[WASM] Use better error message when decoding function body fails.

src/wasm/module-decoder.cc

 // Copyright 2015 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/wasm/module-decoder.h"
 #include "src/wasm/function-body-decoder-impl.h"
 
 #include "src/base/functional.h"
 #include "src/base/platform/platform.h"
 #include "src/counters.h"
@@ skipping 942 matching lines @@
       os << "Verifying WASM function " << func_name << std::endl;
     }
     FunctionBody body = {
         function->sig, start_,
         start_ + GetBufferRelativeOffset(function->code_start_offset),
         start_ + GetBufferRelativeOffset(function->code_end_offset)};
     DecodeResult result = VerifyWasmCode(
         allocator, menv == nullptr ? nullptr : menv->module_env.module, body);
     if (result.failed()) {
       // Wrap the error message from the function decoder.
-      std::ostringstream str;
-      str << "in function " << func_name << ": " << result.error_msg();
+      std::ostringstream wrapped;
+      wrapped << "in function " << func_name << ": " << result.error_msg();
+      result.error(result.error_offset(), wrapped.str());

[Mircea Trofin, 2017/05/31 14:51:42]

Does str() flush?

[bbudge, 2017/06/01 00:53:26]

I think flush is a nop on string streams, since data can be written immediately.
The docs say that str() is a copy of the current contents of the stream.

 
       // Set error code and location, if this is the first error.
       if (intermediate_result_.ok()) {
         intermediate_result_.MoveErrorFrom(result);
       }
     }
   }
 
   uint32_t consume_string(uint32_t* length, bool validate_utf8,
                           const char* name) {
@@ skipping 436 matching lines @@
     result.push_back({section_start, name_offset, name_length, payload_offset,
                       payload_length, section_length});
   }
 
   return result;
 }
 
 }  // namespace wasm
 }  // namespace internal
 }  // namespace v8