Fix SandboxFileStreamWriter Flush() method

- Add DidFlush() method which checks for cancellation
  before flush completes.
- This should fix the behavior of Cancel()

Bug: N/A
Change-Id: I763f5f87eaed80c7a8f1962d781a749d8a63b2f8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2456866Reviewed-by: Marijn Kruisselbrink <mek@chromium.org>
Commit-Queue: Austin Sullivan <asully@chromium.org>
Cr-Commit-Position: refs/heads/master@{#817003}

storage/browser/file_system/sandbox_file_stream_writer.cc

@@ -266,7 +266,23 @@ int SandboxFileStreamWriter::Flush(net::CompletionOnceCallback callback) {
   if (!file_writer_)
     return net::OK;
 
-  return file_writer_->Flush(std::move(callback));
+  has_pending_operation_ = true;
+  int result = file_writer_->Flush(
+      base::BindOnce(&SandboxFileStreamWriter::DidFlush,
+                     weak_factory_.GetWeakPtr(), std::move(callback)));
+  if (result != net::ERR_IO_PENDING)
+    has_pending_operation_ = false;
+  return result;
+}
+
+void SandboxFileStreamWriter::DidFlush(net::CompletionOnceCallback callback,
+                                       int result) {
+  DCHECK(has_pending_operation_);
+
+  if (CancelIfRequested())
+    return;
+  has_pending_operation_ = false;
+  std::move(callback).Run(result);
 }
 
 }  // namespace storage

storage/browser/file_system/sandbox_file_stream_writer.h

@@ -64,6 +64,8 @@ class COMPONENT_EXPORT(STORAGE_BROWSER) SandboxFileStreamWriter
 
   void DidWrite(int write_response);
 
+  void DidFlush(net::CompletionOnceCallback callback, int result);
+
   // Stops the in-flight operation, calls |cancel_callback_| and returns true
   // if there's a pending cancel request.
   bool CancelIfRequested();