Tell V8 about the extra memory being held by FileReader objects.

third_party/WebKit/Source/core/fileapi/FileReaderLoader.cpp

 /*
  * Copyright (C) 2010 Google Inc.  All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 32 matching lines @@
 #include "platform/loader/fetch/ResourceError.h"
 #include "platform/loader/fetch/ResourceRequest.h"
 #include "platform/loader/fetch/ResourceResponse.h"
 #include "platform/wtf/PassRefPtr.h"
 #include "platform/wtf/PtrUtil.h"
 #include "platform/wtf/RefPtr.h"
 #include "platform/wtf/Vector.h"
 #include "platform/wtf/text/Base64.h"
 #include "platform/wtf/text/StringBuilder.h"
 #include "public/platform/WebURLRequest.h"
+#include "v8/include/v8.h"
 
 namespace blink {
 
 FileReaderLoader::FileReaderLoader(ReadType read_type,
                                    FileReaderLoaderClient* client)
     : read_type_(read_type),
       client_(client),
       is_raw_data_converted_(false),
       string_result_(""),
       finished_loading_(false),
       bytes_loaded_(0),
       total_bytes_(-1),
       has_range_(false),
       range_start_(0),
       range_end_(0),
       error_code_(FileError::kOK) {}
 
 FileReaderLoader::~FileReaderLoader() {
   Cleanup();
+  UnreportMemoryUsageToV8();
   if (!url_for_reading_.IsEmpty()) {
     BlobRegistry::RevokePublicBlobURL(url_for_reading_);
   }
 }
 
 void FileReaderLoader::Start(ExecutionContext* execution_context,
                              PassRefPtr<BlobDataHandle> blob_data) {
   DCHECK(execution_context);
   // The blob is read by routing through the request handling layer given a
   // temporary public url.
@@ skipping 54 matching lines @@
     loader_->Cancel();
     loader_ = nullptr;
   }
 
   // If we get any error, we do not need to keep a buffer around.
   if (error_code_) {
     raw_data_.reset();
     string_result_ = "";
     is_raw_data_converted_ = true;
     decoder_.reset();
+    UnreportMemoryUsageToV8();
   }
 }
 
+void FileReaderLoader::ReportAdditionalMemoryUsageToV8(int64_t usage) {
+  memory_usage_reported_to_v8_ += usage;
+  v8::Isolate::GetCurrent()->AdjustAmountOfExternalAllocatedMemory(usage);
+}
+
+void FileReaderLoader::UnreportMemoryUsageToV8() {
+  if (!memory_usage_reported_to_v8_)
+    return;
+  v8::Isolate::GetCurrent()->AdjustAmountOfExternalAllocatedMemory(
+      -memory_usage_reported_to_v8_);
+  memory_usage_reported_to_v8_ = 0;
+}
+
 void FileReaderLoader::DidReceiveResponse(
     unsigned long,
     const ResourceResponse& response,
     std::unique_ptr<WebDataConsumerHandle> handle) {
   DCHECK(!handle);
   if (response.HttpStatusCode() != 200) {
     Failed(HttpStatusCodeToErrorCode(response.HttpStatusCode()));
     return;
   }
 
@@ skipping 64 matching lines @@
 
   unsigned bytes_appended = raw_data_->Append(data, data_length);
   if (!bytes_appended) {
     raw_data_.reset();
     bytes_loaded_ = 0;
     Failed(FileError::kNotReadableErr);
     return;
   }
   bytes_loaded_ += bytes_appended;
   is_raw_data_converted_ = false;
+  ReportAdditionalMemoryUsageToV8(bytes_appended);
 
   if (client_)
     client_->DidReceiveData();
 }
 
 void FileReaderLoader::DidFinishLoading(unsigned long, double) {
   if (read_type_ != kReadByClient && raw_data_) {
     raw_data_->ShrinkToFit();
     is_raw_data_converted_ = false;
   }
@@ skipping 82 matching lines @@
       break;
     case kReadAsDataURL:
       // Partial data is not supported when reading as data URL.
       if (finished_loading_)
         ConvertToDataURL();
       break;
     default:
       NOTREACHED();
   }
 
+  if (finished_loading_)

[dmurph, 2017/04/27 21:57:12]

To make this more clear, can you have the two Convert methods return the string,
which you then std::move into string_result_?

Then it makes more sense in this method that you're allocating that memory right
now, and you want to report it.


Then, if we're finished loading:
* clear raw_data_.
* int64_t size_diff = string_result_.size() - memory_usage_reported_to_v8_;
  AdjustAdditionalV8MemoryUsage(size_diff).

That makes the memory amount accurate, and we clear raw_data_ (which it looks
like we're hanging on to for some reason).

[michaeln, 2017/04/28 00:54:25]

ok, i made it more accurate and thnx for pointing out that we don't need to hang
onto raw_data_ beyond this point

+    ReportAdditionalMemoryUsageToV8(string_result_.CharactersSizeInBytes());
   return string_result_;
 }
 
 void FileReaderLoader::ConvertToText() {
   is_raw_data_converted_ = true;
 
   if (!bytes_loaded_) {
     string_result_ = "";
     return;
   }
@@ skipping 39 matching lines @@
 
   string_result_ = builder.ToString();
 }
 
 void FileReaderLoader::SetEncoding(const String& encoding) {
   if (!encoding.IsEmpty())
     encoding_ = WTF::TextEncoding(encoding);
 }
 
 }  // namespace blink