deno_core (#1827)

A new low-level crate with focus on speed. 
This doesn't yet hook into the existing code base.

26 files changed, 1478 insertions, 120 deletions

diff --git a/BUILD.gn b/BUILD.gn
index cda8d1ffb..461e5fcd1 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -13,6 +13,8 @@ group("default") {
     ":deno",
     ":hyper_hello",
     ":test_rs",
+    "core:deno_core_http_bench",
+    "core:deno_core_test",
     "libdeno:test_cc",
   ]
 }
diff --git a/core/BUILD.gn b/core/BUILD.gn
new file mode 100644
index 000000000..be06a7f4b
--- /dev/null
+++ b/core/BUILD.gn
@@ -0,0 +1,37 @@
+import("//build_extra/rust/rust.gni")
+
+# deno_core does not depend on flatbuffers nor tokio.
+main_extern = [
+  "$rust_build:futures",
+  "$rust_build:libc",
+  "$rust_build:serde_json",
+  "$rust_build:log",
+]
+
+rust_crate("deno_core") {
+  source_root = "lib.rs"
+  extern = main_extern
+  deps = [
+    "../libdeno:libdeno_static_lib",
+  ]
+}
+
+rust_test("deno_core_test") {
+  source_root = "lib.rs"
+  extern = main_extern
+  deps = [
+    "../libdeno:libdeno_static_lib",
+  ]
+}
+
+rust_executable("deno_core_http_bench") {
+  source_root = "http_bench.rs"
+  extern = [
+    "$rust_build:futures",
+    "$rust_build:lazy_static",
+    "$rust_build:libc",
+    "$rust_build:log",
+    "$rust_build:tokio",
+    ":deno_core"
+  ]
+}
diff --git a/core/http_bench.js b/core/http_bench.js
new file mode 100644
index 000000000..b9615e689
--- /dev/null
+++ b/core/http_bench.js
@@ -0,0 +1,150 @@
+// This is not a real HTTP server. We read blindly one time into 'requestBuf',
+// then write this fixed 'responseBuf'. The point of this benchmark is to
+// exercise the event loop in a simple yet semi-realistic way.
+const shared32 = new Int32Array(libdeno.shared);
+
+const INDEX_NUM_RECORDS = 0;
+const INDEX_RECORDS = 1;
+const RECORD_OFFSET_PROMISE_ID = 0;
+const RECORD_OFFSET_OP = 1;
+const RECORD_OFFSET_ARG = 2;
+const RECORD_OFFSET_RESULT = 3;
+const RECORD_SIZE = 4;
+const OP_LISTEN = 1;
+const OP_ACCEPT = 2;
+const OP_READ = 3;
+const OP_WRITE = 4;
+const OP_CLOSE = 5;
+
+const NUM_RECORDS = (shared32.length - INDEX_RECORDS) / RECORD_SIZE;
+if (NUM_RECORDS != 100) {
+  throw Error("expected 100 entries");
+}
+
+const requestBuf = new Uint8Array(64 * 1024);
+const responseBuf = new Uint8Array(
+  "HTTP/1.1 200 OK\r\nContent-Length: 12\r\n\r\nHello World\n"
+    .split("")
+    .map(c => c.charCodeAt(0))
+);
+
+const promiseMap = new Map();
+let nextPromiseId = 1;
+
+function createResolvable() {
+  let methods;
+  const promise = new Promise((resolve, reject) => {
+    methods = { resolve, reject };
+  });
+  return Object.assign(promise, methods);
+}
+
+/** Returns Promise<number> */
+function sendAsync(op, arg, zeroCopyData) {
+  const id = nextPromiseId++;
+  const p = createResolvable();
+  shared32[INDEX_NUM_RECORDS] = 1;
+  setRecord(0, RECORD_OFFSET_PROMISE_ID, id);
+  setRecord(0, RECORD_OFFSET_OP, op);
+  setRecord(0, RECORD_OFFSET_ARG, arg);
+  setRecord(0, RECORD_OFFSET_RESULT, -1);
+  promiseMap.set(id, p);
+  libdeno.send(null, zeroCopyData);
+  return p;
+}
+
+/** Returns u32 number */
+function sendSync(op, arg) {
+  shared32[INDEX_NUM_RECORDS] = 1;
+  setRecord(0, RECORD_OFFSET_PROMISE_ID, 0);
+  setRecord(0, RECORD_OFFSET_OP, op);
+  setRecord(0, RECORD_OFFSET_ARG, arg);
+  setRecord(0, RECORD_OFFSET_RESULT, -1);
+  libdeno.send();
+  return getRecord(0, RECORD_OFFSET_RESULT);
+}
+
+function setRecord(i, off, value) {
+  if (i >= NUM_RECORDS) {
+    throw Error("out of range");
+  }
+  shared32[INDEX_RECORDS + RECORD_SIZE * i + off] = value;
+}
+
+function getRecord(i, off) {
+  if (i >= NUM_RECORDS) {
+    throw Error("out of range");
+  }
+  return shared32[INDEX_RECORDS + RECORD_SIZE * i + off];
+}
+
+function handleAsyncMsgFromRust() {
+  for (let i = 0; i < shared32[INDEX_NUM_RECORDS]; i++) {
+    let id = getRecord(i, RECORD_OFFSET_PROMISE_ID);
+    const p = promiseMap.get(id);
+    promiseMap.delete(id);
+    p.resolve(getRecord(i, RECORD_OFFSET_RESULT));
+  }
+}
+
+/** Listens on 0.0.0.0:4500, returns rid. */
+function listen() {
+  return sendSync(OP_LISTEN, -1);
+}
+
+/** Accepts a connection, returns rid. */
+async function accept(rid) {
+  return await sendAsync(OP_ACCEPT, rid);
+}
+
+/**
+ * Reads a packet from the rid, presumably an http request. data is ignored.
+ * Returns bytes read.
+ */
+async function read(rid, data) {
+  return await sendAsync(OP_READ, rid, data);
+}
+
+/** Writes a fixed HTTP response to the socket rid. Returns bytes written. */
+async function write(rid, data) {
+  return await sendAsync(OP_WRITE, rid, data);
+}
+
+function close(rid) {
+  return sendSync(OP_CLOSE, rid);
+}
+
+async function serve(rid) {
+  while (true) {
+    const nread = await read(rid, requestBuf);
+    if (nread <= 0) {
+      break;
+    }
+
+    const nwritten = await write(rid, responseBuf);
+    if (nwritten < 0) {
+      break;
+    }
+  }
+  close(rid);
+}
+
+async function main() {
+  libdeno.recv(handleAsyncMsgFromRust);
+
+  libdeno.print("http_bench.js start");
+
+  const listener_rid = listen();
+  libdeno.print(`listening http://127.0.0.1:4544/ rid = ${listener_rid}`);
+  while (true) {
+    const rid = await accept(listener_rid);
+    // libdeno.print(`accepted ${rid}`);
+    if (rid < 0) {
+      libdeno.print(`accept error ${rid}`);
+      return;
+    }
+    serve(rid);
+  }
+}
+
+main();
diff --git a/core/http_bench.rs b/core/http_bench.rs
new file mode 100644
index 000000000..3da30433a
--- /dev/null
+++ b/core/http_bench.rs
@@ -0,0 +1,210 @@
+/// To run this benchmark:
+///
+/// > DENO_BUILD_MODE=release ./tools/build.py && \
+///   ./target/release/deno_core_http_bench --multi-thread
+extern crate deno_core;
+extern crate futures;
+extern crate libc;
+extern crate tokio;
+
+#[macro_use]
+extern crate log;
+#[macro_use]
+extern crate lazy_static;
+
+use deno_core::deno_buf;
+use deno_core::AsyncResult;
+use deno_core::Isolate;
+use deno_core::JSError;
+use deno_core::Op;
+use deno_core::RECORD_OFFSET_ARG;
+use deno_core::RECORD_OFFSET_OP;
+use deno_core::RECORD_OFFSET_PROMISE_ID;
+use deno_core::RECORD_OFFSET_RESULT;
+use futures::future::lazy;
+use std::collections::HashMap;
+use std::env;
+use std::net::SocketAddr;
+use std::sync::atomic::AtomicUsize;
+use std::sync::atomic::Ordering;
+use std::sync::Mutex;
+use tokio::prelude::*;
+
+const OP_LISTEN: i32 = 1;
+const OP_ACCEPT: i32 = 2;
+const OP_READ: i32 = 3;
+const OP_WRITE: i32 = 4;
+const OP_CLOSE: i32 = 5;
+
+fn main() {
+  let js_source = include_str!("http_bench.js");
+  let isolate = deno_core::Isolate::new(recv_cb);
+
+  let main_future = lazy(move || {
+    // TODO currently isolate.execute() must be run inside tokio, hence the
+    // lazy(). It would be nice to not have that contraint. Probably requires
+    // using v8::MicrotasksPolicy::kExplicit
+    js_check(isolate.execute("http_bench.js", js_source));
+    isolate.then(|r| {
+      js_check(r);
+      Ok(())
+    })
+  });
+
+  let args: Vec<String> = env::args().collect();
+  if args.len() > 1 && args[1] == "--multi-thread" {
+    println!("multi-thread");
+    tokio::run(main_future);
+  } else {
+    println!("single-thread");
+    tokio::runtime::current_thread::run(main_future);
+  }
+}
+
+enum Repr {
+  TcpListener(tokio::net::TcpListener),
+  TcpStream(tokio::net::TcpStream),
+}
+
+type ResourceTable = HashMap<i32, Repr>;
+lazy_static! {
+  static ref RESOURCE_TABLE: Mutex<ResourceTable> = Mutex::new(HashMap::new());
+  static ref NEXT_RID: AtomicUsize = AtomicUsize::new(3);
+}
+
+fn new_rid() -> i32 {
+  let rid = NEXT_RID.fetch_add(1, Ordering::SeqCst);
+  rid as i32
+}
+
+fn recv_cb(isolate: &mut Isolate, zero_copy_buf: deno_buf) {
+  isolate.test_send_counter += 1; // TODO ideally store this in isolate.state?
+
+  let promise_id = isolate.shared.get_record(0, RECORD_OFFSET_PROMISE_ID);
+  let op_id = isolate.shared.get_record(0, RECORD_OFFSET_OP);
+  let arg = isolate.shared.get_record(0, RECORD_OFFSET_ARG);
+
+  // dbg!(promise_id);
+  // dbg!(op_id);
+  // dbg!(arg);
+
+  let is_sync = promise_id == 0;
+
+  if is_sync {
+    // sync ops
+    match op_id {
+      OP_CLOSE => {
+        debug!("close");
+        assert!(is_sync);
+        let mut table = RESOURCE_TABLE.lock().unwrap();
+        let r = table.remove(&arg);
+        isolate.shared.set_record(
+          0,
+          RECORD_OFFSET_RESULT,
+          if r.is_some() { 0 } else { -1 },
+        );
+      }
+      OP_LISTEN => {
+        debug!("listen");
+        assert!(is_sync);
+
+        let addr = "127.0.0.1:4544".parse::<SocketAddr>().unwrap();
+        let listener = tokio::net::TcpListener::bind(&addr).unwrap();
+        let rid = new_rid();
+        isolate.shared.set_record(0, RECORD_OFFSET_RESULT, rid);
+        let mut guard = RESOURCE_TABLE.lock().unwrap();
+        guard.insert(rid, Repr::TcpListener(listener));
+      }
+      _ => panic!("bad op"),
+    }
+  } else {
+    // async ops
+    let zero_copy_id = zero_copy_buf.zero_copy_id;
+    let op = match op_id {
+      OP_ACCEPT => {
+        let listener_rid = arg;
+        op_accept(listener_rid)
+      }
+      OP_READ => {
+        let rid = arg;
+        op_read(rid, zero_copy_buf)
+      }
+      OP_WRITE => {
+        let rid = arg;
+        op_write(rid, zero_copy_buf)
+      }
+      _ => panic!("bad op"),
+    };
+    isolate.add_op(promise_id, op, zero_copy_id);
+  }
+}
+
+fn op_accept(listener_rid: i32) -> Box<Op> {
+  debug!("accept {}", listener_rid);
+  Box::new(
+    futures::future::poll_fn(move || {
+      let mut table = RESOURCE_TABLE.lock().unwrap();
+      let maybe_repr = table.get_mut(&listener_rid);
+      match maybe_repr {
+        Some(Repr::TcpListener(ref mut listener)) => listener.poll_accept(),
+        _ => panic!("bad rid"),
+      }
+    }).and_then(move |(stream, addr)| {
+      debug!("accept success {}", addr);
+      let rid = new_rid();
+
+      let mut guard = RESOURCE_TABLE.lock().unwrap();
+      guard.insert(rid, Repr::TcpStream(stream));
+
+      Ok(AsyncResult { result: rid })
+    }),
+  )
+}
+
+fn op_read(rid: i32, mut zero_copy_buf: deno_buf) -> Box<Op> {
+  debug!("read rid={}", rid);
+  Box::new(
+    futures::future::poll_fn(move || {
+      let mut table = RESOURCE_TABLE.lock().unwrap();
+      let maybe_repr = table.get_mut(&rid);
+      match maybe_repr {
+        Some(Repr::TcpStream(ref mut stream)) => {
+          stream.poll_read(&mut zero_copy_buf)
+        }
+        _ => panic!("bad rid"),
+      }
+    }).and_then(move |nread| {
+      debug!("read success {}", nread);
+      Ok(AsyncResult {
+        result: nread as i32,
+      })
+    }),
+  )
+}
+
+fn op_write(rid: i32, zero_copy_buf: deno_buf) -> Box<Op> {
+  debug!("write rid={}", rid);
+  Box::new(
+    futures::future::poll_fn(move || {
+      let mut table = RESOURCE_TABLE.lock().unwrap();
+      let maybe_repr = table.get_mut(&rid);
+      match maybe_repr {
+        Some(Repr::TcpStream(ref mut stream)) => {
+          stream.poll_write(&zero_copy_buf)
+        }
+        _ => panic!("bad rid"),
+      }
+    }).and_then(move |nwritten| {
+      debug!("write success {}", nwritten);
+      Ok(AsyncResult {
+        result: nwritten as i32,
+      })
+    }),
+  )
+}
+
+fn js_check(r: Result<(), JSError>) {
+  if let Err(e) = r {
+    panic!(e.to_string());
+  }
+}
diff --git a/core/js_errors.rs b/core/js_errors.rs
new file mode 100644
index 000000000..c07af136f
--- /dev/null
+++ b/core/js_errors.rs
@@ -0,0 +1,416 @@
+// Copyright 2018-2019 the Deno authors. All rights reserved. MIT license.
+
+// Note that source_map_mappings requires 0-indexed line and column numbers but
+// V8 Exceptions are 1-indexed.
+
+// TODO: This currently only applies to uncaught exceptions. It would be nice to
+// also have source maps for situations like this:
+//   const err = new Error("Boo!");
+//   console.log(err.stack);
+// It would require calling into Rust from Error.prototype.prepareStackTrace.
+
+use serde_json;
+use std::fmt;
+use std::str;
+
+#[derive(Debug, PartialEq)]
+pub struct StackFrame {
+  pub line: i64,   // zero indexed
+  pub column: i64, // zero indexed
+  pub script_name: String,
+  pub function_name: String,
+  pub is_eval: bool,
+  pub is_constructor: bool,
+  pub is_wasm: bool,
+}
+
+#[derive(Debug, PartialEq)]
+pub struct JSError {
+  pub message: String,
+
+  pub source_line: Option<String>,
+  pub script_resource_name: Option<String>,
+  pub line_number: Option<i64>,
+  pub start_position: Option<i64>,
+  pub end_position: Option<i64>,
+  pub error_level: Option<i64>,
+  pub start_column: Option<i64>,
+  pub end_column: Option<i64>,
+
+  pub frames: Vec<StackFrame>,
+}
+
+impl fmt::Display for StackFrame {
+  fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+    // Note when we print to string, we change from 0-indexed to 1-indexed.
+    let function_name = self.function_name.clone();
+    let script_line_column =
+      format_script_line_column(&self.script_name, self.line, self.column);
+
+    if !self.function_name.is_empty() {
+      write!(f, "    at {} ({})", function_name, script_line_column)
+    } else if self.is_eval {
+      write!(f, "    at eval ({})", script_line_column)
+    } else {
+      write!(f, "    at {}", script_line_column)
+    }
+  }
+}
+
+fn format_script_line_column(
+  script_name: &str,
+  line: i64,
+  column: i64,
+) -> String {
+  // TODO match this style with how typescript displays errors.
+  let line = (1 + line).to_string();
+  let column = (1 + column).to_string();
+  let script_name = script_name.to_string();
+  format!("{}:{}:{}", script_name, line, column)
+}
+
+impl fmt::Display for JSError {
+  fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+    if self.script_resource_name.is_some() {
+      let script_resource_name = self.script_resource_name.as_ref().unwrap();
+      // Avoid showing internal code from gen/bundle/main.js
+      if script_resource_name != "gen/bundle/main.js"
+        && script_resource_name != "gen/bundle/compiler.js"
+      {
+        if self.line_number.is_some() && self.start_column.is_some() {
+          assert!(self.line_number.is_some());
+          assert!(self.start_column.is_some());
+          let script_line_column = format_script_line_column(
+            script_resource_name,
+            self.line_number.unwrap() - 1,
+            self.start_column.unwrap() - 1,
+          );
+          write!(f, "{}", script_line_column)?;
+        }
+        if self.source_line.is_some() {
+          write!(f, "\n{}\n", self.source_line.as_ref().unwrap())?;
+          let mut s = String::new();
+          for i in 0..self.end_column.unwrap() {
+            if i >= self.start_column.unwrap() {
+              s.push('^');
+            } else {
+              s.push(' ');
+            }
+          }
+          writeln!(f, "{}", s)?;
+        }
+      }
+    }
+
+    write!(f, "{}", self.message.clone())?;
+
+    for frame in &self.frames {
+      write!(f, "\n{}", &frame.to_string())?;
+    }
+    Ok(())
+  }
+}
+
+impl StackFrame {
+  // TODO Maybe use serde_derive?
+  fn from_json_value(v: &serde_json::Value) -> Option<Self> {
+    if !v.is_object() {
+      return None;
+    }
+    let obj = v.as_object().unwrap();
+
+    let line_v = &obj["line"];
+    if !line_v.is_u64() {
+      return None;
+    }
+    let line = line_v.as_u64().unwrap() as i64;
+
+    let column_v = &obj["column"];
+    if !column_v.is_u64() {
+      return None;
+    }
+    let column = column_v.as_u64().unwrap() as i64;
+
+    let script_name_v = &obj["scriptName"];
+    if !script_name_v.is_string() {
+      return None;
+    }
+    let script_name = String::from(script_name_v.as_str().unwrap());
+
+    // Optional fields. See EncodeExceptionAsJSON() in libdeno.
+    // Sometimes V8 doesn't provide all the frame information.
+
+    let mut function_name = String::from(""); // default
+    if obj.contains_key("functionName") {
+      let function_name_v = &obj["functionName"];
+      if function_name_v.is_string() {
+        function_name = String::from(function_name_v.as_str().unwrap());
+      }
+    }
+
+    let mut is_eval = false; // default
+    if obj.contains_key("isEval") {
+      let is_eval_v = &obj["isEval"];
+      if is_eval_v.is_boolean() {
+        is_eval = is_eval_v.as_bool().unwrap();
+      }
+    }
+
+    let mut is_constructor = false; // default
+    if obj.contains_key("isConstructor") {
+      let is_constructor_v = &obj["isConstructor"];
+      if is_constructor_v.is_boolean() {
+        is_constructor = is_constructor_v.as_bool().unwrap();
+      }
+    }
+
+    let mut is_wasm = false; // default
+    if obj.contains_key("isWasm") {
+      let is_wasm_v = &obj["isWasm"];
+      if is_wasm_v.is_boolean() {
+        is_wasm = is_wasm_v.as_bool().unwrap();
+      }
+    }
+
+    Some(StackFrame {
+      line: line - 1,
+      column: column - 1,
+      script_name,
+      function_name,
+      is_eval,
+      is_constructor,
+      is_wasm,
+    })
+  }
+}
+
+impl JSError {
+  /// Creates a new JSError by parsing the raw exception JSON string from V8.
+  pub fn from_v8_exception(json_str: &str) -> Option<Self> {
+    let v = serde_json::from_str::<serde_json::Value>(json_str);
+    if v.is_err() {
+      return None;
+    }
+    let v = v.unwrap();
+
+    if !v.is_object() {
+      return None;
+    }
+    let obj = v.as_object().unwrap();
+
+    let message_v = &obj["message"];
+    if !message_v.is_string() {
+      return None;
+    }
+    let message = String::from(message_v.as_str().unwrap());
+
+    let source_line = obj
+      .get("sourceLine")
+      .and_then(|v| v.as_str().map(String::from));
+    let script_resource_name = obj
+      .get("scriptResourceName")
+      .and_then(|v| v.as_str().map(String::from));
+    let line_number = obj.get("lineNumber").and_then(|v| v.as_i64());
+    let start_position = obj.get("startPosition").and_then(|v| v.as_i64());
+    let end_position = obj.get("endPosition").and_then(|v| v.as_i64());
+    let error_level = obj.get("errorLevel").and_then(|v| v.as_i64());
+    let start_column = obj.get("startColumn").and_then(|v| v.as_i64());
+    let end_column = obj.get("endColumn").and_then(|v| v.as_i64());
+
+    let frames_v = &obj["frames"];
+    if !frames_v.is_array() {
+      return None;
+    }
+    let frame_values = frames_v.as_array().unwrap();
+
+    let mut frames = Vec::<StackFrame>::new();
+    for frame_v in frame_values {
+      match StackFrame::from_json_value(frame_v) {
+        None => return None,
+        Some(frame) => frames.push(frame),
+      }
+    }
+
+    Some(JSError {
+      message,
+      source_line,
+      script_resource_name,
+      line_number,
+      start_position,
+      end_position,
+      error_level,
+      start_column,
+      end_column,
+      frames,
+    })
+  }
+}
+
+#[cfg(test)]
+mod tests {
+  use super::*;
+
+  fn error1() -> JSError {
+    JSError {
+      message: "Error: foo bar".to_string(),
+      source_line: None,
+      script_resource_name: None,
+      line_number: None,
+      start_position: None,
+      end_position: None,
+      error_level: None,
+      start_column: None,
+      end_column: None,
+      frames: vec![
+        StackFrame {
+          line: 4,
+          column: 16,
+          script_name: "foo_bar.ts".to_string(),
+          function_name: "foo".to_string(),
+          is_eval: false,
+          is_constructor: false,
+          is_wasm: false,
+        },
+        StackFrame {
+          line: 5,
+          column: 20,
+          script_name: "bar_baz.ts".to_string(),
+          function_name: "qat".to_string(),
+          is_eval: false,
+          is_constructor: false,
+          is_wasm: false,
+        },
+        StackFrame {
+          line: 1,
+          column: 1,
+          script_name: "deno_main.js".to_string(),
+          function_name: "".to_string(),
+          is_eval: false,
+          is_constructor: false,
+          is_wasm: false,
+        },
+      ],
+    }
+  }
+
+  #[test]
+  fn stack_frame_from_json_value_1() {
+    let v = serde_json::from_str::<serde_json::Value>(
+      r#"{
+        "line":2,
+        "column":11,
+        "functionName":"foo",
+        "scriptName":"/Users/rld/src/deno/tests/error_001.ts",
+        "isEval":true,
+        "isConstructor":false,
+        "isWasm":false
+      }"#,
+    ).unwrap();
+    let r = StackFrame::from_json_value(&v);
+    assert_eq!(
+      r,
+      Some(StackFrame {
+        line: 1,
+        column: 10,
+        script_name: "/Users/rld/src/deno/tests/error_001.ts".to_string(),
+        function_name: "foo".to_string(),
+        is_eval: true,
+        is_constructor: false,
+        is_wasm: false,
+      })
+    );
+  }
+
+  #[test]
+  fn stack_frame_from_json_value_2() {
+    let v = serde_json::from_str::<serde_json::Value>(
+      r#"{
+        "scriptName": "/Users/rld/src/deno/tests/error_001.ts",
+        "line": 2,
+        "column": 11
+      }"#,
+    ).unwrap();
+    let r = StackFrame::from_json_value(&v);
+    assert!(r.is_some());
+    let f = r.unwrap();
+    assert_eq!(f.line, 1);
+    assert_eq!(f.column, 10);
+    assert_eq!(f.script_name, "/Users/rld/src/deno/tests/error_001.ts");
+  }
+
+  #[test]
+  fn js_error_from_v8_exception() {
+    let r = JSError::from_v8_exception(
+      r#"{
+        "message":"Uncaught Error: bad",
+        "frames":[
+          {
+            "line":2,
+            "column":11,
+            "functionName":"foo",
+            "scriptName":"/Users/rld/src/deno/tests/error_001.ts",
+            "isEval":true,
+            "isConstructor":false,
+            "isWasm":false
+          }, {
+            "line":5,
+            "column":5,
+            "functionName":"bar",
+            "scriptName":"/Users/rld/src/deno/tests/error_001.ts",
+            "isEval":true,
+            "isConstructor":false,
+            "isWasm":false
+          }
+        ]}"#,
+    );
+    assert!(r.is_some());
+    let e = r.unwrap();
+    assert_eq!(e.message, "Uncaught Error: bad");
+    assert_eq!(e.frames.len(), 2);
+    assert_eq!(
+      e.frames[0],
+      StackFrame {
+        line: 1,
+        column: 10,
+        script_name: "/Users/rld/src/deno/tests/error_001.ts".to_string(),
+        function_name: "foo".to_string(),
+        is_eval: true,
+        is_constructor: false,
+        is_wasm: false,
+      }
+    )
+  }
+
+  #[test]
+  fn js_error_from_v8_exception2() {
+    let r = JSError::from_v8_exception(
+      "{\"message\":\"Error: boo\",\"sourceLine\":\"throw Error('boo');\",\"scriptResourceName\":\"a.js\",\"lineNumber\":3,\"startPosition\":8,\"endPosition\":9,\"errorLevel\":8,\"startColumn\":6,\"endColumn\":7,\"isSharedCrossOrigin\":false,\"isOpaque\":false,\"frames\":[{\"line\":3,\"column\":7,\"functionName\":\"\",\"scriptName\":\"a.js\",\"isEval\":false,\"isConstructor\":false,\"isWasm\":false}]}"
+    );
+    assert!(r.is_some());
+    let e = r.unwrap();
+    assert_eq!(e.message, "Error: boo");
+    assert_eq!(e.source_line, Some("throw Error('boo');".to_string()));
+    assert_eq!(e.script_resource_name, Some("a.js".to_string()));
+    assert_eq!(e.line_number, Some(3));
+    assert_eq!(e.start_position, Some(8));
+    assert_eq!(e.end_position, Some(9));
+    assert_eq!(e.error_level, Some(8));
+    assert_eq!(e.start_column, Some(6));
+    assert_eq!(e.end_column, Some(7));
+    assert_eq!(e.frames.len(), 1);
+  }
+
+  #[test]
+  fn stack_frame_to_string() {
+    let e = error1();
+    assert_eq!("    at foo (foo_bar.ts:5:17)", &e.frames[0].to_string());
+    assert_eq!("    at qat (bar_baz.ts:6:21)", &e.frames[1].to_string());
+  }
+
+  #[test]
+  fn js_error_to_string() {
+    let e = error1();
+    let expected = "Error: foo bar\n    at foo (foo_bar.ts:5:17)\n    at qat (bar_baz.ts:6:21)\n    at deno_main.js:2:2";
+    assert_eq!(expected, &e.to_string());
+  }
+}
diff --git a/core/lib.rs b/core/lib.rs
new file mode 100644
index 000000000..d13339ee5
--- /dev/null
+++ b/core/lib.rs
@@ -0,0 +1,364 @@
+#[macro_use]
+extern crate log;
+extern crate futures;
+extern crate libc;
+
+mod js_errors;
+mod libdeno;
+mod shared;
+
+pub use crate::js_errors::JSError;
+pub use crate::libdeno::deno_buf;
+pub use crate::shared::*;
+use futures::Async;
+use futures::Future;
+use futures::Poll;
+use libc::c_void;
+use std::collections::HashMap;
+use std::ffi::CStr;
+use std::ffi::CString;
+use std::sync::{Once, ONCE_INIT};
+
+pub struct Isolate {
+  libdeno_isolate: *const libdeno::isolate,
+  pending_ops: HashMap<i32, PendingOp>, // promise_id -> op
+  polled_recently: bool,
+  recv_cb: RecvCallback,
+
+  pub shared: Shared,
+  pub test_send_counter: u32, // TODO only used for testing- REMOVE.
+}
+
+pub type RecvCallback = fn(isolate: &mut Isolate, zero_copy_buf: deno_buf);
+
+pub const NUM_RECORDS: usize = 100;
+
+// TODO rename to AsyncResult
+pub struct AsyncResult {
+  pub result: i32,
+}
+
+pub type Op = dyn Future<Item = AsyncResult, Error = std::io::Error> + Send;
+
+struct PendingOp {
+  op: Box<Op>,
+  polled_recently: bool,
+  zero_copy_id: usize, // non-zero if associated zero-copy buffer.
+}
+
+static DENO_INIT: Once = ONCE_INIT;
+
+unsafe impl Send for Isolate {}
+
+impl Isolate {
+  pub fn new(recv_cb: RecvCallback) -> Self {
+    DENO_INIT.call_once(|| {
+      unsafe { libdeno::deno_init() };
+    });
+
+    // Allocate unmanaged memory for the shared buffer by creating a Vec<u8>,
+    // grabbing the raw pointer, and then leaking the Vec so it is never freed.
+    let mut shared = Shared::new();
+    let shared_deno_buf = shared.as_deno_buf();
+
+    let config = libdeno::deno_config {
+      will_snapshot: 0,
+      load_snapshot: deno_buf::empty(), // TODO
+      shared: shared_deno_buf,
+      recv_cb: pre_dispatch,
+    };
+    let libdeno_isolate = unsafe { libdeno::deno_new(config) };
+
+    Self {
+      pending_ops: HashMap::new(),
+      polled_recently: false,
+      libdeno_isolate,
+      test_send_counter: 0,
+      recv_cb,
+      shared,
+    }
+  }
+
+  fn zero_copy_release(&self, zero_copy_id: usize) {
+    unsafe {
+      libdeno::deno_zero_copy_release(self.libdeno_isolate, zero_copy_id)
+    }
+  }
+
+  pub fn add_op(
+    self: &mut Self,
+    promise_id: i32,
+    op: Box<Op>,
+    zero_copy_id: usize,
+  ) {
+    debug!("add_op {}", zero_copy_id);
+    self.pending_ops.insert(
+      promise_id,
+      PendingOp {
+        op,
+        polled_recently: false,
+        zero_copy_id,
+      },
+    );
+    self.polled_recently = false;
+  }
+
+  #[inline]
+  pub unsafe fn from_raw_ptr<'a>(ptr: *const c_void) -> &'a mut Self {
+    let ptr = ptr as *mut _;
+    &mut *ptr
+  }
+
+  #[inline]
+  pub fn as_raw_ptr(&self) -> *const c_void {
+    self as *const _ as *const c_void
+  }
+
+  pub fn execute(
+    &self,
+    js_filename: &str,
+    js_source: &str,
+  ) -> Result<(), JSError> {
+    let filename = CString::new(js_filename).unwrap();
+    let source = CString::new(js_source).unwrap();
+    unsafe {
+      libdeno::deno_execute(
+        self.libdeno_isolate,
+        self.as_raw_ptr(),
+        filename.as_ptr(),
+        source.as_ptr(),
+      )
+    };
+    if let Some(err) = self.last_exception() {
+      return Err(err);
+    }
+    Ok(())
+  }
+
+  pub fn last_exception(&self) -> Option<JSError> {
+    let ptr = unsafe { libdeno::deno_last_exception(self.libdeno_isolate) };
+    if ptr.is_null() {
+      None
+    } else {
+      let cstr = unsafe { CStr::from_ptr(ptr) };
+      let v8_exception = cstr.to_str().unwrap();
+      debug!("v8_exception\n{}\n", v8_exception);
+      let js_error = JSError::from_v8_exception(v8_exception).unwrap();
+      Some(js_error)
+    }
+  }
+
+  fn check_promise_errors(&self) {
+    unsafe {
+      libdeno::deno_check_promise_errors(self.libdeno_isolate);
+    }
+  }
+
+  fn respond(&mut self) -> Result<(), JSError> {
+    let buf = deno_buf::empty();
+    unsafe {
+      libdeno::deno_respond(self.libdeno_isolate, self.as_raw_ptr(), buf)
+    }
+    if let Some(err) = self.last_exception() {
+      Err(err)
+    } else {
+      Ok(())
+    }
+  }
+}
+
+struct LockerScope {
+  libdeno_isolate: *const libdeno::isolate,
+}
+
+impl LockerScope {
+  fn new(isolate: &Isolate) -> LockerScope {
+    let libdeno_isolate = isolate.libdeno_isolate;
+    unsafe { libdeno::deno_lock(libdeno_isolate) }
+    LockerScope { libdeno_isolate }
+  }
+}
+
+impl Drop for LockerScope {
+  fn drop(&mut self) {
+    unsafe { libdeno::deno_unlock(self.libdeno_isolate) }
+  }
+}
+
+impl Future for Isolate {
+  type Item = ();
+  type Error = JSError;
+
+  fn poll(&mut self) -> Poll<(), JSError> {
+    // Lock the current thread for V8.
+    let _locker = LockerScope::new(self);
+
+    // Clear
+    self.polled_recently = false;
+    for (_, pending) in self.pending_ops.iter_mut() {
+      pending.polled_recently = false;
+    }
+
+    while !self.polled_recently {
+      let mut complete = HashMap::<i32, AsyncResult>::new();
+
+      self.polled_recently = true;
+      for (promise_id, pending) in self.pending_ops.iter_mut() {
+        // Do not call poll on futures we've already polled this turn.
+        if pending.polled_recently {
+          continue;
+        }
+        pending.polled_recently = true;
+
+        let promise_id = *promise_id;
+        let op = &mut pending.op;
+        match op.poll() {
+          Err(op_err) => {
+            eprintln!("op err {:?}", op_err);
+            complete.insert(promise_id, AsyncResult { result: -1 });
+            debug!("pending op {} complete err", promise_id);
+          }
+          Ok(Async::Ready(async_result)) => {
+            complete.insert(promise_id, async_result);
+            debug!("pending op {} complete ready", promise_id);
+          }
+          Ok(Async::NotReady) => {
+            debug!("pending op {} not ready", promise_id);
+            continue;
+          }
+        }
+      }
+
+      self.shared.set_num_records(complete.len() as i32);
+      if complete.len() > 0 {
+        // self.zero_copy_release() and self.respond() need Locker.
+        let mut i = 0;
+        for (promise_id, async_result) in complete.iter_mut() {
+          let pending = self.pending_ops.remove(promise_id).unwrap();
+
+          if pending.zero_copy_id > 0 {
+            self.zero_copy_release(pending.zero_copy_id);
+          }
+
+          self
+            .shared
+            .set_record(i, RECORD_OFFSET_PROMISE_ID, *promise_id);
+          self
+            .shared
+            .set_record(i, RECORD_OFFSET_RESULT, async_result.result);
+          i += 1;
+        }
+        self.respond()?;
+      }
+    }
+
+    self.check_promise_errors();
+    if let Some(err) = self.last_exception() {
+      return Err(err);
+    }
+
+    // We're idle if pending_ops is empty.
+    if self.pending_ops.is_empty() {
+      Ok(futures::Async::Ready(()))
+    } else {
+      Ok(futures::Async::NotReady)
+    }
+  }
+}
+
+extern "C" fn pre_dispatch(
+  user_data: *mut c_void,
+  control_buf: deno_buf,
+  zero_copy_buf: deno_buf,
+) {
+  let isolate = unsafe { Isolate::from_raw_ptr(user_data) };
+  assert_eq!(control_buf.len(), 0);
+  (isolate.recv_cb)(isolate, zero_copy_buf);
+}
+
+#[cfg(test)]
+mod tests {
+  use super::*;
+
+  fn inc_counter(isolate: &mut Isolate, zero_copy_buf: deno_buf) {
+    assert_eq!(zero_copy_buf.len(), 0);
+    isolate.test_send_counter += 1; // TODO ideally store this in isolate.state?
+  }
+
+  fn js_check(r: Result<(), JSError>) {
+    if let Err(e) = r {
+      panic!(e.to_string());
+    }
+  }
+
+  #[test]
+  fn test_execute() {
+    let isolate = Isolate::new(inc_counter);
+    js_check(isolate.execute(
+      "filename.js",
+      r#"
+        libdeno.send();
+        async function main() {
+          libdeno.send();
+        }
+        main();
+        "#,
+    ));
+    // We expect that main is executed even tho we didn't poll.
+    assert_eq!(isolate.test_send_counter, 2);
+  }
+
+  fn async_immediate(isolate: &mut Isolate, zero_copy_buf: deno_buf) {
+    assert_eq!(zero_copy_buf.len(), 0);
+    isolate.test_send_counter += 1; // TODO ideally store this in isolate.state?
+
+    let promise_id = 0;
+    let op = Box::new(futures::future::ok(AsyncResult { result: 0 }));
+    isolate.add_op(promise_id, op, zero_copy_buf.zero_copy_id);
+  }
+
+  #[test]
+  fn test_poll_async_immediate_ops() {
+    let mut isolate = Isolate::new(async_immediate);
+    js_check(isolate.execute(
+      "setup.js",
+      r#"
+        let nrecv = 0;
+        libdeno.recv(() => {
+          nrecv++;
+        });
+        function assertEq(actual, expected) {
+          if (expected != actual) {
+            throw Error(`actual ${actual} expected ${expected} `);
+          }
+        }
+        "#,
+    ));
+    assert_eq!(isolate.test_send_counter, 0);
+    js_check(isolate.execute(
+      "check1.js",
+      r#"
+        assertEq(nrecv, 0);
+        libdeno.send();
+        assertEq(nrecv, 0);
+        "#,
+    ));
+    assert_eq!(isolate.test_send_counter, 1);
+    assert_eq!(Ok(Async::Ready(())), isolate.poll());
+    assert_eq!(isolate.test_send_counter, 1);
+    js_check(isolate.execute(
+      "check2.js",
+      r#"
+        assertEq(nrecv, 1);
+        libdeno.send();
+        assertEq(nrecv, 1);
+        "#,
+    ));
+    assert_eq!(isolate.test_send_counter, 2);
+    assert_eq!(Ok(Async::Ready(())), isolate.poll());
+    js_check(isolate.execute("check3.js", "assertEq(nrecv, 2)"));
+    assert_eq!(isolate.test_send_counter, 2);
+    // We are idle, so the next poll should be the last.
+    assert_eq!(Ok(Async::Ready(())), isolate.poll());
+  }
+}
diff --git a/core/libdeno.rs b/core/libdeno.rs
new file mode 120000
index 000000000..32688906e
--- /dev/null
+++ b/core/libdeno.rs
@@ -0,0 +1 @@
+../src/libdeno.rs
\ No newline at end of file
diff --git a/core/shared.rs b/core/shared.rs
new file mode 100644
index 000000000..40d83db73
--- /dev/null
+++ b/core/shared.rs
@@ -0,0 +1,49 @@
+use crate::libdeno::deno_buf;
+use std::mem;
+
+// TODO this is where we abstract flatbuffers at.
+// TODO make these constants private to this file.
+const INDEX_NUM_RECORDS: usize = 0;
+const INDEX_RECORDS: usize = 1;
+pub const RECORD_OFFSET_PROMISE_ID: usize = 0;
+pub const RECORD_OFFSET_OP: usize = 1;
+pub const RECORD_OFFSET_ARG: usize = 2;
+pub const RECORD_OFFSET_RESULT: usize = 3;
+const RECORD_SIZE: usize = 4;
+const NUM_RECORDS: usize = 100;
+
+/// Represents the shared buffer between JS and Rust.
+/// Used for FFI.
+pub struct Shared(Vec<i32>);
+
+impl Shared {
+  pub fn new() -> Shared {
+    let mut vec = Vec::<i32>::new();
+    vec.resize(INDEX_RECORDS + RECORD_SIZE * NUM_RECORDS, 0);
+    Shared(vec)
+  }
+
+  pub fn set_record(&mut self, i: usize, off: usize, value: i32) {
+    assert!(i < NUM_RECORDS);
+    self.0[INDEX_RECORDS + RECORD_SIZE * i + off] = value;
+  }
+
+  pub fn get_record(&self, i: usize, off: usize) -> i32 {
+    assert!(i < NUM_RECORDS);
+    return self.0[INDEX_RECORDS + RECORD_SIZE * i + off];
+  }
+
+  pub fn set_num_records(&mut self, num_records: i32) {
+    self.0[INDEX_NUM_RECORDS] = num_records;
+  }
+
+  pub fn get_num_records(&self) -> i32 {
+    return self.0[INDEX_NUM_RECORDS];
+  }
+
+  pub fn as_deno_buf(&mut self) -> deno_buf {
+    let ptr = self.0.as_mut_ptr() as *mut u8;
+    let len = mem::size_of::<i32>() * self.0.len();
+    unsafe { deno_buf::from_raw_parts(ptr, len) }
+  }
+}
diff --git a/js/dispatch.ts b/js/dispatch.ts
index e52a6f64c..55ea682fc 100644
--- a/js/dispatch.ts
+++ b/js/dispatch.ts
@@ -17,7 +17,7 @@ export function setFireTimersCallback(fn: () => void) {
 export function handleAsyncMsgFromRust(ui8: Uint8Array) {
   // If a the buffer is empty, recv() on the native side timed out and we
   // did not receive a message.
-  if (ui8.length) {
+  if (ui8 && ui8.length) {
     const bb = new flatbuffers.ByteBuffer(ui8);
     const base = msg.Base.getRootAsBase(bb);
     const cmdId = base.cmdId();
diff --git a/libdeno/api.cc b/libdeno/api.cc
index 2601862a2..4cb7b846b 100644
--- a/libdeno/api.cc
+++ b/libdeno/api.cc
@@ -78,6 +78,19 @@ deno::DenoIsolate* unwrap(Deno* d_) {
   return reinterpret_cast<deno::DenoIsolate*>(d_);
 }
 
+void deno_lock(Deno* d_) {
+  auto* d = unwrap(d_);
+  CHECK_NULL(d->locker_);
+  d->locker_ = new v8::Locker(d->isolate_);
+}
+
+void deno_unlock(Deno* d_) {
+  auto* d = unwrap(d_);
+  CHECK_NOT_NULL(d->locker_);
+  delete d->locker_;
+  d->locker_ = nullptr;
+}
+
 deno_buf deno_get_snapshot(Deno* d_) {
   auto* d = unwrap(d_);
   CHECK_NOT_NULL(d->snapshot_creator_);
@@ -87,7 +100,7 @@ deno_buf deno_get_snapshot(Deno* d_) {
   auto blob = d->snapshot_creator_->CreateBlob(
       v8::SnapshotCreator::FunctionCodeHandling::kKeep);
   return {nullptr, 0, reinterpret_cast<uint8_t*>(const_cast<char*>(blob.data)),
-          blob.raw_size};
+          blob.raw_size, 0};
 }
 
 static std::unique_ptr<v8::Platform> platform;
@@ -127,12 +140,23 @@ void deno_execute(Deno* d_, void* user_data, const char* js_filename,
   deno::Execute(context, js_filename, js_source);
 }
 
-void deno_respond(Deno* d_, void* user_data, int32_t req_id, deno_buf buf) {
+void deno_zero_copy_release(Deno* d_, size_t zero_copy_id) {
+  auto* d = unwrap(d_);
+  v8::Isolate::Scope isolate_scope(d->isolate_);
+  v8::Locker locker(d->isolate_);
+  v8::HandleScope handle_scope(d->isolate_);
+  d->DeleteZeroCopyRef(zero_copy_id);
+}
+
+void deno_respond(Deno* d_, void* user_data, deno_buf buf) {
   auto* d = unwrap(d_);
   if (d->current_args_ != nullptr) {
     // Synchronous response.
-    auto ab = deno::ImportBuf(d, buf);
-    d->current_args_->GetReturnValue().Set(ab);
+    if (buf.data_ptr != nullptr) {
+      DCHECK_EQ(buf.zero_copy_id, 0);
+      auto ab = deno::ImportBuf(d, buf);
+      d->current_args_->GetReturnValue().Set(ab);
+    }
     d->current_args_ = nullptr;
     return;
   }
@@ -148,8 +172,6 @@ void deno_respond(Deno* d_, void* user_data, int32_t req_id, deno_buf buf) {
 
   v8::TryCatch try_catch(d->isolate_);
 
-  deno::DeleteDataRef(d, req_id);
-
   auto recv_ = d->recv_.Get(d->isolate_);
   if (recv_.IsEmpty()) {
     d->last_exception_ = "libdeno.recv_ has not been called.";
@@ -157,8 +179,17 @@ void deno_respond(Deno* d_, void* user_data, int32_t req_id, deno_buf buf) {
   }
 
   v8::Local<v8::Value> args[1];
-  args[0] = deno::ImportBuf(d, buf);
-  auto v = recv_->Call(context, context->Global(), 1, args);
+  int argc = 0;
+
+  // You cannot use zero_copy_buf with deno_respond(). Use
+  // deno_zero_copy_release() instead.
+  DCHECK_EQ(buf.zero_copy_id, 0);
+  if (buf.data_ptr != nullptr) {
+    args[0] = deno::ImportBuf(d, buf);
+    argc = 1;
+  }
+
+  auto v = recv_->Call(context, context->Global(), argc, args);
 
   if (try_catch.HasCaught()) {
     CHECK(v.IsEmpty());
diff --git a/libdeno/binding.cc b/libdeno/binding.cc
index f640fe83c..d4849db7b 100644
--- a/libdeno/binding.cc
+++ b/libdeno/binding.cc
@@ -44,20 +44,6 @@ v8::StartupData SerializeInternalFields(v8::Local<v8::Object> holder, int index,
   return {payload, size};
 }
 
-void AddDataRef(DenoIsolate* d, int32_t req_id, v8::Local<v8::Value> data_v) {
-  d->async_data_map_.emplace(std::piecewise_construct, std::make_tuple(req_id),
-                             std::make_tuple(d->isolate_, data_v));
-}
-
-void DeleteDataRef(DenoIsolate* d, int32_t req_id) {
-  // Delete persistent reference to data ArrayBuffer.
-  auto it = d->async_data_map_.find(req_id);
-  if (it != d->async_data_map_.end()) {
-    it->second.Reset();
-    d->async_data_map_.erase(it);
-  }
-}
-
 // Extracts a C string from a v8::V8 Utf8Value.
 const char* ToCString(const v8::String::Utf8Value& value) {
   return *value ? *value : "<string conversion failed>";
@@ -131,6 +117,13 @@ void ErrorToJSON(const v8::FunctionCallbackInfo<v8::Value>& args) {
 }
 
 v8::Local<v8::Uint8Array> ImportBuf(DenoIsolate* d, deno_buf buf) {
+  // Do not use ImportBuf with zero_copy buffers.
+  DCHECK_EQ(buf.zero_copy_id, 0);
+
+  if (buf.data_ptr == nullptr) {
+    return v8::Local<v8::Uint8Array>();
+  }
+
   if (buf.alloc_ptr == nullptr) {
     // If alloc_ptr isn't set, we memcpy.
     // This is currently used for flatbuffers created in Rust.
@@ -209,42 +202,44 @@ void Send(const v8::FunctionCallbackInfo<v8::Value>& args) {
   DenoIsolate* d = DenoIsolate::FromIsolate(isolate);
   DCHECK_EQ(d->isolate_, isolate);
 
-  v8::Locker locker(d->isolate_);
+  deno_buf control = {nullptr, 0u, nullptr, 0u, 0u};
+  deno_buf zero_copy = {nullptr, 0u, nullptr, 0u, 0u};
+
   v8::HandleScope handle_scope(isolate);
 
-  CHECK_NULL(d->current_args_);  // libdeno.send re-entry forbidden.
-  int32_t req_id = d->next_req_id_++;
+  if (args.Length() > 0) {
+    v8::Local<v8::Value> control_v = args[0];
+    if (control_v->IsArrayBufferView()) {
+      control =
+          GetContents(isolate, v8::Local<v8::ArrayBufferView>::Cast(control_v));
+    }
+  }
 
-  v8::Local<v8::Value> control_v = args[0];
-  CHECK(control_v->IsArrayBufferView());
-  deno_buf control =
-      GetContents(isolate, v8::Local<v8::ArrayBufferView>::Cast(control_v));
-  deno_buf data = {nullptr, 0u, nullptr, 0u};
-  v8::Local<v8::Value> data_v;
+  v8::Local<v8::Value> zero_copy_v;
   if (args.Length() == 2) {
     if (args[1]->IsArrayBufferView()) {
-      data_v = args[1];
-      data = GetContents(isolate, v8::Local<v8::ArrayBufferView>::Cast(data_v));
+      zero_copy_v = args[1];
+      zero_copy = GetContents(
+          isolate, v8::Local<v8::ArrayBufferView>::Cast(zero_copy_v));
+      size_t zero_copy_id = d->next_zero_copy_id_++;
+      DCHECK_GT(zero_copy_id, 0);
+      zero_copy.zero_copy_id = zero_copy_id;
+      // If the zero_copy ArrayBuffer was given, we must maintain a strong
+      // reference to it until deno_zero_copy_release is called.
+      d->AddZeroCopyRef(zero_copy_id, zero_copy_v);
     }
-  } else {
-    CHECK_EQ(args.Length(), 1);
   }
 
   DCHECK_NULL(d->current_args_);
   d->current_args_ = &args;
 
-  d->recv_cb_(d->user_data_, req_id, control, data);
+  d->recv_cb_(d->user_data_, control, zero_copy);
 
   if (d->current_args_ == nullptr) {
     // This indicates that deno_repond() was called already.
   } else {
     // Asynchronous.
     d->current_args_ = nullptr;
-    // If the data ArrayBuffer was given, we must maintain a strong reference
-    // to it until deno_respond is called.
-    if (!data_v.IsEmpty()) {
-      AddDataRef(d, req_id, data_v);
-    }
   }
 }
 
diff --git a/libdeno/deno.h b/libdeno/deno.h
index 6be0b5625..f3902985e 100644
--- a/libdeno/deno.h
+++ b/libdeno/deno.h
@@ -15,6 +15,7 @@ typedef struct {
   size_t alloc_len;    // Length of the memory allocation.
   uint8_t* data_ptr;   // Start of logical contents (within the allocation).
   size_t data_len;     // Length of logical contents.
+  size_t zero_copy_id;  // 0 = normal, 1 = must call deno_zero_copy_release.
 } deno_buf;
 
 typedef struct deno_s Deno;
@@ -22,8 +23,8 @@ typedef struct deno_s Deno;
 // A callback to receive a message from a libdeno.send() javascript call.
 // control_buf is valid for only for the lifetime of this callback.
 // data_buf is valid until deno_respond() is called.
-typedef void (*deno_recv_cb)(void* user_data, int32_t req_id,
-                             deno_buf control_buf, deno_buf data_buf);
+typedef void (*deno_recv_cb)(void* user_data, deno_buf control_buf,
+                             deno_buf zerop_copy_buf);
 
 void deno_init();
 const char* deno_v8_version();
@@ -47,6 +48,9 @@ deno_buf deno_get_snapshot(Deno* d);
 
 void deno_delete(Deno* d);
 
+void deno_lock(Deno* d);
+void deno_unlock(Deno* d);
+
 // Compile and execute a traditional JavaScript script that does not use
 // module import statements.
 // If it succeeded deno_last_exception() will return NULL.
@@ -66,11 +70,13 @@ void deno_execute(Deno* d, void* user_data, const char* js_filename,
 // longer owns `buf` and must not use it; deno_respond() is responsible for
 // releasing its memory.)
 //
-// Calling this function more than once with the same req_id will result in
-// an error.
-//
 // If a JS exception was encountered, deno_last_exception() will be non-NULL.
-void deno_respond(Deno* d, void* user_data, int32_t req_id, deno_buf buf);
+void deno_respond(Deno* d, void* user_data, deno_buf buf);
+
+// consumes zero_copy
+// Calling this function more than once with the same zero_copy_id will result
+// in an error.
+void deno_zero_copy_release(Deno* d, size_t zero_copy_id);
 
 void deno_check_promise_errors(Deno* d);
 
diff --git a/libdeno/internal.h b/libdeno/internal.h
index 720966407..563043085 100644
--- a/libdeno/internal.h
+++ b/libdeno/internal.h
@@ -30,12 +30,13 @@ class DenoIsolate {
  public:
   explicit DenoIsolate(deno_config config)
       : isolate_(nullptr),
+        locker_(nullptr),
         shared_(config.shared),
         current_args_(nullptr),
         snapshot_creator_(nullptr),
         global_import_buf_ptr_(nullptr),
         recv_cb_(config.recv_cb),
-        next_req_id_(0),
+        next_zero_copy_id_(1),  // zero_copy_id must not be zero.
         user_data_(nullptr),
         resolve_cb_(nullptr) {
     array_buffer_allocator_ = v8::ArrayBuffer::Allocator::NewDefaultAllocator();
@@ -48,6 +49,9 @@ class DenoIsolate {
 
   ~DenoIsolate() {
     shared_ab_.Reset();
+    if (locker_) {
+      delete locker_;
+    }
     if (snapshot_creator_) {
       delete snapshot_creator_;
     } else {
@@ -78,14 +82,31 @@ class DenoIsolate {
     }
   }
 
+  void DeleteZeroCopyRef(size_t zero_copy_id) {
+    DCHECK_NE(zero_copy_id, 0);
+    // Delete persistent reference to data ArrayBuffer.
+    auto it = zero_copy_map_.find(zero_copy_id);
+    if (it != zero_copy_map_.end()) {
+      it->second.Reset();
+      zero_copy_map_.erase(it);
+    }
+  }
+
+  void AddZeroCopyRef(size_t zero_copy_id, v8::Local<v8::Value> zero_copy_v) {
+    zero_copy_map_.emplace(std::piecewise_construct,
+                           std::make_tuple(zero_copy_id),
+                           std::make_tuple(isolate_, zero_copy_v));
+  }
+
   v8::Isolate* isolate_;
+  v8::Locker* locker_;
   v8::ArrayBuffer::Allocator* array_buffer_allocator_;
   deno_buf shared_;
   const v8::FunctionCallbackInfo<v8::Value>* current_args_;
   v8::SnapshotCreator* snapshot_creator_;
   void* global_import_buf_ptr_;
   deno_recv_cb recv_cb_;
-  int32_t next_req_id_;
+  size_t next_zero_copy_id_;
   void* user_data_;
 
   v8::Persistent<v8::Object> builtin_modules_;
@@ -94,7 +115,7 @@ class DenoIsolate {
   deno_resolve_cb resolve_cb_;
 
   v8::Persistent<v8::Context> context_;
-  std::map<int32_t, v8::Persistent<v8::Value>> async_data_map_;
+  std::map<size_t, v8::Persistent<v8::Value>> zero_copy_map_;
   std::map<int, v8::Persistent<v8::Value>> pending_promise_map_;
   std::string last_exception_;
   v8::Persistent<v8::Function> recv_;
@@ -152,7 +173,7 @@ static intptr_t external_references[] = {
     reinterpret_cast<intptr_t>(MessageCallback),
     0};
 
-static const deno_buf empty_buf = {nullptr, 0, nullptr, 0};
+static const deno_buf empty_buf = {nullptr, 0, nullptr, 0, 0};
 
 Deno* NewFromSnapshot(void* user_data, deno_recv_cb cb);
 
@@ -166,8 +187,6 @@ v8::StartupData SerializeInternalFields(v8::Local<v8::Object> holder, int index,
 
 v8::Local<v8::Uint8Array> ImportBuf(DenoIsolate* d, deno_buf buf);
 
-void DeleteDataRef(DenoIsolate* d, int32_t req_id);
-
 bool Execute(v8::Local<v8::Context> context, const char* js_filename,
              const char* js_source);
 bool ExecuteMod(v8::Local<v8::Context> context, const char* js_filename,
diff --git a/libdeno/libdeno_test.cc b/libdeno/libdeno_test.cc
index 3193e7677..90fceef73 100644
--- a/libdeno/libdeno_test.cc
+++ b/libdeno/libdeno_test.cc
@@ -26,9 +26,11 @@ TEST(LibDenoTest, Snapshotter) {
 
 TEST(LibDenoTest, CanCallFunction) {
   Deno* d = deno_new(deno_config{0, snapshot, empty, nullptr});
+  deno_lock(d);
   deno_execute(d, nullptr, "a.js",
                "if (CanCallFunction() != 'foo') throw Error();");
   EXPECT_EQ(nullptr, deno_last_exception(d));
+  deno_unlock(d);
   deno_delete(d);
 }
 
@@ -47,6 +49,7 @@ deno_buf strbuf(const char* str) {
   buf.alloc_len = len + 1;
   buf.data_ptr = buf.alloc_ptr;
   buf.data_len = len;
+  buf.zero_copy_id = 0;
 
   return buf;
 }
@@ -71,8 +74,8 @@ void assert_null(deno_buf b) {
 
 TEST(LibDenoTest, RecvReturnEmpty) {
   static int count = 0;
-  auto recv_cb = [](auto _, int req_id, auto buf, auto data_buf) {
-    assert_null(data_buf);
+  auto recv_cb = [](auto _, auto buf, auto zero_copy_buf) {
+    assert_null(zero_copy_buf);
     count++;
     EXPECT_EQ(static_cast<size_t>(3), buf.data_len);
     EXPECT_EQ(buf.data_ptr[0], 'a');
@@ -88,15 +91,17 @@ TEST(LibDenoTest, RecvReturnEmpty) {
 
 TEST(LibDenoTest, RecvReturnBar) {
   static int count = 0;
-  auto recv_cb = [](auto user_data, int req_id, auto buf, auto data_buf) {
+  auto recv_cb = [](auto user_data, auto buf, auto zero_copy_buf) {
     auto d = reinterpret_cast<Deno*>(user_data);
-    assert_null(data_buf);
+    assert_null(zero_copy_buf);
     count++;
     EXPECT_EQ(static_cast<size_t>(3), buf.data_len);
     EXPECT_EQ(buf.data_ptr[0], 'a');
     EXPECT_EQ(buf.data_ptr[1], 'b');
     EXPECT_EQ(buf.data_ptr[2], 'c');
-    deno_respond(d, user_data, req_id, strbuf("bar"));
+    EXPECT_EQ(zero_copy_buf.zero_copy_id, 0u);
+    EXPECT_EQ(zero_copy_buf.data_ptr, nullptr);
+    deno_respond(d, user_data, strbuf("bar"));
   };
   Deno* d = deno_new(deno_config{0, snapshot, empty, recv_cb});
   deno_execute(d, d, "a.js", "RecvReturnBar()");
@@ -114,9 +119,9 @@ TEST(LibDenoTest, DoubleRecvFails) {
 
 TEST(LibDenoTest, SendRecvSlice) {
   static int count = 0;
-  auto recv_cb = [](auto user_data, int req_id, auto buf, auto data_buf) {
+  auto recv_cb = [](auto user_data, auto buf, auto zero_copy_buf) {
     auto d = reinterpret_cast<Deno*>(user_data);
-    assert_null(data_buf);
+    assert_null(zero_copy_buf);
     static const size_t alloc_len = 1024;
     size_t i = count++;
     // Check the size and offset of the slice.
@@ -134,12 +139,12 @@ TEST(LibDenoTest, SendRecvSlice) {
     memcpy(alloc_ptr, buf.alloc_ptr, alloc_len);
     // Make a slice that is a bit shorter than the original.
     deno_buf buf2{alloc_ptr, alloc_len, alloc_ptr + data_offset,
-                  buf.data_len - 19};
+                  buf.data_len - 19, 0};
     // Place some values into the buffer for the JS side to verify.
     buf2.data_ptr[0] = 200 + i;
     buf2.data_ptr[buf2.data_len - 1] = 200 - i;
     // Send back.
-    deno_respond(d, user_data, req_id, buf2);
+    deno_respond(d, user_data, buf2);
   };
   Deno* d = deno_new(deno_config{0, snapshot, empty, recv_cb});
   deno_execute(d, d, "a.js", "SendRecvSlice()");
@@ -150,8 +155,8 @@ TEST(LibDenoTest, SendRecvSlice) {
 
 TEST(LibDenoTest, JSSendArrayBufferViewTypes) {
   static int count = 0;
-  auto recv_cb = [](auto _, int req_id, auto buf, auto data_buf) {
-    assert_null(data_buf);
+  auto recv_cb = [](auto _, auto buf, auto zero_copy_buf) {
+    assert_null(zero_copy_buf);
     count++;
     size_t data_offset = buf.data_ptr - buf.alloc_ptr;
     EXPECT_EQ(data_offset, 2468u);
@@ -197,33 +202,39 @@ TEST(LibDenoTest, GlobalErrorHandling) {
   deno_delete(d);
 }
 
-TEST(LibDenoTest, DataBuf) {
+TEST(LibDenoTest, ZeroCopyBuf) {
   static int count = 0;
-  static deno_buf data_buf_copy;
-  auto recv_cb = [](auto _, int req_id, deno_buf buf, deno_buf data_buf) {
+  static deno_buf zero_copy_buf2;
+  auto recv_cb = [](auto user_data, deno_buf buf, deno_buf zero_copy_buf) {
     count++;
-    data_buf.data_ptr[0] = 4;
-    data_buf.data_ptr[1] = 2;
-    data_buf_copy = data_buf;
+    EXPECT_GT(zero_copy_buf.zero_copy_id, 0u);
+    zero_copy_buf.data_ptr[0] = 4;
+    zero_copy_buf.data_ptr[1] = 2;
+    zero_copy_buf2 = zero_copy_buf;
     EXPECT_EQ(2u, buf.data_len);
-    EXPECT_EQ(2u, data_buf.data_len);
+    EXPECT_EQ(2u, zero_copy_buf.data_len);
     EXPECT_EQ(buf.data_ptr[0], 1);
     EXPECT_EQ(buf.data_ptr[1], 2);
+    // Note zero_copy_buf won't actually be freed here because in
+    // libdeno_test.js zeroCopyBuf is a rooted global. We just want to exercise
+    // the API here.
+    auto d = reinterpret_cast<Deno*>(user_data);
+    deno_zero_copy_release(d, zero_copy_buf.zero_copy_id);
   };
   Deno* d = deno_new(deno_config{0, snapshot, empty, recv_cb});
-  deno_execute(d, nullptr, "a.js", "DataBuf()");
+  deno_execute(d, d, "a.js", "ZeroCopyBuf()");
   EXPECT_EQ(nullptr, deno_last_exception(d));
   EXPECT_EQ(count, 1);
-  // data_buf was subsequently changed in JS, let's check that our copy reflects
-  // that.
-  EXPECT_EQ(data_buf_copy.data_ptr[0], 9);
-  EXPECT_EQ(data_buf_copy.data_ptr[1], 8);
+  // zero_copy_buf was subsequently changed in JS, let's check that our copy
+  // reflects that.
+  EXPECT_EQ(zero_copy_buf2.data_ptr[0], 9);
+  EXPECT_EQ(zero_copy_buf2.data_ptr[1], 8);
   deno_delete(d);
 }
 
 TEST(LibDenoTest, CheckPromiseErrors) {
   static int count = 0;
-  auto recv_cb = [](auto _, int req_id, auto buf, auto data_buf) { count++; };
+  auto recv_cb = [](auto _, auto buf, auto zero_copy_buf) { count++; };
   Deno* d = deno_new(deno_config{0, snapshot, empty, recv_cb});
   EXPECT_EQ(deno_last_exception(d), nullptr);
   deno_execute(d, nullptr, "a.js", "CheckPromiseErrors()");
@@ -271,7 +282,7 @@ TEST(LibDenoTest, EncodeErrorBug) {
 
 TEST(LibDenoTest, Shared) {
   uint8_t s[] = {0, 1, 2};
-  deno_buf shared = {nullptr, 0, s, 3};
+  deno_buf shared = {nullptr, 0, s, 3, 0};
   Deno* d = deno_new(deno_config{0, snapshot, shared, nullptr});
   deno_execute(d, nullptr, "a.js", "Shared()");
   EXPECT_EQ(nullptr, deno_last_exception(d));
@@ -306,7 +317,7 @@ TEST(LibDenoTest, LibDenoEvalContextError) {
 
 TEST(LibDenoTest, SharedAtomics) {
   int32_t s[] = {0, 1, 2};
-  deno_buf shared = {nullptr, 0, reinterpret_cast<uint8_t*>(s), sizeof s};
+  deno_buf shared = {nullptr, 0, reinterpret_cast<uint8_t*>(s), sizeof s, 0};
   Deno* d = deno_new(deno_config{0, empty, shared, nullptr});
   deno_execute(d, nullptr, "a.js",
                "Atomics.add(new Int32Array(libdeno.shared), 0, 1)");
diff --git a/libdeno/libdeno_test.js b/libdeno/libdeno_test.js
index d6ea5f983..8b1ad2e04 100644
--- a/libdeno/libdeno_test.js
+++ b/libdeno/libdeno_test.js
@@ -103,11 +103,11 @@ global.GlobalErrorHandling = () => {
 };
 
 // Allocate this buf at the top level to avoid GC.
-const dataBuf = new Uint8Array([3, 4]);
+const zeroCopyBuf = new Uint8Array([3, 4]);
 
-global.DataBuf = () => {
+global.ZeroCopyBuf = () => {
   const a = new Uint8Array([1, 2]);
-  const b = dataBuf;
+  const b = zeroCopyBuf;
   // The second parameter of send should modified by the
   // privileged side.
   const r = libdeno.send(a, b);
diff --git a/libdeno/modules_test.cc b/libdeno/modules_test.cc
index d41c38b8e..cb800e89a 100644
--- a/libdeno/modules_test.cc
+++ b/libdeno/modules_test.cc
@@ -2,10 +2,12 @@
 #include "test.h"
 
 static int exec_count = 0;
-void recv_cb(void* user_data, int req_id, deno_buf buf, deno_buf data_buf) {
+void recv_cb(void* user_data, deno_buf buf, deno_buf zero_copy_buf) {
   // We use this to check that scripts have executed.
   EXPECT_EQ(1u, buf.data_len);
   EXPECT_EQ(buf.data_ptr[0], 4);
+  EXPECT_EQ(zero_copy_buf.zero_copy_id, 0u);
+  EXPECT_EQ(zero_copy_buf.data_ptr, nullptr);
   exec_count++;
 }
 
diff --git a/libdeno/test.cc b/libdeno/test.cc
index a8fcbc63b..1340fe8c3 100644
--- a/libdeno/test.cc
+++ b/libdeno/test.cc
@@ -3,7 +3,7 @@
 #include <string>
 #include "file_util.h"
 
-deno_buf snapshot = {nullptr, 0, nullptr, 0};
+deno_buf snapshot = {nullptr, 0, nullptr, 0, 0};
 
 int main(int argc, char** argv) {
   // Locate the snapshot.
diff --git a/libdeno/test.h b/libdeno/test.h
index 25ca93988..2f7c32384 100644
--- a/libdeno/test.h
+++ b/libdeno/test.h
@@ -6,6 +6,6 @@
 #include "testing/gtest/include/gtest/gtest.h"
 
 extern deno_buf snapshot;  // Loaded in libdeno/test.cc
-const deno_buf empty = {nullptr, 0, nullptr, 0};
+const deno_buf empty = {nullptr, 0, nullptr, 0, 0};
 
 #endif  // TEST_H_
diff --git a/src/isolate.rs b/src/isolate.rs
index b723738d7..440c168f2 100644
--- a/src/isolate.rs
+++ b/src/isolate.rs
@@ -48,15 +48,17 @@ pub type Buf = Box<[u8]>;
 pub type Op = dyn Future<Item = Buf, Error = DenoError> + Send;
 
 // Returns (is_sync, op)
-pub type Dispatch =
-  fn(isolate: &Isolate, buf: libdeno::deno_buf, data_buf: libdeno::deno_buf)
-    -> (bool, Box<Op>);
+pub type Dispatch = fn(
+  isolate: &Isolate,
+  buf: libdeno::deno_buf,
+  zero_copy_buf: libdeno::deno_buf,
+) -> (bool, Box<Op>);
 
 pub struct Isolate {
   libdeno_isolate: *const libdeno::isolate,
   dispatch: Dispatch,
-  rx: mpsc::Receiver<(i32, Buf)>,
-  tx: mpsc::Sender<(i32, Buf)>,
+  rx: mpsc::Receiver<(usize, Buf)>,
+  tx: mpsc::Sender<(usize, Buf)>,
   ntasks: Cell<i32>,
   timeout_due: Cell<Option<Instant>>,
   pub modules: RefCell<Modules>,
@@ -204,7 +206,7 @@ impl Isolate {
     };
     let libdeno_isolate = unsafe { libdeno::deno_new(config) };
     // This channel handles sending async messages back to the runtime.
-    let (tx, rx) = mpsc::channel::<(i32, Buf)>();
+    let (tx, rx) = mpsc::channel::<(usize, Buf)>();
 
     Self {
       libdeno_isolate,
@@ -404,37 +406,39 @@ impl Isolate {
     Ok(())
   }
 
-  pub fn respond(&self, req_id: i32, buf: Buf) {
+  pub fn respond(&self, zero_copy_id: usize, buf: Buf) {
     self.state.metrics_op_completed(buf.len());
+
+    // This will be cleaned up in the future.
+    if zero_copy_id > 0 {
+      unsafe {
+        libdeno::deno_zero_copy_release(self.libdeno_isolate, zero_copy_id)
+      }
+    }
+
     // deno_respond will memcpy the buf into V8's heap,
     // so borrowing a reference here is sufficient.
     unsafe {
       libdeno::deno_respond(
         self.libdeno_isolate,
         self.as_raw_ptr(),
-        req_id,
         buf.as_ref().into(),
       )
     }
   }
 
-  fn complete_op(&self, req_id: i32, buf: Buf) {
+  fn complete_op(&self, zero_copy_id: usize, buf: Buf) {
     // Receiving a message on rx exactly corresponds to an async task
     // completing.
     self.ntasks_decrement();
     // Call into JS with the buf.
-    self.respond(req_id, buf);
+    self.respond(zero_copy_id, buf);
   }
 
   fn timeout(&self) {
     let dummy_buf = libdeno::deno_buf::empty();
     unsafe {
-      libdeno::deno_respond(
-        self.libdeno_isolate,
-        self.as_raw_ptr(),
-        -1,
-        dummy_buf,
-      )
+      libdeno::deno_respond(self.libdeno_isolate, self.as_raw_ptr(), dummy_buf)
     }
   }
 
@@ -450,7 +454,7 @@ impl Isolate {
     // Main thread event loop.
     while !self.is_idle() {
       match recv_deadline(&self.rx, self.get_timeout_due()) {
-        Ok((req_id, buf)) => self.complete_op(req_id, buf),
+        Ok((zero_copy_id, buf)) => self.complete_op(zero_copy_id, buf),
         Err(mpsc::RecvTimeoutError::Timeout) => self.timeout(),
         Err(e) => panic!("recv_deadline() failed: {:?}", e),
       }
@@ -532,23 +536,24 @@ extern "C" fn resolve_cb(
 // Dereferences the C pointer into the Rust Isolate object.
 extern "C" fn pre_dispatch(
   user_data: *mut c_void,
-  req_id: i32,
   control_buf: libdeno::deno_buf,
-  data_buf: libdeno::deno_buf,
+  zero_copy_buf: libdeno::deno_buf,
 ) {
   // for metrics
   let bytes_sent_control = control_buf.len();
-  let bytes_sent_data = data_buf.len();
+  let bytes_sent_zero_copy = zero_copy_buf.len();
+
+  let zero_copy_id = zero_copy_buf.zero_copy_id;
 
   // We should ensure that there is no other `&mut Isolate` exists.
   // And also, it should be in the same thread with other `&Isolate`s.
   let isolate = unsafe { Isolate::from_raw_ptr(user_data) };
   let dispatch = isolate.dispatch;
-  let (is_sync, op) = dispatch(isolate, control_buf, data_buf);
+  let (is_sync, op) = dispatch(isolate, control_buf, zero_copy_buf);
 
   isolate
     .state
-    .metrics_op_dispatched(bytes_sent_control, bytes_sent_data);
+    .metrics_op_dispatched(bytes_sent_control, bytes_sent_zero_copy);
 
   if is_sync {
     // Execute op synchronously.
@@ -560,7 +565,7 @@ extern "C" fn pre_dispatch(
       isolate.state.metrics_op_completed(buf.len());
     } else {
       // Set the synchronous response, the value returned from isolate.send().
-      isolate.respond(req_id, buf);
+      isolate.respond(zero_copy_id, buf);
     }
   } else {
     // Execute op asynchronously.
@@ -574,7 +579,7 @@ extern "C" fn pre_dispatch(
     let task = op
       .and_then(move |buf| {
         let sender = tx; // tx is moved to new thread
-        sender.send((req_id, buf)).expect("tx.send error");
+        sender.send((zero_copy_id, buf)).expect("tx.send error");
         Ok(())
       }).map_err(|_| ());
     tokio::spawn(task);
diff --git a/src/libdeno.rs b/src/libdeno.rs
index 204f817b5..6696a382b 100644
--- a/src/libdeno.rs
+++ b/src/libdeno.rs
@@ -1,4 +1,9 @@
 // Copyright 2018-2019 the Deno authors. All rights reserved. MIT license.
+
+// TODO Remove. While core is being developed, it may not use the complete
+// libdeno API. Thus we allow dead code until things settle.
+#![allow(dead_code)]
+
 use libc::c_char;
 use libc::c_int;
 use libc::c_void;
@@ -25,6 +30,7 @@ pub struct deno_buf {
   alloc_len: usize,
   data_ptr: *const u8,
   data_len: usize,
+  pub zero_copy_id: usize,
 }
 
 /// `deno_buf` can not clone, and there is no interior mutability.
@@ -39,6 +45,7 @@ impl deno_buf {
       alloc_len: 0,
       data_ptr: null(),
       data_len: 0,
+      zero_copy_id: 0,
     }
   }
 
@@ -49,6 +56,7 @@ impl deno_buf {
       alloc_len: 0,
       data_ptr: ptr,
       data_len: len,
+      zero_copy_id: 0,
     }
   }
 }
@@ -62,6 +70,7 @@ impl<'a> From<&'a [u8]> for deno_buf {
       alloc_len: 0,
       data_ptr: x.as_ref().as_ptr(),
       data_len: x.len(),
+      zero_copy_id: 0,
     }
   }
 }
@@ -106,9 +115,8 @@ impl AsMut<[u8]> for deno_buf {
 #[allow(non_camel_case_types)]
 type deno_recv_cb = unsafe extern "C" fn(
   user_data: *mut c_void,
-  req_id: i32,
-  buf: deno_buf,
-  data_buf: deno_buf,
+  control_buf: deno_buf, // deprecated
+  zero_copy_buf: deno_buf,
 );
 
 #[allow(non_camel_case_types)]
@@ -137,12 +145,14 @@ extern "C" {
   pub fn deno_delete(i: *const isolate);
   pub fn deno_last_exception(i: *const isolate) -> *const c_char;
   pub fn deno_check_promise_errors(i: *const isolate);
+  pub fn deno_lock(i: *const isolate);
+  pub fn deno_unlock(i: *const isolate);
   pub fn deno_respond(
     i: *const isolate,
     user_data: *const c_void,
-    req_id: i32,
     buf: deno_buf,
   );
+  pub fn deno_zero_copy_release(i: *const isolate, zero_copy_id: usize);
   pub fn deno_execute(
     i: *const isolate,
     user_data: *const c_void,
diff --git a/tools/benchmark.py b/tools/benchmark.py
index c9a3e0243..53037b1e7 100755
--- a/tools/benchmark.py
+++ b/tools/benchmark.py
@@ -204,8 +204,10 @@ def main(argv):
     # pipe.
     if os.name != 'nt':
         hyper_hello_path = os.path.join(build_dir, "hyper_hello")
+        core_http_bench_exe = os.path.join(build_dir, "deno_core_http_bench")
         new_data["throughput"] = run_throughput(deno_path)
-        new_data["req_per_sec"] = http_benchmark(deno_path, hyper_hello_path)
+        new_data["req_per_sec"] = http_benchmark(deno_path, hyper_hello_path,
+                                                 core_http_bench_exe)
     if "linux" in sys.platform:
         # Thread count test, only on linux
         new_data["thread_count"] = run_thread_count_benchmark(deno_path)
diff --git a/tools/format.py b/tools/format.py
index fe053eaa3..083640c22 100755
--- a/tools/format.py
+++ b/tools/format.py
@@ -39,7 +39,7 @@ qrun(
 print "prettier"
 qrun(["node", prettier, "--write", "--loglevel=error"] + ["rollup.config.js"] +
      glob("*.json") + glob("*.md") +
-     find_exts([".github", "js", "tests", "tools", "website"],
+     find_exts([".github", "js", "tests", "tools", "website", "core"],
                [".js", ".json", ".ts", ".md"],
                skip=["tools/clang", "js/deps"]))
 
@@ -47,4 +47,4 @@ print "rustfmt"
 qrun([
     "third_party/rustfmt/" + platform() +
     "/rustfmt", "--config-path", rustfmt_config, "build.rs"
-] + find_exts(["src"], [".rs"]))
+] + find_exts(["src", "core"], [".rs"]))
diff --git a/tools/http_benchmark.py b/tools/http_benchmark.py
index 312e61da6..d84a24de6 100755
--- a/tools/http_benchmark.py
+++ b/tools/http_benchmark.py
@@ -30,6 +30,16 @@ def deno_net_http_benchmark(deno_exe):
         })
 
 
+def deno_core_single(exe):
+    print "http_benchmark testing deno_core_single"
+    return run([exe, "--single-thread"])
+
+
+def deno_core_multi(exe):
+    print "http_benchmark testing deno_core_multi"
+    return run([exe, "--multi-thread"])
+
+
 def node_http_benchmark():
     node_cmd = ["node", "tools/node_http.js", ADDR.split(":")[1]]
     print "http_benchmark testing NODE."
@@ -48,11 +58,13 @@ def hyper_http_benchmark(hyper_hello_exe):
     return run(hyper_cmd)
 
 
-def http_benchmark(deno_exe, hyper_hello_exe):
+def http_benchmark(deno_exe, hyper_hello_exe, core_http_bench_exe):
     r = {}
     # TODO Rename to "deno_tcp"
     r["deno"] = deno_http_benchmark(deno_exe)
     r["deno_net_http"] = deno_net_http_benchmark(deno_exe)
+    r["deno_core_single"] = deno_core_single(core_http_bench_exe)
+    r["deno_core_multi"] = deno_core_multi(core_http_bench_exe)
     r["node"] = node_http_benchmark()
     r["node_tcp"] = node_tcp_benchmark()
     r["hyper"] = hyper_http_benchmark(hyper_hello_exe)
@@ -68,8 +80,14 @@ def run(server_cmd, merge_env=None):
         for key, value in merge_env.iteritems():
             env[key] = value
 
+    # Wait for port 4544 to become available.
+    # TODO Need to use SO_REUSEPORT with tokio::net::TcpListener.
+    time.sleep(5)
+
     server = subprocess.Popen(server_cmd, env=env)
+
     time.sleep(5)  # wait for server to wake up. TODO racy.
+
     try:
         cmd = "third_party/wrk/%s/wrk -d %s http://%s/" % (util.platform(),
                                                            DURATION, ADDR)
diff --git a/tools/lint.py b/tools/lint.py
index 148cc4728..e3e765387 100755
--- a/tools/lint.py
+++ b/tools/lint.py
@@ -21,8 +21,8 @@ run([
 
 run(["node", tslint, "-p", ".", "--exclude", "**/gen/**/*.ts"])
 run([
-    "node", tslint, "./js/**/*_test.ts", "./tests/**/*.ts", "--exclude",
-    "**/gen/**/*.ts", "--project", "tsconfig.json"
+    "node", tslint, "./js/**/*_test.ts", "./tests/**/*.ts", "./core/*.js",
+    "--exclude", "**/gen/**/*.ts", "--project", "tsconfig.json"
 ])
 
 run([sys.executable, "third_party/depot_tools/pylint.py"] +
diff --git a/tools/node_tcp_promise.js b/tools/node_tcp_promise.js
new file mode 100644
index 000000000..c8fc54aba
--- /dev/null
+++ b/tools/node_tcp_promise.js
@@ -0,0 +1,25 @@
+// Copyright 2018-2019 the Deno authors. All rights reserved. MIT license.
+// Note: this is a keep-alive server.
+const { Server } = require("net");
+const port = process.argv[2] || "4544";
+console.log("port", port);
+
+const response = Buffer.from(
+  "HTTP/1.1 200 OK\r\nContent-Length: 12\r\n\r\nHello World\n"
+);
+
+async function write(socket, buffer) {
+  let p = new Promise((resolve, reject) => {
+    socket.write(buffer, resolve);
+  });
+  return p;
+}
+
+Server(async socket => {
+  socket.on("error", _ => {
+    socket.destroy();
+  });
+  for await (const data of socket) {
+    write(socket, response);
+  }
+}).listen(port);
diff --git a/tools/test.py b/tools/test.py
index 5d08e59d1..5a8c67519 100755
--- a/tools/test.py
+++ b/tools/test.py
@@ -85,6 +85,11 @@ def main(argv):
     check_exists(test_rs)
     run([test_rs])
 
+    deno_core_test = os.path.join(build_dir,
+                                  "deno_core_test" + executable_suffix)
+    check_exists(deno_core_test)
+    run([deno_core_test])
+
     unit_tests(deno_exe)
 
     prefetch_test(deno_exe)