Move buffers between V8 and native

* send()/recv() now operate on TypedArrays rather than ArrayBuffers.

* Remove a copy (through ArrayBuffer.slice()) from the send path.

* Remove a copy (through v8::ArrayBuffer::New()) from the return path.

* After moving a buffer from JS to native, the ArrayBuffer object and
  it's views are made inaccessible ('neutered').

* `struct deno_buf` now holds two [ptr, length] tuples, one for the actual
  memory allocation, and one for the logical data contained therein.
  This is necessary because flatbuffers fills it's buffer bottom-up, so
  the serialized blob doesn't start at beginning of the buffer, but
  somewhere in the middle.

12 files changed, 438 insertions, 70 deletions

diff --git a/BUILD.gn b/BUILD.gn
index 163df2607..488a98300 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -76,6 +76,7 @@ executable("mock_runtime_test") {
   testonly = true
   sources = [
     "src/file_util_test.cc",
+    "src/flatbuffer_builder_test.cc",
     "src/from_snapshot.cc",
     "src/mock_runtime_test.cc",
   ]
@@ -106,11 +107,16 @@ v8_source_set("deno_bindings") {
     "src/deno.h",
     "src/file_util.cc",
     "src/file_util.h",
+    "src/flatbuffer_builder.cc",
+    "src/flatbuffer_builder.h",
     "src/internal.h",
   ]
   deps = [
     "third_party/v8:v8_monolith",
   ]
+  public_deps = [
+    "build_extra/flatbuffers:flatbuffers",
+  ]
   configs = [ ":deno_config" ]
 }
 
diff --git a/js/deno.d.ts b/js/deno.d.ts
index f554135bc..a54dab851 100644
--- a/js/deno.d.ts
+++ b/js/deno.d.ts
@@ -1,10 +1,10 @@
 // Copyright 2018 Ryan Dahl <ry@tinyclouds.org>
 // All rights reserved. MIT License.
-type MessageCallback = (msg: ArrayBuffer) => void;
+type MessageCallback = (msg: Uint8Array) => void;
 
 interface Deno {
   recv(cb: MessageCallback): void;
-  send(msg: ArrayBuffer): null | ArrayBuffer;
+  send(msg: ArrayBufferView): null | Uint8Array;
   print(x: string): void;
 }
 
diff --git a/js/main.ts b/js/main.ts
index 027baa3cf..379b69c97 100644
--- a/js/main.ts
+++ b/js/main.ts
@@ -19,7 +19,7 @@ function assignCmdId(): number {
   return cmdId;
 }
 
-function startMsg(cmdId: number): ArrayBuffer {
+function startMsg(cmdId: number): Uint8Array {
   const builder = new flatbuffers.Builder();
   const msg = fbs.Start.createStart(builder, 0);
   fbs.Base.startBase(builder);
@@ -27,7 +27,7 @@ function startMsg(cmdId: number): ArrayBuffer {
   fbs.Base.addMsg(builder, msg);
   fbs.Base.addMsgType(builder, fbs.Any.Start);
   builder.finish(fbs.Base.endBase(builder));
-  return typedArrayToArrayBuffer(builder.asUint8Array());
+  return builder.asUint8Array();
 }
 
 window["denoMain"] = () => {
@@ -47,7 +47,7 @@ window["denoMain"] = () => {
   }
 
   // Deserialize res into startResMsg.
-  const bb = new flatbuffers.ByteBuffer(new Uint8Array(res));
+  const bb = new flatbuffers.ByteBuffer(res);
   const base = fbs.Base.getRootAsBase(bb);
   assert(base.cmdId() === cmdId);
   assert(fbs.Any.StartRes === base.msgType());
@@ -69,10 +69,3 @@ window["denoMain"] = () => {
   mod.compileAndRun();
   */
 };
-
-function typedArrayToArrayBuffer(ta: Uint8Array): ArrayBuffer {
-  return ta.buffer.slice(
-    ta.byteOffset,
-    ta.byteOffset + ta.byteLength
-  ) as ArrayBuffer;
-}
diff --git a/js/mock_runtime.js b/js/mock_runtime.js
index 66a34657e..934fbeab4 100644
--- a/js/mock_runtime.js
+++ b/js/mock_runtime.js
@@ -7,10 +7,6 @@ function assert(cond) {
   if (!cond) throw Error("mock_runtime.js assert failed");
 }
 
-global.typedArrayToArrayBuffer = ta => {
-  return ta.buffer.slice(ta.byteOffset, ta.byteOffset + ta.byteLength);
-};
-
 global.CanCallFunction = () => {
   deno.print("Hello world from foo");
   return "foo";
@@ -41,22 +37,20 @@ global.SendByteLength = () => {
 };
 
 global.RecvReturnEmpty = () => {
-  const ui8 = new Uint8Array("abc".split("").map(c => c.charCodeAt(0)));
-  const ab = typedArrayToArrayBuffer(ui8);
-  let r = deno.send(ab);
-  assert(r == null);
-  r = deno.send(ab);
-  assert(r == null);
+  const m1 = new Uint8Array("abc".split("").map(c => c.charCodeAt(0)));
+  const m2 = m1.slice();
+  const r1 = deno.send(m1);
+  assert(r1 == null);
+  const r2 = deno.send(m2);
+  assert(r2 == null);
 };
 
 global.RecvReturnBar = () => {
-  const ui8 = new Uint8Array("abc".split("").map(c => c.charCodeAt(0)));
-  const ab = typedArrayToArrayBuffer(ui8);
-  const r = deno.send(ab);
-  assert(r instanceof ArrayBuffer);
+  const m = new Uint8Array("abc".split("").map(c => c.charCodeAt(0)));
+  const r = deno.send(m);
+  assert(r instanceof Uint8Array);
   assert(r.byteLength === 3);
-  const rui8 = new Uint8Array(r);
-  const rstr = String.fromCharCode(...rui8);
+  const rstr = String.fromCharCode(...r);
   assert(rstr === "bar");
 };
 
@@ -67,6 +61,59 @@ global.DoubleRecvFails = () => {
   deno.recv((channel, msg) => assert(false));
 };
 
+global.SendRecvSlice = () => {
+  const abLen = 1024;
+  let buf = new Uint8Array(abLen);
+  for (let i = 0; i < 5; i++) {
+    // Set first and last byte, for verification by the native side.
+    buf[0] = 100 + i;
+    buf[buf.length - 1] = 100 - i;
+    // On the native side, the slice is shortened by 19 bytes.
+    buf = deno.send(buf);
+    assert(buf.byteOffset === i * 11);
+    assert(buf.byteLength === abLen - i * 30 - 19);
+    assert(buf.buffer.byteLength == abLen);
+    // Look for values written by the backend.
+    assert(buf[0] === 200 + i);
+    assert(buf[buf.length - 1] === 200 - i);
+    // On the JS side, the start of the slice is moved up by 11 bytes.
+    buf = buf.subarray(11);
+    assert(buf.byteOffset === (i + 1) * 11);
+    assert(buf.byteLength === abLen - (i + 1) * 30);
+  }
+};
+
+global.JSSendArrayBufferViewTypes = () => {
+  // Test that ArrayBufferView slices are transferred correctly.
+  // Send Uint8Array.
+  const ab1 = new ArrayBuffer(4321);
+  const u8 = new Uint8Array(ab1, 2468, 1000);
+  u8[0] = 1;
+  deno.send(u8);
+  // Send Uint32Array.
+  const ab2 = new ArrayBuffer(4321);
+  const u32 = new Uint32Array(ab2, 2468, 1000 / Uint32Array.BYTES_PER_ELEMENT);
+  u32[0] = 0x02020202;
+  deno.send(u32);
+  // Send DataView.
+  const ab3 = new ArrayBuffer(4321);
+  const dv = new DataView(ab3, 2468, 1000);
+  dv.setUint8(0, 3);
+  deno.send(dv);
+};
+
+global.JSSendNeutersBuffer = () => {
+  // Buffer should be neutered after transferring it to the native side.
+  const u8 = new Uint8Array([42]);
+  assert(u8.byteLength === 1);
+  assert(u8.buffer.byteLength === 1);
+  assert(u8[0] === 42);
+  const r = deno.send(u8);
+  assert(u8.byteLength === 0);
+  assert(u8.buffer.byteLength === 0);
+  assert(u8[0] === undefined);
+};
+
 // The following join has caused SnapshotBug to segfault when using kKeep.
 [].join("");
 
@@ -82,7 +129,7 @@ global.ErrorHandling = () => {
     assert(line === 3);
     assert(col === 1);
     assert(error instanceof Error);
-    deno.send(typedArrayToArrayBuffer(new Uint8Array([42])));
+    deno.send(new Uint8Array([42]));
   };
   eval("\n\n notdefined()\n//# sourceURL=helloworld.js");
 };
diff --git a/src/binding.cc b/src/binding.cc
index 50b029b90..2c4b6f722 100644
--- a/src/binding.cc
+++ b/src/binding.cc
@@ -139,6 +139,34 @@ void Print(const v8::FunctionCallbackInfo<v8::Value>& args) {
   fflush(stdout);
 }
 
+static v8::Local<v8::Uint8Array> ImportBuf(v8::Isolate* isolate, deno_buf buf) {
+  auto ab = v8::ArrayBuffer::New(
+      isolate, reinterpret_cast<void*>(buf.alloc_ptr), buf.alloc_len,
+      v8::ArrayBufferCreationMode::kInternalized);
+  auto view =
+      v8::Uint8Array::New(ab, buf.data_ptr - buf.alloc_ptr, buf.data_len);
+  return view;
+}
+
+static deno_buf ExportBuf(v8::Isolate* isolate,
+                          v8::Local<v8::ArrayBufferView> view) {
+  auto ab = view->Buffer();
+  auto contents = ab->Externalize();
+
+  deno_buf buf;
+  buf.alloc_ptr = reinterpret_cast<uint8_t*>(contents.Data());
+  buf.alloc_len = contents.ByteLength();
+  buf.data_ptr = buf.alloc_ptr + view->ByteOffset();
+  buf.data_len = view->ByteLength();
+
+  // Prevent JS from modifying buffer contents after exporting.
+  ab->Neuter();
+
+  return buf;
+}
+
+static void FreeBuf(deno_buf buf) { free(buf.alloc_ptr); }
+
 // Sets the recv callback.
 void Recv(const v8::FunctionCallbackInfo<v8::Value>& args) {
   v8::Isolate* isolate = args.GetIsolate();
@@ -169,20 +197,21 @@ void Send(const v8::FunctionCallbackInfo<v8::Value>& args) {
 
   CHECK_EQ(args.Length(), 1);
   v8::Local<v8::Value> ab_v = args[0];
-  CHECK(ab_v->IsArrayBuffer());
-  auto ab = v8::Local<v8::ArrayBuffer>::Cast(ab_v);
-  auto contents = ab->GetContents();
-
-  // data is only a valid pointer until the end of this call.
-  const char* data =
-      const_cast<const char*>(reinterpret_cast<char*>(contents.Data()));
-  deno_buf buf{data, contents.ByteLength()};
+  CHECK(ab_v->IsArrayBufferView());
+  auto buf = ExportBuf(isolate, v8::Local<v8::ArrayBufferView>::Cast(ab_v));
 
   DCHECK_EQ(d->currentArgs, nullptr);
   d->currentArgs = &args;
 
   d->cb(d, buf);
 
+  // Buffer is only valid until the end of the callback.
+  // TODO(piscisaureus):
+  //   It's possible that data in the buffer is needed after the callback
+  //   returns, e.g. when the handler offloads work to a thread pool, therefore
+  //   make the callback responsible for releasing the buffer.
+  FreeBuf(buf);
+
   d->currentArgs = nullptr;
 }
 
@@ -303,12 +332,8 @@ int deno_send(Deno* d, deno_buf buf) {
     return 0;
   }
 
-  // TODO(ry) support zero-copy.
-  auto ab = v8::ArrayBuffer::New(d->isolate, buf.len);
-  memcpy(ab->GetContents().Data(), buf.data, buf.len);
-
   v8::Local<v8::Value> args[1];
-  args[0] = ab;
+  args[0] = deno::ImportBuf(d->isolate, buf);
   recv->Call(context->Global(), 1, args);
 
   if (try_catch.HasCaught()) {
@@ -320,9 +345,7 @@ int deno_send(Deno* d, deno_buf buf) {
 }
 
 void deno_set_response(Deno* d, deno_buf buf) {
-  // TODO(ry) Support zero-copy.
-  auto ab = v8::ArrayBuffer::New(d->isolate, buf.len);
-  memcpy(ab->GetContents().Data(), buf.data, buf.len);
+  auto ab = deno::ImportBuf(d->isolate, buf);
   d->currentArgs->GetReturnValue().Set(ab);
 }
 
diff --git a/src/deno.h b/src/deno.h
index fd2b4766a..ab214ab03 100644
--- a/src/deno.h
+++ b/src/deno.h
@@ -3,6 +3,7 @@
 #ifndef DENO_H_
 #define DENO_H_
 #include <stddef.h>
+#include <stdint.h>
 // Neither Rust nor Go support calling directly into C++ functions, therefore
 // the public interface to libdeno is done in C.
 #ifdef __cplusplus
@@ -11,8 +12,10 @@ extern "C" {
 
 // Data that gets transmitted.
 typedef struct {
-  const char* data;
-  size_t len;
+  uint8_t* alloc_ptr;  // Start of memory allocation (returned from `malloc()`).
+  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.
 } deno_buf;
 
 struct deno_s;
@@ -37,11 +40,17 @@ int deno_execute(Deno* d, const char* js_filename, const char* js_source);
 // Routes message to the javascript callback set with deno.recv(). A false
 // return value indicates error. Check deno_last_exception() for exception text.
 // 0 = fail, 1 = success
+// After calling deno_send(), the caller no longer owns `buf` and must not use
+// it; deno_send() is responsible for releasing it's memory.
+// TODO(piscisaureus) In C++ and/or Rust, use a smart pointer or similar to
+// enforce this rule.
 int deno_send(Deno* d, deno_buf buf);
 
 // Call this inside a deno_recv_cb to respond synchronously to messages.
 // If this is not called during the life time of a deno_recv_cb callback
 // the deno.send() call in javascript will return null.
+// After calling deno_set_response(), the caller no longer owns `buf` and must
+// not access it; deno_set_response() is responsible for releasing it's memory.
 void deno_set_response(Deno* d, deno_buf buf);
 
 const char* deno_last_exception(Deno* d);
diff --git a/src/flatbuffer_builder.cc b/src/flatbuffer_builder.cc
new file mode 100644
index 000000000..8a7a0c649
--- /dev/null
+++ b/src/flatbuffer_builder.cc
@@ -0,0 +1,78 @@
+// Copyright 2018 Bert Belder <bertbelder@gmail.com>
+// All rights reserved. MIT License.
+
+#include <stddef.h>
+#include <stdint.h>
+#include <stdlib.h>
+
+#include "deno.h"
+#include "flatbuffer_builder.h"
+#include "flatbuffers/flatbuffers.h"
+
+namespace deno {
+
+deno_buf FlatBufferBuilder::ExportBuf() {
+  uint8_t* data_ptr = GetBufferPointer();
+  size_t data_len = GetSize();
+  return allocator_.GetAndKeepBuf(data_ptr, data_len);
+}
+
+deno_buf FlatBufferBuilder::Allocator::GetAndKeepBuf(uint8_t* data_ptr,
+                                                     size_t data_len) {
+  // The builder will typically allocate one chunk of memory with some
+  // default size. After that, it'll only call allocate() again if the
+  // initial allocation wasn't big enough, which is then immediately
+  // followed by deallocate() to release the buffer that was too small.
+  //
+  // Therefore we can assume that the `data_ptr` points somewhere inside
+  // the last allocation, and that we never have to protect earlier
+  // allocations from being released.
+  //
+  // Each builder gets it's own Allocator instance, so multiple builders
+  // can be exist at the same time without conflicts.
+
+  assert(last_alloc_ptr_ != nullptr);   // Must have allocated.
+  assert(keep_alloc_ptr_ == nullptr);   // Didn't export any buffer so far.
+  assert(data_ptr >= last_alloc_ptr_);  // Data must be within allocation.
+  assert(data_ptr + data_len <= last_alloc_ptr_ + last_alloc_len_);
+
+  keep_alloc_ptr_ = last_alloc_ptr_;
+
+  deno_buf buf;
+  buf.alloc_ptr = last_alloc_ptr_;
+  buf.alloc_len = last_alloc_len_;
+  buf.data_ptr = data_ptr;
+  buf.data_len = data_len;
+  return buf;
+}
+
+uint8_t* FlatBufferBuilder::Allocator::allocate(size_t size) {
+  auto ptr = reinterpret_cast<uint8_t*>(malloc(size));
+  if (ptr == nullptr) {
+    return nullptr;
+  }
+
+  last_alloc_ptr_ = ptr;
+  last_alloc_len_ = size;
+
+  return ptr;
+}
+
+void FlatBufferBuilder::Allocator::deallocate(uint8_t* ptr, size_t size) {
+  if (ptr == last_alloc_ptr_) {
+    last_alloc_ptr_ = nullptr;
+    last_alloc_len_ = 0;
+  }
+
+  if (ptr == keep_alloc_ptr_) {
+    // This allocation became an exported buffer, so don't free it.
+    // Clearing keep_alloc_ptr_ makes it possible to export another
+    // buffer later (after the builder is reset with `Reset()`).
+    keep_alloc_ptr_ = nullptr;
+    return;
+  }
+
+  free(ptr);
+}
+
+}  // namespace deno
diff --git a/src/flatbuffer_builder.h b/src/flatbuffer_builder.h
new file mode 100644
index 000000000..1bc973afe
--- /dev/null
+++ b/src/flatbuffer_builder.h
@@ -0,0 +1,64 @@
+// Copyright 2018 Bert Belder <bertbelder@gmail.com>
+// All rights reserved. MIT License.
+#ifndef FLATBUFFER_BUILDER_H_
+#define FLATBUFFER_BUILDER_H_
+
+#include <stddef.h>
+#include <stdint.h>
+#include <stdlib.h>
+
+#include "deno.h"
+#include "flatbuffers/flatbuffers.h"
+
+namespace deno {
+
+// Wrap the default FlatBufferBuilder class, because the default one can't give
+// us a pointer to the output buffer that we own. Nominally,
+// FlatBufferBuilder::Release() should do that, but it returns some
+// smart-pointer-like object (DetachedBuffer) that frees the buffer when it goes
+// out of scope.
+//
+// This wrapper adds the `ExportBuf` method that returns a deno_buf, which
+// is really not owned at all -- the caller is responsible for releasing the
+// allocation with free().
+//
+// The alternative allocator also uses malloc()/free(), rather than
+// new/delete[], so that the exported buffer can be later be converted to an
+// ArrayBuffer; the (default) V8 ArrayBuffer allocator also uses free().
+class FlatBufferBuilder : public flatbuffers::FlatBufferBuilder {
+  static const size_t kDefaultInitialSize = 1024;
+
+  class Allocator : public flatbuffers::Allocator {
+    uint8_t* keep_alloc_ptr_ = nullptr;
+    uint8_t* last_alloc_ptr_ = nullptr;
+    size_t last_alloc_len_ = 0;
+
+   public:
+    deno_buf GetAndKeepBuf(uint8_t* data_ptr, size_t data_len);
+
+   protected:
+    virtual uint8_t* allocate(size_t size);
+    virtual void deallocate(uint8_t* ptr, size_t size);
+  };
+
+  Allocator allocator_;
+
+ public:
+  explicit FlatBufferBuilder(size_t initial_size = kDefaultInitialSize)
+      : flatbuffers::FlatBufferBuilder(initial_size, &allocator_) {}
+
+  // Export the finalized flatbuffer as a deno_buf structure. The caller takes
+  // ownership of the underlying memory allocation, which must be released with
+  // free().
+  // Afer calling ExportBuf() the FlatBufferBuilder should no longer be used;
+  // However it can be used again once it is reset with the Reset() method.
+  deno_buf ExportBuf();
+
+  // Don't use these.
+  flatbuffers::DetachedBuffer Release() = delete;
+  flatbuffers::DetachedBuffer ReleaseBufferPointer() = delete;
+};
+
+}  // namespace deno
+
+#endif  // FLATBUFFER_BUILDER_H_
diff --git a/src/flatbuffer_builder_test.cc b/src/flatbuffer_builder_test.cc
new file mode 100644
index 000000000..cfa12f0a0
--- /dev/null
+++ b/src/flatbuffer_builder_test.cc
@@ -0,0 +1,78 @@
+// Copyright 2018 Bert Belder <bertbelder@gmail.com>
+// All rights reserved. MIT License.
+
+#include <stdint.h>
+
+#include "testing/gtest/include/gtest/gtest.h"
+
+#include "deno.h"
+#include "flatbuffer_builder.h"
+
+template <typename T, std::size_t N>
+constexpr std::size_t countof(T const (&)[N]) noexcept {
+  return N;
+}
+
+TEST(FlatBufferBuilderTest, ExportBuf) {
+  const uint32_t nums[] = {1, 2, 3};
+  const char str[] = "hello mars";
+  deno_buf nums_buf;
+  deno_buf str_buf;
+  // Use scope so builder gets destroyed after building buffers.
+  {
+    deno::FlatBufferBuilder builder;
+    // Build first flatbuffer.
+    auto nums_fb = builder.CreateVector(nums, countof(nums));
+    builder.Finish(nums_fb);
+    nums_buf = builder.ExportBuf();
+    // Reset builder.
+    builder.Reset();
+    // Build second flatbuffer using the same builder.
+    auto str_fb = builder.CreateString(str);
+    builder.Finish(str_fb);
+    str_buf = builder.ExportBuf();
+  }
+  // Allocations should be different.
+  EXPECT_NE(nums_buf.alloc_ptr, str_buf.alloc_ptr);
+  // Logical buffer data should be contained inside their allocations.
+  EXPECT_GE(nums_buf.data_ptr, nums_buf.alloc_ptr);
+  EXPECT_LE(nums_buf.data_ptr + nums_buf.data_len,
+            nums_buf.alloc_ptr + nums_buf.alloc_len);
+  EXPECT_GE(str_buf.data_ptr, str_buf.alloc_ptr);
+  EXPECT_LE(str_buf.data_ptr + str_buf.data_len,
+            str_buf.alloc_ptr + str_buf.alloc_len);
+  // Since there is no way to parse these buffers without generating code,
+  // just check whether the data is contained in the raw content.
+  // Both the numbers vector and the string start at offset 8 in the flatbuffer.
+  auto nums_data = reinterpret_cast<uint32_t*>(nums_buf.data_ptr + 8);
+  for (size_t i = 0; i < countof(nums); i++) {
+    EXPECT_EQ(nums_data[i], nums[i]);
+  }
+  auto str_data = str_buf.data_ptr + 8;
+  for (size_t i = 0; i < countof(str); i++) {
+    EXPECT_EQ(str_data[i], str[i]);
+  }
+}
+
+TEST(FlatBufferBuilderTest, CanGrowBuffer) {
+  static const size_t kSmallInitialSize = 32;
+  static const char zeroes[1024] = {0};
+  {
+    // Create buffer with small initial size.
+    deno::FlatBufferBuilder builder(kSmallInitialSize);
+    // Write 1 byte and export buffer.
+    builder.Finish(builder.CreateVector(zeroes, 1));
+    auto buf = builder.ExportBuf();
+    // Exported buffer should have initial size.
+    EXPECT_EQ(buf.alloc_len, kSmallInitialSize);
+  }
+  {
+    // Create buffer with small initial size.
+    deno::FlatBufferBuilder builder(kSmallInitialSize);
+    // Write 1024 bytes and export buffer.
+    builder.Finish(builder.CreateVector(zeroes, countof(zeroes)));
+    auto buf = builder.ExportBuf();
+    // Exported buffer have grown.
+    EXPECT_GT(buf.alloc_len, kSmallInitialSize);
+  }
+}
diff --git a/src/main.cc b/src/main.cc
index a801bc292..81b8a2710 100644
--- a/src/main.cc
+++ b/src/main.cc
@@ -11,6 +11,7 @@
 #endif
 
 #include "deno.h"
+#include "flatbuffer_builder.h"
 #include "flatbuffers/flatbuffers.h"
 #include "src/handlers.h"
 #include "src/msg_generated.h"
@@ -23,7 +24,7 @@ static int global_argc;
 
 // Sends StartRes message
 void HandleStart(Deno* d, uint32_t cmd_id) {
-  flatbuffers::FlatBufferBuilder builder;
+  deno::FlatBufferBuilder builder;
 
   char cwdbuf[1024];
   // TODO(piscisaureus): support unicode on windows.
@@ -39,9 +40,7 @@ void HandleStart(Deno* d, uint32_t cmd_id) {
   auto start_msg = CreateStartRes(builder, start_cwd, start_argv);
   auto base = CreateBase(builder, cmd_id, 0, Any_StartRes, start_msg.Union());
   builder.Finish(base);
-  deno_buf bufout{reinterpret_cast<const char*>(builder.GetBufferPointer()),
-                  builder.GetSize()};
-  deno_set_response(d, bufout);
+  deno_set_response(d, builder.ExportBuf());
 }
 
 void HandleCodeFetch(Deno* d, uint32_t cmd_id, const CodeFetch* msg) {
@@ -54,11 +53,10 @@ void HandleCodeFetch(Deno* d, uint32_t cmd_id, const CodeFetch* msg) {
 }
 
 void MessagesFromJS(Deno* d, deno_buf buf) {
-  auto data = reinterpret_cast<const uint8_t*>(buf.data);
-  flatbuffers::Verifier verifier(data, buf.len);
+  flatbuffers::Verifier verifier(buf.data_ptr, buf.data_len);
   DCHECK(verifier.VerifyBuffer<Base>());
 
-  auto base = flatbuffers::GetRoot<Base>(buf.data);
+  auto base = flatbuffers::GetRoot<Base>(buf.data_ptr);
   auto cmd_id = base->cmdId();
   auto msg_type = base->msg_type();
   const char* msg_type_name = EnumNamesAny()[msg_type];
diff --git a/src/main.rs b/src/main.rs
index ece311a4f..2755a3a9c 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -9,8 +9,10 @@ use std::ptr;
 
 #[repr(C)]
 struct deno_buf {
-    data: *const c_char,
-    len: c_int, // TODO(ry) should be size_t.
+    alloc_ptr: *mut u8,
+    alloc_len: usize,
+    data_ptr: *mut u8,
+    data_len: usize,
 }
 
 #[repr(C)]
@@ -120,9 +122,10 @@ fn main() {
 
     let mut d = Deno::new();
 
-    d.execute("deno_main.js", "denoMain();")
-        .unwrap_or_else(|err| {
+    d.execute("deno_main.js", "denoMain();").unwrap_or_else(
+        |err| {
             println!("Error {}\n", err);
             std::process::exit(1);
-        });
+        },
+    );
 }
diff --git a/src/mock_runtime_test.cc b/src/mock_runtime_test.cc
index 67b097e8d..19b224950 100644
--- a/src/mock_runtime_test.cc
+++ b/src/mock_runtime_test.cc
@@ -23,7 +23,17 @@ TEST(MockRuntimeTest, ErrorsCorrectly) {
   deno_delete(d);
 }
 
-deno_buf strbuf(const char* str) { return deno_buf{str, strlen(str)}; }
+deno_buf strbuf(const char* str) {
+  auto len = strlen(str);
+
+  deno_buf buf;
+  buf.alloc_ptr = reinterpret_cast<uint8_t*>(strdup(str));
+  buf.alloc_len = len + 1;
+  buf.data_ptr = buf.alloc_ptr;
+  buf.data_len = len;
+
+  return buf;
+}
 
 TEST(MockRuntimeTest, SendSuccess) {
   Deno* d = deno_new(nullptr, nullptr);
@@ -51,10 +61,10 @@ TEST(MockRuntimeTest, RecvReturnEmpty) {
   static int count = 0;
   Deno* d = deno_new(nullptr, [](auto _, auto buf) {
     count++;
-    EXPECT_EQ(static_cast<size_t>(3), buf.len);
-    EXPECT_EQ(buf.data[0], 'a');
-    EXPECT_EQ(buf.data[1], 'b');
-    EXPECT_EQ(buf.data[2], 'c');
+    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');
   });
   EXPECT_TRUE(deno_execute(d, "a.js", "RecvReturnEmpty()"));
   EXPECT_EQ(count, 2);
@@ -65,10 +75,10 @@ TEST(MockRuntimeTest, RecvReturnBar) {
   static int count = 0;
   Deno* d = deno_new(nullptr, [](auto deno, auto buf) {
     count++;
-    EXPECT_EQ(static_cast<size_t>(3), buf.len);
-    EXPECT_EQ(buf.data[0], 'a');
-    EXPECT_EQ(buf.data[1], 'b');
-    EXPECT_EQ(buf.data[2], 'c');
+    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_set_response(deno, strbuf("bar"));
   });
   EXPECT_TRUE(deno_execute(d, "a.js", "RecvReturnBar()"));
@@ -82,6 +92,65 @@ TEST(MockRuntimeTest, DoubleRecvFails) {
   deno_delete(d);
 }
 
+TEST(MockRuntimeTest, SendRecvSlice) {
+  static int count = 0;
+  Deno* d = deno_new(nullptr, [](auto deno, auto buf) {
+    static const size_t alloc_len = 1024;
+    size_t i = count++;
+    // Check the size and offset of the slice.
+    size_t data_offset = buf.data_ptr - buf.alloc_ptr;
+    EXPECT_EQ(data_offset, i * 11);
+    EXPECT_EQ(buf.data_len, alloc_len - i * 30);
+    EXPECT_EQ(buf.alloc_len, alloc_len);
+    // Check values written by the JS side.
+    EXPECT_EQ(buf.data_ptr[0], 100 + i);
+    EXPECT_EQ(buf.data_ptr[buf.data_len - 1], 100 - i);
+    // Make copy of the backing buffer -- this is currently necessary because
+    // deno_set_response() takes ownership over the buffer, but we are not given
+    // ownership of `buf` by our caller.
+    uint8_t* alloc_ptr = reinterpret_cast<uint8_t*>(malloc(alloc_len));
+    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};
+    // 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_set_response(deno, buf2);
+  });
+  EXPECT_TRUE(deno_execute(d, "a.js", "SendRecvSlice()"));
+  EXPECT_EQ(count, 5);
+  deno_delete(d);
+}
+
+TEST(MockRuntimeTest, JSSendArrayBufferViewTypes) {
+  static int count = 0;
+  Deno* d = deno_new(nullptr, [](auto _, auto buf) {
+    count++;
+    size_t data_offset = buf.data_ptr - buf.alloc_ptr;
+    EXPECT_EQ(data_offset, 2468);
+    EXPECT_EQ(buf.data_len, 1000);
+    EXPECT_EQ(buf.alloc_len, 4321);
+    EXPECT_EQ(buf.data_ptr[0], count);
+  });
+  EXPECT_TRUE(deno_execute(d, "a.js", "JSSendArrayBufferViewTypes()"));
+  EXPECT_EQ(count, 3);
+  deno_delete(d);
+}
+
+TEST(MockRuntimeTest, JSSendNeutersBuffer) {
+  static int count = 0;
+  Deno* d = deno_new(nullptr, [](auto _, auto buf) {
+    count++;
+    EXPECT_EQ(buf.data_len, 1);
+    EXPECT_EQ(buf.data_ptr[0], 42);
+  });
+  EXPECT_TRUE(deno_execute(d, "a.js", "JSSendNeutersBuffer()"));
+  EXPECT_EQ(count, 1);
+  deno_delete(d);
+}
+
 TEST(MockRuntimeTest, TypedArraySnapshots) {
   Deno* d = deno_new(nullptr, nullptr);
   EXPECT_TRUE(deno_execute(d, "a.js", "TypedArraySnapshots()"));
@@ -98,8 +167,8 @@ TEST(MockRuntimeTest, ErrorHandling) {
   static int count = 0;
   Deno* d = deno_new(nullptr, [](auto deno, auto buf) {
     count++;
-    EXPECT_EQ(static_cast<size_t>(1), buf.len);
-    EXPECT_EQ(buf.data[0], 42);
+    EXPECT_EQ(static_cast<size_t>(1), buf.data_len);
+    EXPECT_EQ(buf.data_ptr[0], 42);
   });
   EXPECT_FALSE(deno_execute(d, "a.js", "ErrorHandling()"));
   EXPECT_EQ(count, 1);