Refactor zero-copy buffers for performance and to prevent memory leaks

* In order to prevent ArrayBuffers from getting garbage collected by V8,
  we used to store a v8::Persistent<ArrayBuffer> in a map. This patch
  introduces a custom ArrayBuffer allocator which doesn't use Persistent
  handles, but instead stores a pointer to the actual ArrayBuffer data
  alongside with a reference count. Since creating Persistent handles
  has quite a bit of overhead, this change significantly increases
  performance. Various HTTP server benchmarks report about 5-10% more
  requests per second than before.

* Previously the Persistent handle that prevented garbage collection had
  to be released manually, and this wasn't always done, which was
  causing memory leaks. This has been resolved by introducing a new
  `PinnedBuf` type in both Rust and C++ that automatically re-enables
  garbage collection when it goes out of scope.

* Zero-copy buffers are now correctly wrapped in an Option if there is a
  possibility that they're not present. This clears up a correctness
  issue where we were creating zero-length slices from a null pointer,
  which is against the rules.

1 files changed, 4 insertions, 3 deletions

diff --git a/core/libdeno/modules_test.cc b/core/libdeno/modules_test.cc
index 0eaa9e8eb..f3a97396f 100644
--- a/core/libdeno/modules_test.cc
+++ b/core/libdeno/modules_test.cc
@@ -1,13 +1,14 @@
-// Copyright 2018 the Deno authors. All rights reserved. MIT license.
+// Copyright 2018-2019 the Deno authors. All rights reserved. MIT license.
 #include "test.h"
 
 static int exec_count = 0;
-void recv_cb(void* user_data, deno_buf buf, deno_buf zero_copy_buf) {
+void recv_cb(void* user_data, deno_buf buf, deno_pinned_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);
+  EXPECT_EQ(zero_copy_buf.data_len, 0u);
+  EXPECT_EQ(zero_copy_buf.pin, nullptr);
   exec_count++;
 }