Avoid allocing a new buffer for each protobuf payload (#192)

This commit avoids creating a new buffer for every protobuf payload
the native source receives. There's still some temporary objects
being made--the implicit CodedInputStream could be reused--but it's
a start.

Signed-off-by: Brian L. Troutwine <blt@postmates.com>

src/source/native.rs

@@ -1,8 +1,8 @@
 use byteorder::{BigEndian, ReadBytesExt};
 use hopper;
 use metric;
-use protobuf::parse_from_bytes;
 use protocols::native::{AggregationMethod, Payload};
+use protobuf;
 use std::io;
 use std::io::Read;
 use std::net::{TcpListener, TcpStream, ToSocketAddrs};
@@ -63,16 +63,17 @@ fn handle_tcp(chans: util::Channel,
 fn handle_stream(mut chans: util::Channel, tags: metric::TagMap, stream: TcpStream) {
  thread::spawn(move || {
  let mut reader = io::BufReader::new(stream);
+ let mut buf = Vec::with_capacity(4000);
  loop {
  let payload_size_in_bytes = match reader.read_u32::<BigEndian>() {
- Ok(i) => i,
+ Ok(i) => i as usize,
  Err(_) => return,
  };
- let mut buf = vec![0; payload_size_in_bytes as usize];
+ buf.resize(payload_size_in_bytes, 0);
  if reader.read_exact(&mut buf).is_err() {
  return;
  }
- match parse_from_bytes::<Payload>(&buf) {
+ match protobuf::parse_from_bytes::<Payload>(&buf) {
  Ok(pyld) => {
  for point in pyld.get_points() {
  let name: &str = point.get_name();