network: correct data modeling for headers messages

We modeled a Bitcoin `headers` message as being a list of block headers.
However, the actual data structure is slightly different: it's a list of (block
header, transaction count) pairs.  This caused zcashd to reject our headers
messages.

To fix this, introduce a new `CountedHeader` struct with a `block::Header` and
transaction count `usize`, then thread it through the inbound service and the
state.

I tested this locally by running Zebra with these changes and inspecting a
trace-level log of the span of a peer connection that requested a nontrivial
headers packet from us, and verified that it did not reject our message.

zebra-chain/src/block.rs

@@ -18,7 +18,7 @@ use std::fmt;
 
 pub use hash::Hash;
 pub use header::BlockTimeError;
-pub use header::Header;
+pub use header::{Header, CountedHeader};
 pub use height::Height;
 pub use root_hash::RootHash;
 

zebra-chain/src/block/header.rs

@@ -109,3 +109,12 @@ impl Header {
         }
     }
 }
+
+/// A header with a count of the number of transactions in its block.
+///
+/// This structure is used in the Bitcoin network protocol.
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct CountedHeader {
+    pub header: Header,
+    pub transaction_count: usize,
+}

zebra-chain/src/block/serialize.rs

@@ -1,12 +1,17 @@
+use std::{convert::TryInto, io};
+
 use byteorder::{LittleEndian, ReadBytesExt, WriteBytesExt};
 use chrono::{TimeZone, Utc};
-use std::io;
 
-use crate::serialization::ZcashDeserializeInto;
-use crate::serialization::{ReadZcashExt, SerializationError, ZcashDeserialize, ZcashSerialize};
-use crate::work::{difficulty::CompactDifficulty, equihash};
+use crate::{
+    serialization::{
+        ReadZcashExt, SerializationError, WriteZcashExt, ZcashDeserialize, ZcashDeserializeInto,
+        ZcashSerialize,
+    },
+    work::{difficulty::CompactDifficulty, equihash},
+};
 
-use super::{merkle, Block, Hash, Header};
+use super::{merkle, Block, CountedHeader, Hash, Header};
 
 /// The maximum size of a Zcash block, in bytes.
 ///
@@ -79,6 +84,23 @@ impl ZcashDeserialize for Header {
     }
 }
 
+impl ZcashSerialize for CountedHeader {
+    fn zcash_serialize<W: io::Write>(&self, mut writer: W) -> Result<(), io::Error> {
+        self.header.zcash_serialize(&mut writer)?;
+        writer.write_compactsize(self.transaction_count as u64)?;
+        Ok(())
+    }
+}
+
+impl ZcashDeserialize for CountedHeader {
+    fn zcash_deserialize<R: io::Read>(mut reader: R) -> Result<Self, SerializationError> {
+        Ok(CountedHeader {
+            header: (&mut reader).zcash_deserialize_into()?,
+            transaction_count: reader.read_compactsize()?.try_into().unwrap(),
+        })
+    }
+}
+
 impl ZcashSerialize for Block {
     fn zcash_serialize<W: io::Write>(&self, mut writer: W) -> Result<(), io::Error> {
         // All block structs are validated when they are parsed.

zebra-network/src/protocol/external/message.rs

@@ -183,12 +183,10 @@ pub enum Message {
     ///
     /// Returns block headers in response to a getheaders packet.
     ///
+    /// Each block header is accompanied by a transaction count.
+    ///
     /// [Bitcoin reference](https://en.bitcoin.it/wiki/Protocol_documentation#headers)
-    // Note that the block headers in this packet include a
-    // transaction count (a var_int, so there can be more than 81
-    // bytes per header) as opposed to the block headers that are
-    // hashed by miners.
-    Headers(Vec<block::Header>),
+    Headers(Vec<block::CountedHeader>),
 
     /// A `getdata` message.
     ///

zebra-network/src/protocol/internal/response.rs

@@ -22,7 +22,7 @@ pub enum Response {
     BlockHashes(Vec<block::Hash>),
 
     /// A list of block headers.
-    BlockHeaders(Vec<block::Header>),
+    BlockHeaders(Vec<block::CountedHeader>),
 
     /// A list of transactions.
     Transactions(Vec<Arc<Transaction>>),

zebra-state/src/response.rs

@@ -40,5 +40,5 @@ pub enum Response {
     BlockHashes(Vec<block::Hash>),
 
     /// The response to a `FindBlockHeaders` request.
-    BlockHeaders(Vec<block::Header>),
+    BlockHeaders(Vec<block::CountedHeader>),
 }

zebra-state/src/service.rs

@@ -644,9 +644,13 @@ impl Service<Request> for StateService {
                 let res: Vec<_> = res
                     .iter()
                     .map(|&hash| {
-                        self.best_block(hash.into())
-                            .expect("block for found hash is in the best chain")
-                            .header
+                        let block = self
+                            .best_block(hash.into())
+                            .expect("block for found hash is in the best chain");
+                        block::CountedHeader {
+                            transaction_count: block.transactions.len(),
+                            header: block.header,
+                        }
                     })
                     .collect();
                 async move { Ok(Response::BlockHeaders(res)) }.boxed()