Include string size when passing from WASM to JS

Summary:
This is the first part of a fix for unicode handling within the Hermes parser compiled to WASM. JS strings may contain null bytes (e.g. if they are escaped using `\0`), but the current decoding utility (emscripten's `UTF8ToString`) is built to stop at the first null byte.

In the next diff we will be introducing a new decoder to handle this issue with null bytes (along with handling of surrogate pairs from Hermes). To set up for this we need to refactor the portion of the JS/WASM bridge logic that pertains to strings. Instead of just sending a string's pointer over from WASM to JS, we must now send both the pointer and its size in bytes. These will be passed to a new `buildString` method in the JS library that decodes the string of a given length and returns a reference to it on the JS side. This reference can then be passed to an AST node builder function, just like references for nodes and node lists.

Reviewed By: gkz

Differential Revision: D24521408

fbshipit-source-id: a69d4c284aad1bba437ba6976d73f4879ba1585d

tools/hermes-parser/HermesParserJSBuilder.cpp

@@ -43,15 +43,17 @@ class HermesParserJSBuilder {
     return num;
   }
 
-  /// Pointers to null-terminated strings can be passed directly to a JS library
-  /// call. The caller in JS must then read the null-terminated string off the
-  /// WASM heap.
-  const char *buildNode(NodeLabel label) {
+  /// Decode a string within JS and return a reference to it from this function.
+  /// We must pass the size along with the pointer as the string may contain
+  /// null bytes.
+  JSReference buildNode(NodeLabel label) {
     if (label == nullptr) {
-      return nullptr;
+      return 0;
     }
 
-    return label->c_str();
+    auto str = label->str();
+
+    return buildString(str.begin(), str.size());
   }
 
   /// Arrays cannot be passed directly to a JS library call. Instead, an array

tools/hermes-parser/HermesParserJSLibrary.h

@@ -10,7 +10,7 @@
 
 using JSReference = int;
 
-using NodeLabel = const char *;
+using NodeLabel = JSReference;
 using NodeBoolean = bool;
 using NodeNumber = double;
 using NodePtr = JSReference;
@@ -19,6 +19,7 @@ using NodeList = JSReference;
 /// Definitions of JS Library functions that can be called from WASM
 
 extern "C" {
+JSReference buildString(const char *, size_t);
 JSReference buildArray();
 void appendToArray(JSReference, JSReference);
 JSReference buildSourceLocation(int, int, int, int, int, int);

tools/hermes-parser/HermesParserJSLibrary.js.template

@@ -41,18 +41,18 @@ const HermesParserJSLibrary = {
 
       return value;
     },
+  },
 
-    /**
-     * Return the string at a given pointer, returning null if the pointer is 0
-     * (corresponding to the null pointer).
-     */
-    getString: function(pointer) {
-      if (pointer == 0) {
-        return null;
-      }
+  /**
+   * Return a reference to the decoded string at a given pointer, returning 0
+   * if the pointer is 0 (corresponding to the null pointer).
+   */
+  buildString: function(pointer, size) {
+    if (pointer == 0) {
+      return 0;
+    }
 
-      return UTF8ToString(pointer);
-    },
+    return JSReferences.store(UTF8ToString(pointer, size));
   },
 
   buildArray: function() {
@@ -104,7 +104,7 @@ const HermesParserJSLibrary = {
 
 // Define wrapper functions for each node type. Numbers do not need to be wrapped.
 #define NodeNumber
-#define NodeLabel JSReferences.getString
+#define NodeLabel JSReferences.pop
 #define NodeBoolean Boolean
 #define NodePtr JSReferences.pop
 #define NodeList JSReferences.pop