HeaderEntry for storing subset of header info in the header MMR (#2137)

* HeaderEntry for storing subset of header info in the header MMR

* cleanup

* cleanup

chain/src/txhashset/txhashset.rs

@@ -204,8 +204,8 @@ impl TxHashSet {
 		let pos = pmmr::insertion_to_pmmr_index(height + 1);
 		let header_pmmr =
 			ReadonlyPMMR::at(&self.header_pmmr_h.backend, self.header_pmmr_h.last_pos);
-		if let Some(hash) = header_pmmr.get_data(pos) {
+		if let Some(entry) = header_pmmr.get_data(pos) {
-			let header = self.commit_index.get_block_header(&hash)?;
+			let header = self.commit_index.get_block_header(&entry.hash())?;
 			Ok(header)
 		} else {
 			Err(ErrorKind::Other(format!("get header by height")).into())
@@ -613,7 +613,7 @@ impl<'a> HeaderExtension<'a> {
 
 	/// Get the header hash for the specified pos from the underlying MMR backend.
 	fn get_header_hash(&self, pos: u64) -> Option<Hash> {
-		self.pmmr.get_data(pos)
+		self.pmmr.get_data(pos).map(|x| x.hash())
 	}
 
 	/// Get the header at the specified height based on the current state of the header extension.
@@ -989,7 +989,7 @@ impl<'a> Extension<'a> {
 
 	/// Get the header hash for the specified pos from the underlying MMR backend.
 	fn get_header_hash(&self, pos: u64) -> Option<Hash> {
-		self.header_pmmr.get_data(pos)
+		self.header_pmmr.get_data(pos).map(|x| x.hash())
 	}
 
 	/// Get the header at the specified height based on the current state of the extension.

core/src/core/block.rs

@@ -34,7 +34,7 @@ use crate::core::{
 use crate::global;
 use crate::keychain::{self, BlindingFactor};
 use crate::pow::{Difficulty, Proof, ProofOfWork};
-use crate::ser::{self, PMMRable, Readable, Reader, Writeable, Writer};
+use crate::ser::{self, FixedLength, PMMRable, Readable, Reader, Writeable, Writer};
 use crate::util::{secp, static_secp_instance};
 
 /// Errors thrown by Block validation
@@ -109,6 +109,65 @@ impl fmt::Display for Error {
 	}
 }
 
+/// Header entry for storing in the header MMR.
+/// Note: we hash the block header itself and maintain the hash in the entry.
+/// This allows us to lookup the original header from the db as necessary.
+pub struct HeaderEntry {
+	hash: Hash,
+	timestamp: u64,
+	total_difficulty: Difficulty,
+	secondary_scaling: u32,
+	is_secondary: bool,
+}
+
+impl Readable for HeaderEntry {
+	fn read(reader: &mut Reader) -> Result<HeaderEntry, ser::Error> {
+		let hash = Hash::read(reader)?;
+		let timestamp = reader.read_u64()?;
+		let total_difficulty = Difficulty::read(reader)?;
+		let secondary_scaling = reader.read_u32()?;
+
+		// Using a full byte to represent the bool for now.
+		let is_secondary = reader.read_u8()? != 0;
+
+		Ok(HeaderEntry {
+			hash,
+			timestamp,
+			total_difficulty,
+			secondary_scaling,
+			is_secondary,
+		})
+	}
+}
+
+impl Writeable for HeaderEntry {
+	fn write<W: Writer>(&self, writer: &mut W) -> Result<(), ser::Error> {
+		self.hash.write(writer)?;
+		writer.write_u64(self.timestamp)?;
+		self.total_difficulty.write(writer)?;
+		writer.write_u32(self.secondary_scaling)?;
+
+		// Using a full byte to represent the bool for now.
+		if self.is_secondary {
+			writer.write_u8(1)?;
+		} else {
+			writer.write_u8(0)?;
+		}
+		Ok(())
+	}
+}
+
+impl FixedLength for HeaderEntry {
+	const LEN: usize = Hash::LEN + 8 + Difficulty::LEN + 4 + 1;
+}
+
+impl HeaderEntry {
+	/// The hash of the underlying block.
+	pub fn hash(&self) -> Hash {
+		self.hash
+	}
+}
+
 /// Block header, fairly standard compared to other blockchains.
 #[derive(Clone, Debug, PartialEq)]
 pub struct BlockHeader {
@@ -160,10 +219,16 @@ impl Default for BlockHeader {
 }
 
 impl PMMRable for BlockHeader {
-	type E = Hash;
+	type E = HeaderEntry;
 
 	fn as_elmt(&self) -> Self::E {
-		self.hash()
+		HeaderEntry {
+			hash: self.hash(),
+			timestamp: self.timestamp.timestamp() as u64,
+			total_difficulty: self.total_difficulty(),
+			secondary_scaling: self.pow.secondary_scaling,
+			is_secondary: self.pow.is_secondary(),
+		}
 	}
 }