
+					let last_non_zero_byte = flags.iter().rposition(|&byte| byte != 0);
+					let size = if let Some(offset) = last_non_zero_byte { offset + 1 } else { 0 };
+					flags.resize(size, 0u8);
+				}
+			}
+
+			$(
+				impl $feature for $context {
+					// EVEN_BIT % 2 == 0
+					const ASSERT_EVEN_BIT_PARITY: usize = 0 - (<Self as $feature>::EVEN_BIT % 2);
+
+					// ODD_BIT % 2 == 1
+					const ASSERT_ODD_BIT_PARITY: usize = (<Self as $feature>::ODD_BIT % 2) - 1;
+				}
+			)*
+		}
+	}
+
+	define_feature!(1, DataLossProtect, [InitContext, NodeContext],
+		"Feature flags for `option_data_loss_protect`.");
+	// NOTE: Per Bolt #9, initial_routing_sync has no even bit.
+	define_feature!(3, InitialRoutingSync, [InitContext],
+		"Feature flags for `initial_routing_sync`.");
+	define_feature!(5, UpfrontShutdownScript, [InitContext, NodeContext],
+		"Feature flags for `option_upfront_shutdown_script`.");
+	define_feature!(9, VariableLengthOnion, [InitContext, NodeContext],
+		"Feature flags for `var_onion_optin`.");
+	define_feature!(13, StaticRemoteKey, [InitContext, NodeContext],
+		"Feature flags for `option_static_remotekey`.");
+	define_feature!(15, PaymentSecret, [InitContext, NodeContext],
+		"Feature flags for `payment_secret`.");
+	define_feature!(17, BasicMPP, [InitContext, NodeContext],
+		"Feature flags for `basic_mpp`.");
+
+	#[cfg(test)]
+	define_context!(TestingContext {
+		required_features: [
+			// Byte 0
+			,
+			// Byte 1
+			,
+			// Byte 2
+			UnknownFeature,
+		],
+		optional_features: [
+			// Byte 0
+			,
+			// Byte 1
+			,
+			// Byte 2
+			,
+		],
+	});
+
+	#[cfg(test)]
+	define_feature!(23, UnknownFeature, [TestingContext],
+		"Feature flags for an unknown feature used in testing.");
+}
 
 /// Tracks the set of features which a node implements, templated by the context in which it
 /// appears.
-pub struct Features<T: FeatureContext> {
-	/// Note that, for convinience, flags is LITTLE endian (despite being big-endian on the wire)
+pub struct Features<T: sealed::Context> {
+	/// Note that, for convenience, flags is LITTLE endian (despite being big-endian on the wire)
 	flags: Vec<u8>,
 	mark: PhantomData<T>,
 }
 
-impl<T: FeatureContext> Clone for Features<T> {
+impl<T: sealed::Context> Clone for Features<T> {
 	fn clone(&self) -> Self {
 		Self {
 			flags: self.flags.clone(),
@@ -72,67 +284,91 @@ impl<T: FeatureContext> Clone for Features<T> {
 		}
 	}
 }
-impl<T: FeatureContext> PartialEq for Features<T> {
+impl<T: sealed::Context> PartialEq for Features<T> {
 	fn eq(&self, o: &Self) -> bool {
 		self.flags.eq(&o.flags)
 	}
 }
-impl<T: FeatureContext> fmt::Debug for Features<T> {
+impl<T: sealed::Context> fmt::Debug for Features<T> {
 	fn fmt(&self, fmt: &mut fmt::Formatter) -> Result<(), fmt::Error> {
 		self.flags.fmt(fmt)
 	}
 }
 
-/// A feature message as it appears in an init message
-pub type InitFeatures = Features<FeatureContextInit>;
-/// A feature message as it appears in a node_announcement message
-pub type NodeFeatures = Features<FeatureContextNode>;
-/// A feature message as it appears in a channel_announcement message
-pub type ChannelFeatures = Features<FeatureContextChannel>;
-
-impl<T: FeatureContextInitNode> Features<T> {
-	/// Create a Features with the features we support
-	#[cfg(not(feature = "fuzztarget"))]
-	pub(crate) fn supported() -> Features<T> {
-		Features {
-			flags: vec![2 | 1 << 5],
-			mark: PhantomData,
+/// Features used within an `init` message.
+pub type InitFeatures = Features<sealed::InitContext>;
+/// Features used within a `node_announcement` message.
+pub type NodeFeatures = Features<sealed::NodeContext>;
+/// Features used within a `channel_announcement` message.
+pub type ChannelFeatures = Features<sealed::ChannelContext>;
+
+impl InitFeatures {
+	/// Writes all features present up to, and including, 13.
+	pub(crate) fn write_up_to_13<W: Writer>(&self, w: &mut W) -> Result<(), ::std::io::Error> {
+		let len = cmp::min(2, self.flags.len());
+		w.size_hint(len + 2);
+		(len as u16).write(w)?;
+		for i in (0..len).rev() {
+			if i == 0 {
+				self.flags[i].write(w)?;
+			} else {
+				// On byte 1, we want up-to-and-including-bit-13, 0-indexed, which is
+				// up-to-and-including-bit-5, 0-indexed, on this byte:
+				(self.flags[i] & 0b00_11_11_11).write(w)?;
+			}
 		}
+		Ok(())
 	}
-	#[cfg(feature = "fuzztarget")]
-	pub fn supported() -> Features<T> {
-		Features {
-			flags: vec![2 | 1 << 5],
-			mark: PhantomData,
+
+	/// or's another InitFeatures into this one.
+	pub(crate) fn or(mut self, o: InitFeatures) -> InitFeatures {
+		let total_feature_len = cmp::max(self.flags.len(), o.flags.len());
+		self.flags.resize(total_feature_len, 0u8);
+		for (byte, o_byte) in self.flags.iter_mut().zip(o.flags.iter()) {
+			*byte |= *o_byte;
 		}
+		self
+	}
+
+	/// Converts `InitFeatures` to `Features<C>`. Only known `InitFeatures` relevant to context `C`
+	/// are included in the result.
+	pub(crate) fn to_context<C: sealed::Context>(&self) -> Features<C> {
+		self.to_context_internal()
 	}
 }
 
-impl Features<FeatureContextChannel> {
-	/// Create a Features with the features we support
-	#[cfg(not(feature = "fuzztarget"))]
-	pub(crate) fn supported() -> Features<FeatureContextChannel> {
+impl<T: sealed::Context> Features<T> {
+	/// Create a blank Features with no features set
+	pub fn empty() -> Features<T> {
 		Features {
 			flags: Vec::new(),
 			mark: PhantomData,
 		}
 	}
-	#[cfg(feature = "fuzztarget")]
-	pub fn supported() -> Features<FeatureContextChannel> {
-		Features {
-			flags: Vec::new(),
+
+	/// Creates features known by the implementation as defined by [`T::KNOWN_FEATURE_FLAGS`].
+	///
+	/// [`T::KNOWN_FEATURE_FLAGS`]: sealed/trait.Context.html#associatedconstant.KNOWN_FEATURE_FLAGS
+	pub fn known() -> Features<T> {
+		Self {
+			flags: T::KNOWN_FEATURE_FLAGS.to_vec(),
 			mark: PhantomData,
 		}
 	}
-}
 
-impl<T: FeatureContext> Features<T> {
-	/// Create a blank Features with no features set
-	pub fn empty() -> Features<T> {
-		Features {
-			flags: Vec::new(),
-			mark: PhantomData,
+	/// Converts `Features<T>` to `Features<C>`. Only known `T` features relevant to context `C` are
+	/// included in the result.
+	fn to_context_internal<C: sealed::Context>(&self) -> Features<C> {
+		let byte_count = C::KNOWN_FEATURE_MASK.len();
+		let mut flags = Vec::new();
+		for (i, byte) in self.flags.iter().enumerate() {
+			if i < byte_count {
+				let known_source_features = T::KNOWN_FEATURE_MASK[i];
+				let known_target_features = C::KNOWN_FEATURE_MASK[i];
+				flags.push(byte & known_source_features & known_target_features);
+			}
 		}
+		Features::<C> { flags, mark: PhantomData, }
 	}
 
 	#[cfg(test)]
@@ -151,14 +387,31 @@ impl<T: FeatureContext> Features<T> {
 	}
 
 	pub(crate) fn requires_unknown_bits(&self) -> bool {
-		self.flags.iter().enumerate().any(|(idx, &byte)| {
-			( idx != 0 && (byte & 0x55) != 0 ) || ( idx == 0 && (byte & 0x14) != 0 )
+		// Bitwise AND-ing with all even bits set except for known features will select required
+		// unknown features.
+		let byte_count = T::KNOWN_FEATURE_MASK.len();
+		self.flags.iter().enumerate().any(|(i, &byte)| {
+			let required_features = 0b01_01_01_01;
+			let unknown_features = if i < byte_count {
+				!T::KNOWN_FEATURE_MASK[i]
+			} else {
+				0b11_11_11_11
+			};
+			(byte & (required_features & unknown_features)) != 0
 		})
 	}
 
 	pub(crate) fn supports_unknown_bits(&self) -> bool {
-		self.flags.iter().enumerate().any(|(idx, &byte)| {
-			( idx != 0 && byte != 0 ) || ( idx == 0 && (byte & 0xc4) != 0 )
+		// Bitwise AND-ing with all even and odd bits set except for known features will select
+		// both required and optional unknown features.
+		let byte_count = T::KNOWN_FEATURE_MASK.len();
+		self.flags.iter().enumerate().any(|(i, &byte)| {
+			let unknown_features = if i < byte_count {
+				!T::KNOWN_FEATURE_MASK[i]
+			} else {
+				0b11_11_11_11
+			};
+			(byte & unknown_features) != 0
 		})
 	}
 
@@ -169,76 +422,102 @@ impl<T: FeatureContext> Features<T> {
 	}
 
 	#[cfg(test)]
-	pub(crate) fn set_require_unknown_bits(&mut self) {
-		let newlen = cmp::max(2, self.flags.len());
-		self.flags.resize(newlen, 0u8);
-		self.flags[1] |= 0x40;
+	pub(crate) fn set_required_unknown_bits(&mut self) {
+		<sealed::TestingContext as sealed::UnknownFeature>::set_required_bit(&mut self.flags);
 	}
 
 	#[cfg(test)]
-	pub(crate) fn clear_require_unknown_bits(&mut self) {
-		let newlen = cmp::max(2, self.flags.len());
-		self.flags.resize(newlen, 0u8);
-		self.flags[1] &= !0x40;
-		if self.flags.len() == 2 && self.flags[1] == 0 {
-			self.flags.resize(1, 0u8);
-		}
+	pub(crate) fn set_optional_unknown_bits(&mut self) {
+		<sealed::TestingContext as sealed::UnknownFeature>::set_optional_bit(&mut self.flags);
+	}
+
+	#[cfg(test)]
+	pub(crate) fn clear_unknown_bits(&mut self) {
+		<sealed::TestingContext as sealed::UnknownFeature>::clear_bits(&mut self.flags);
 	}
 }
 
-impl<T: FeatureContextInitNode> Features<T> {
+impl<T: sealed::DataLossProtect> Features<T> {
+	#[cfg(test)]
+	pub(crate) fn requires_data_loss_protect(&self) -> bool {
+		<T as sealed::DataLossProtect>::requires_feature(&self.flags)
+	}
 	pub(crate) fn supports_data_loss_protect(&self) -> bool {
-		self.flags.len() > 0 && (self.flags[0] & 3) != 0
+		<T as sealed::DataLossProtect>::supports_feature(&self.flags)
 	}
+}
 
+impl<T: sealed::UpfrontShutdownScript> Features<T> {
+	#[cfg(test)]
+	pub(crate) fn requires_upfront_shutdown_script(&self) -> bool {
+		<T as sealed::UpfrontShutdownScript>::requires_feature(&self.flags)
+	}
 	pub(crate) fn supports_upfront_shutdown_script(&self) -> bool {
-		self.flags.len() > 0 && (self.flags[0] & (3 << 4)) != 0
+		<T as sealed::UpfrontShutdownScript>::supports_feature(&self.flags)
 	}
 	#[cfg(test)]
-	pub(crate) fn unset_upfront_shutdown_script(&mut self) {
-		self.flags[0] ^= 1 << 5;
+	pub(crate) fn clear_upfront_shutdown_script(mut self) -> Self {
+		<T as sealed::UpfrontShutdownScript>::clear_bits(&mut self.flags);
+		self
+	}
+}
+
+impl<T: sealed::VariableLengthOnion> Features<T> {
+	#[cfg(test)]
+	pub(crate) fn requires_variable_length_onion(&self) -> bool {
+		<T as sealed::VariableLengthOnion>::requires_feature(&self.flags)
+	}
+	pub(crate) fn supports_variable_length_onion(&self) -> bool {
+		<T as sealed::VariableLengthOnion>::supports_feature(&self.flags)
 	}
 }
 
-impl Features<FeatureContextInit> {
+impl<T: sealed::StaticRemoteKey> Features<T> {
+	pub(crate) fn supports_static_remote_key(&self) -> bool {
+		<T as sealed::StaticRemoteKey>::supports_feature(&self.flags)
+	}
+	#[cfg(test)]
+	pub(crate) fn requires_static_remote_key(&self) -> bool {
+		<T as sealed::StaticRemoteKey>::requires_feature(&self.flags)
+	}
+}
+
+impl<T: sealed::InitialRoutingSync> Features<T> {
 	pub(crate) fn initial_routing_sync(&self) -> bool {
-		self.flags.len() > 0 && (self.flags[0] & (1 << 3)) != 0
+		<T as sealed::InitialRoutingSync>::supports_feature(&self.flags)
 	}
-	pub(crate) fn set_initial_routing_sync(&mut self) {
-		if self.flags.len() == 0 {
-			self.flags.resize(1, 1 << 3);
-		} else {
-			self.flags[0] |= 1 << 3;
-		}
+	pub(crate) fn clear_initial_routing_sync(&mut self) {
+		<T as sealed::InitialRoutingSync>::clear_bits(&mut self.flags)
 	}
+}
 
-	/// Writes all features present up to, and including, 13.
-	pub(crate) fn write_up_to_13<W: Writer>(&self, w: &mut W) -> Result<(), ::std::io::Error> {
-		let len = cmp::min(2, self.flags.len());
-		w.size_hint(len + 2);
-		(len as u16).write(w)?;
-		for i in (0..len).rev() {
-			if i == 0 {
-				self.flags[i].write(w)?;
-			} else {
-				(self.flags[i] & ((1 << (14 - 8)) - 1)).write(w)?;
-			}
-		}
-		Ok(())
+impl<T: sealed::PaymentSecret> Features<T> {
+	#[cfg(test)]
+	pub(crate) fn requires_payment_secret(&self) -> bool {
+		<T as sealed::PaymentSecret>::requires_feature(&self.flags)
 	}
+	// Note that we never need to test this since what really matters is the invoice - iff the
+	// invoice provides a payment_secret, we assume that we can use it (ie that the recipient
+	// supports payment_secret).
+	#[allow(dead_code)]
+	pub(crate) fn supports_payment_secret(&self) -> bool {
+		<T as sealed::PaymentSecret>::supports_feature(&self.flags)
+	}
+}
 
-	/// or's another InitFeatures into this one.
-	pub(crate) fn or(mut self, o: InitFeatures) -> InitFeatures {
-		let total_feature_len = cmp::max(self.flags.len(), o.flags.len());
-		self.flags.resize(total_feature_len, 0u8);
-		for (feature, o_feature) in self.flags.iter_mut().zip(o.flags.iter()) {
-			*feature |= *o_feature;
-		}
-		self
+impl<T: sealed::BasicMPP> Features<T> {
+	#[cfg(test)]
+	pub(crate) fn requires_basic_mpp(&self) -> bool {
+		<T as sealed::BasicMPP>::requires_feature(&self.flags)
+	}
+	// We currently never test for this since we don't actually *generate* multipath routes.
+	#[allow(dead_code)]
+	pub(crate) fn supports_basic_mpp(&self) -> bool {
+		<T as sealed::BasicMPP>::supports_feature(&self.flags)
 	}
 }
 
-impl<T: FeatureContext> Writeable for Features<T> {
+impl<T: sealed::Context> Writeable for Features<T> {
 	fn write<W: Writer>(&self, w: &mut W) -> Result<(), ::std::io::Error> {
 		w.size_hint(self.flags.len() + 2);
 		(self.flags.len() as u16).write(w)?;
@@ -249,8 +528,8 @@ impl<T: FeatureContext> Writeable for Features<T> {
 	}
 }
 
-impl<R: ::std::io::Read, T: FeatureContext> Readable<R> for Features<T> {
-	fn read(r: &mut R) -> Result<Self, DecodeError> {
+impl<T: sealed::Context> Readable for Features<T> {
+	fn read<R: ::std::io::Read>(r: &mut R) -> Result<Self, DecodeError> {
 		let mut flags: Vec<u8> = Readable::read(r)?;
 		flags.reverse(); // Swap to little-endian
 		Ok(Self {
@@ -259,3 +538,98 @@ impl<R: ::std::io::Read, T: FeatureContext> Readable<R> for Features<T> {
 		})
 	}
 }
+
+#[cfg(test)]
+mod tests {
+	use super::{ChannelFeatures, InitFeatures, NodeFeatures};
+
+	#[test]
+	fn sanity_test_known_features() {
+		assert!(!ChannelFeatures::known().requires_unknown_bits());
+		assert!(!ChannelFeatures::known().supports_unknown_bits());
+		assert!(!InitFeatures::known().requires_unknown_bits());
+		assert!(!InitFeatures::known().supports_unknown_bits());
+		assert!(!NodeFeatures::known().requires_unknown_bits());
+		assert!(!NodeFeatures::known().supports_unknown_bits());
+
+		assert!(InitFeatures::known().supports_upfront_shutdown_script());
+		assert!(NodeFeatures::known().supports_upfront_shutdown_script());
+		assert!(!InitFeatures::known().requires_upfront_shutdown_script());
+		assert!(!NodeFeatures::known().requires_upfront_shutdown_script());
+
+		assert!(InitFeatures::known().supports_data_loss_protect());
+		assert!(NodeFeatures::known().supports_data_loss_protect());
+		assert!(!InitFeatures::known().requires_data_loss_protect());
+		assert!(!NodeFeatures::known().requires_data_loss_protect());
+
+		assert!(InitFeatures::known().supports_variable_length_onion());
+		assert!(NodeFeatures::known().supports_variable_length_onion());
+		assert!(!InitFeatures::known().requires_variable_length_onion());
+		assert!(!NodeFeatures::known().requires_variable_length_onion());
+
+		assert!(!InitFeatures::known().supports_static_remote_key());
+		assert!(!NodeFeatures::known().supports_static_remote_key());
+		assert!(!InitFeatures::known().requires_static_remote_key());
+		assert!(!NodeFeatures::known().requires_static_remote_key());
+
+		assert!(InitFeatures::known().supports_payment_secret());
+		assert!(NodeFeatures::known().supports_payment_secret());
+		assert!(!InitFeatures::known().requires_payment_secret());
+		assert!(!NodeFeatures::known().requires_payment_secret());
+
+		assert!(InitFeatures::known().supports_basic_mpp());
+		assert!(NodeFeatures::known().supports_basic_mpp());
+		assert!(!InitFeatures::known().requires_basic_mpp());
+		assert!(!NodeFeatures::known().requires_basic_mpp());
+
+		let mut init_features = InitFeatures::known();
+		assert!(init_features.initial_routing_sync());
+		init_features.clear_initial_routing_sync();
+		assert!(!init_features.initial_routing_sync());
+	}
+
+	#[test]
+	fn sanity_test_unknown_bits() {
+		let mut features = ChannelFeatures::empty();
+		assert!(!features.requires_unknown_bits());
+		assert!(!features.supports_unknown_bits());
+
+		features.set_required_unknown_bits();
+		assert!(features.requires_unknown_bits());
+		assert!(features.supports_unknown_bits());
+
+		features.clear_unknown_bits();
+		assert!(!features.requires_unknown_bits());
+		assert!(!features.supports_unknown_bits());
+
+		features.set_optional_unknown_bits();
+		assert!(!features.requires_unknown_bits());
+		assert!(features.supports_unknown_bits());
+	}
+
+	#[test]
+	fn convert_to_context_with_relevant_flags() {
+		let init_features = InitFeatures::known().clear_upfront_shutdown_script();
+		assert!(init_features.initial_routing_sync());
+		assert!(!init_features.supports_upfront_shutdown_script());
+
+		let node_features: NodeFeatures = init_features.to_context();
+		{
+			// Check that the flags are as expected:
+			// - option_data_loss_protect
+			// - var_onion_optin | payment_secret
+			// - basic_mpp
+			assert_eq!(node_features.flags.len(), 3);
+			assert_eq!(node_features.flags[0], 0b00000010);
+			assert_eq!(node_features.flags[1], 0b10000010);
+			assert_eq!(node_features.flags[2], 0b00000010);
+		}
+
+		// Check that cleared flags are kept blank when converting back:
+		// - initial_routing_sync was not applicable to NodeContext
+		// - upfront_shutdown_script was cleared before converting
+		let features: InitFeatures = node_features.to_context_internal();
+		assert!(!features.initial_routing_sync());
+		assert!(!features.supports_upfront_shutdown_script());
+	}
+}