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, 8 insertions, 3 deletions

diff --git a/core/examples/http_bench.rs b/core/examples/http_bench.rs
index 3e02cdd46..b355f5568 100644
--- a/core/examples/http_bench.rs
+++ b/core/examples/http_bench.rs
@@ -111,7 +111,10 @@ fn test_record_from() {
 
 pub type HttpBenchOp = dyn Future<Item = i32, Error = std::io::Error> + Send;
 
-fn dispatch(control: &[u8], zero_copy_buf: deno_buf) -> (bool, Box<Op>) {
+fn dispatch(
+  control: &[u8],
+  zero_copy_buf: Option<PinnedBuf>,
+) -> (bool, Box<Op>) {
   let record = Record::from(control);
   let is_sync = record.promise_id == 0;
   let http_bench_op = match record.op_id {
@@ -266,8 +269,9 @@ fn op_close(rid: i32) -> Box<HttpBenchOp> {
   }))
 }
 
-fn op_read(rid: i32, mut zero_copy_buf: deno_buf) -> Box<HttpBenchOp> {
+fn op_read(rid: i32, zero_copy_buf: Option<PinnedBuf>) -> Box<HttpBenchOp> {
   debug!("read rid={}", rid);
+  let mut zero_copy_buf = zero_copy_buf.unwrap();
   Box::new(
     futures::future::poll_fn(move || {
       let mut table = RESOURCE_TABLE.lock().unwrap();
@@ -285,8 +289,9 @@ fn op_read(rid: i32, mut zero_copy_buf: deno_buf) -> Box<HttpBenchOp> {
   )
 }
 
-fn op_write(rid: i32, zero_copy_buf: deno_buf) -> Box<HttpBenchOp> {
+fn op_write(rid: i32, zero_copy_buf: Option<PinnedBuf>) -> Box<HttpBenchOp> {
   debug!("write rid={}", rid);
+  let zero_copy_buf = zero_copy_buf.unwrap();
   Box::new(
     futures::future::poll_fn(move || {
       let mut table = RESOURCE_TABLE.lock().unwrap();