Compute the expected body length to reduce heap allocations (#1773)

* Compute the expected body length to reduce heap allocations

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

@@ -4,7 +4,7 @@ use std::fmt;
 use std::io::{Cursor, Read, Write};
 
 use byteorder::{LittleEndian, ReadBytesExt, WriteBytesExt};
-use bytes::BytesMut;
+use bytes::{BufMut, BytesMut};
 use chrono::{TimeZone, Utc};
 use tokio_util::codec::{Decoder, Encoder};
 
@@ -109,20 +109,15 @@ impl Encoder<Message> for Codec {
 
     fn encode(&mut self, item: Message, dst: &mut BytesMut) -> Result<(), Self::Error> {
         use Error::Parse;
-        // XXX(HACK): this is inefficient and does an extra allocation.
-        // instead, we should have a size estimator for the message, reserve
-        // that much space, write the header (with zeroed checksum), then the body,
-        // then write the computed checksum in-place.  for now, just do an extra alloc.
 
-        let mut body = Vec::new();
-        self.write_body(&item, &mut body)?;
+        let body_length = self.body_length(&item);
 
-        if body.len() > self.builder.max_len {
+        if body_length > self.builder.max_len {
             return Err(Parse("body length exceeded maximum size"));
         }
 
         if let Some(label) = self.builder.metrics_label.clone() {
-            metrics::counter!("bytes.written", (body.len() + HEADER_LEN) as u64, "addr" =>  label);
+            metrics::counter!("bytes.written", (body_length + HEADER_LEN) as u64, "addr" => label);
         }
 
         use Message::*;
@@ -152,26 +147,58 @@ impl Encoder<Message> for Codec {
             FilterAdd { .. } => b"filteradd\0\0\0",
             FilterClear { .. } => b"filterclear\0",
         };
-        trace!(?item, len = body.len());
+        trace!(?item, len = body_length);
 
-        // XXX this should write directly into the buffer,
-        // but leave it for now until we fix the issue above.
-        let mut header = [0u8; HEADER_LEN];
-        let mut header_writer = Cursor::new(&mut header[..]);
-        header_writer.write_all(&Magic::from(self.builder.network).0[..])?;
-        header_writer.write_all(command)?;
-        header_writer.write_u32::<LittleEndian>(body.len() as u32)?;
-        header_writer.write_all(&sha256d::Checksum::from(&body[..]).0)?;
+        dst.reserve(HEADER_LEN + body_length);
+        let start_len = dst.len();
+        {
+            let dst = &mut dst.writer();
+            dst.write_all(&Magic::from(self.builder.network).0[..])?;
+            dst.write_all(command)?;
+            dst.write_u32::<LittleEndian>(body_length as u32)?;
 
-        dst.reserve(HEADER_LEN + body.len());
-        dst.extend_from_slice(&header);
-        dst.extend_from_slice(&body);
+            // We zero the checksum at first, and compute it later
+            // after the body has been written.
+            dst.write_u32::<LittleEndian>(0)?;
+
+            self.write_body(&item, dst)?;
+        }
+        let checksum = sha256d::Checksum::from(&dst[start_len + HEADER_LEN..]);
+        dst[start_len + 20..][..4].copy_from_slice(&checksum.0);
 
         Ok(())
     }
 }
 
 impl Codec {
+    /// Obtain the size of the body of a given message. This will match the
+    /// number of bytes written to the writer provided to `write_body` for the
+    /// same message.
+    ///
+    /// TODO: Replace with a size estimate, to avoid multiple serializations
+    /// for large data structures like lists, blocks, and transactions.
+    /// See #1774.
+    fn body_length(&self, msg: &Message) -> usize {
+        struct FakeWriter(usize);
+
+        impl std::io::Write for FakeWriter {
+            fn write(&mut self, buf: &[u8]) -> std::io::Result<usize> {
+                self.0 += buf.len();
+
+                Ok(buf.len())
+            }
+
+            fn flush(&mut self) -> std::io::Result<()> {
+                Ok(())
+            }
+        }
+
+        let mut writer = FakeWriter(0);
+        self.write_body(msg, &mut writer)
+            .expect("writer should never fail");
+        writer.0
+    }
+
     /// Write the body of the message into the given writer. This allows writing
     /// the message body prior to writing the header, so that the header can
     /// contain a checksum of the message body.