[wasm] Update WasmMemoryObject correctly when module memory is exported.

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 f0598556b4838f9606108e7f4298b6968da72d50..32d365e388ca2d3f65d6f689c00e5f3080928583 100644
--- a/src/wasm/wasm-module.cc
+++ b/src/wasm/wasm-module.cc
@@ -1791,10 +1791,26 @@ class WasmInstanceBuilder {
             // If there was no imported WebAssembly.Memory object, create one.
             Handle<JSArrayBuffer> buffer(instance->get_memory_buffer(),
                                          isolate_);
-            memory_object = WasmMemoryObject::New(
-                isolate_, buffer,
-                (module_->max_mem_pages != 0) ? module_->max_mem_pages : -1);
-            instance->set_memory_object(*memory_object);
+            if (!buffer.is_null() && buffer->has_guard_region()) {

[Eric Holk, 2016/12/05 19:54:52]

We talked about this offline, but I'm summarizing here.

It's probably best to do this branch using wasm::NewArrayBuffer. To match the
old behavior, I think the right thing to do is to copy the old buffer into a
newly allocated buffer. Otherwise we'll end up with inconsistencies where once
the buffer grows sometimes it looks like the old size and sometimes it looks
like the new size.

[gdeepti, 2016/12/07 07:10:33]

Summarizing offline discussions, previous interpretation of the spec was
incorrect, exported memory objects reference instance memory, and should reflect
new size when grow_memory is called. Removed this block of code. 

+              // If guard pages are enabled, use a different buffer to create
+              // the exported memory object. Using the same buffer will result
+              // in the exported memory object having a handle to the buffer
+              // associated with a live instance.
+              Handle<JSArrayBuffer> new_buffer =
+                  isolate_->factory()->NewJSArrayBuffer();
+              JSArrayBuffer::Setup(new_buffer, isolate_, true,
+                                   buffer->backing_store(),
+                                   buffer->byte_length()->Number());
+              new_buffer->set_is_neuterable(false);
+              new_buffer->set_has_guard_region(true);
+              memory_object = WasmMemoryObject::New(
+                  isolate_, new_buffer,
+                  (module_->max_mem_pages != 0) ? module_->max_mem_pages : -1);
+            } else {
+              memory_object = WasmMemoryObject::New(
+                  isolate_, buffer,
+                  (module_->max_mem_pages != 0) ? module_->max_mem_pages : -1);
+            }
           } else {
             memory_object = Handle<WasmMemoryObject>(
                 instance->get_memory_object(), isolate_);