Use FILE thread for disk operations in TraceSubscriberStdio

content/browser/trace_subscriber_stdio_unittest.cc

 // Copyright (c) 2011 The Chromium 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 "content/browser/trace_subscriber_stdio.h"
 
 #include "base/scoped_temp_dir.h"
+#include "base/threading/thread_restrictions.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
 namespace {
 
+class ScopedDisallowIO {
+ public:
+  ScopedDisallowIO() {
+    previous_value_ = base::ThreadRestrictions::SetIOAllowed(false);
+  }
+
+  ~ScopedDisallowIO() {
+    base::ThreadRestrictions::SetIOAllowed(previous_value_);
+  }
+
+ private:
+  bool previous_value_;
+
+  DISALLOW_COPY_AND_ASSIGN(ScopedDisallowIO);
+};
+
 class TraceSubscriberStdioTest : public testing::Test {
  public:
   virtual void SetUp() {
     ASSERT_TRUE(trace_dir_.CreateUniqueTempDir());
     trace_file_ = trace_dir_.path().AppendASCII("trace.txt");
   }
 
   std::string ReadTraceFile() {
     std::string result;
     EXPECT_TRUE(file_util::ReadFileToString(trace_file_, &result));
     return result;
   }
 
   ScopedTempDir trace_dir_;
   FilePath trace_file_;
 };
 
 }  // namespace
 
 TEST_F(TraceSubscriberStdioTest, CanWriteDataToFile) {
+  // TraceSubscriber callbacks are documented to occur on the UI thread,
+  // so this class must work with ThreadRestictions::SetIOAllowed(false);
+  ScopedDisallowIO scoped_disallow_io;
+
   TraceSubscriberStdio subscriber(trace_file_);
   subscriber.OnTraceDataCollected("[foo]");
   subscriber.OnTraceDataCollected("[bar]");
   EXPECT_TRUE(subscriber.IsValid());
 
   subscriber.OnEndTracingComplete();
   EXPECT_FALSE(subscriber.IsValid());
 }
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