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, 2 insertions, 2 deletions

diff --git a/cli/state.rs b/cli/state.rs
index 5aefe7d90..2bfc641d5 100644
--- a/cli/state.rs
+++ b/cli/state.rs
@@ -8,9 +8,9 @@ use crate::permissions::DenoPermissions;
 use crate::resources;
 use crate::resources::ResourceId;
 use crate::worker::Worker;
-use deno::deno_buf;
 use deno::Buf;
 use deno::Op;
+use deno::PinnedBuf;
 use futures::future::Shared;
 use std;
 use std::collections::HashMap;
@@ -84,7 +84,7 @@ impl ThreadSafeState {
   pub fn dispatch(
     &self,
     control: &[u8],
-    zero_copy: deno_buf,
+    zero_copy: Option<PinnedBuf>,
   ) -> (bool, Box<Op>) {
     ops::dispatch_all(self, control, zero_copy, self.dispatch_selector)
   }