[wasm] Do not unregister an ArrayBuffer if it is already external

src/wasm/wasm-module.cc

Index: src/wasm/wasm-module.cc
diff --git a/src/wasm/wasm-module.cc b/src/wasm/wasm-module.cc
index f406ee5f83fd7c0bc5f2b4232e3c4ca5aeb0129a..348a9a915a34486f3bbc23820ab9422f6f804354 100644
--- a/src/wasm/wasm-module.cc
+++ b/src/wasm/wasm-module.cc
@@ -855,7 +855,7 @@ Handle<JSArrayBuffer> wasm::SetupArrayBuffer(Isolate* isolate,
   buffer->set_is_wasm_buffer(true);
   buffer->set_has_guard_region(enable_guard_regions);
 
-  if (is_external) {
+  if (is_external && enable_guard_regions) {
     // We mark the buffer as external if we allocated it here with guard
     // pages. That means we need to arrange for it to be freed.
 
@@ -2231,7 +2231,8 @@ bool wasm::IsWasmCodegenAllowed(Isolate* isolate, Handle<Context> context) {
 }
 
 void wasm::DetachWebAssemblyMemoryBuffer(Isolate* isolate,
-                                         Handle<JSArrayBuffer> buffer) {
+                                         Handle<JSArrayBuffer> buffer,
+                                         uint32_t pages) {

[ahaas, 2017/05/10 08:33:48]

could you just pass in a boolean flag here? I think this would make the purpose
of this parameter more obvious.

[gdeepti, 2017/05/10 15:50:35]

Done.

   int64_t byte_length =
       buffer->byte_length()->IsNumber()
           ? static_cast<uint32_t>(buffer->byte_length()->Number())
@@ -2247,6 +2248,9 @@ void wasm::DetachWebAssemblyMemoryBuffer(Isolate* isolate,
   }
   buffer->set_is_neuterable(true);
   buffer->Neuter();
+  // Neuter but do not free, as when pages == 0, the backing store is being used
+  // by the new buffer.
+  if (pages == 0) return;
   if (has_guard_regions) {
     base::OS::Free(backing_store, RoundUp(i::wasm::kWasmMaxHeapOffset,
                                           base::OS::CommitPageSize()));