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diff --git a/base/threading/thread_restrictions.h b/base/threading/thread_restrictions.h
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+// Copyright (c) 2010 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.
+
+#ifndef BASE_THREADING_THREAD_RESTRICTIONS_H_
+#define BASE_THREADING_THREAD_RESTRICTIONS_H_
+
+#include "base/basictypes.h"
+
+namespace base {
+
+// Certain behavior is disallowed on certain threads.  ThreadRestrictions helps
+// enforce these rules.  Examples of such rules:
+//
+// * Do not do blocking IO (makes the thread janky)
+// * Do not access Singleton/LazyInstance (may lead to shutdown crashes)
+//
+// Here's more about how the protection works:
+//
+// 1) If a thread should not be allowed to make IO calls, mark it:
+//      base::ThreadRestrictions::SetIOAllowed(false);
+//    By default, threads *are* allowed to make IO calls.
+//    In Chrome browser code, IO calls should be proxied to the File thread.
+//
+// 2) If a function makes a call that will go out to disk, check whether the
+//    current thread is allowed:
+//      base::ThreadRestrictions::AssertIOAllowed();
+//
+// ThreadRestrictions does nothing in release builds; it is debug-only.
+//
+// Style tip: where should you put AssertIOAllowed checks?  It's best
+// if you put them as close to the disk access as possible, at the
+// lowest level.  This rule is simple to follow and helps catch all
+// callers.  For example, if your function GoDoSomeBlockingDiskCall()
+// only calls other functions in Chrome and not fopen(), you should go
+// add the AssertIOAllowed checks in the helper functions.
+
+class ThreadRestrictions {
+ public:
+  // Constructing a ScopedAllowIO temporarily allows IO for the current
+  // thread.  Doing this is almost certainly always incorrect.
+  class ScopedAllowIO {
+   public:
+    ScopedAllowIO() { previous_value_ = SetIOAllowed(true); }
+    ~ScopedAllowIO() { SetIOAllowed(previous_value_); }
+   private:
+    // Whether IO is allowed when the ScopedAllowIO was constructed.
+    bool previous_value_;
+
+    DISALLOW_COPY_AND_ASSIGN(ScopedAllowIO);
+  };
+
+  // Constructing a ScopedAllowSingleton temporarily allows accessing for the
+  // current thread.  Doing this is almost always incorrect.
+  class ScopedAllowSingleton {
+   public:
+    ScopedAllowSingleton() { previous_value_ = SetSingletonAllowed(true); }
+    ~ScopedAllowSingleton() { SetSingletonAllowed(previous_value_); }
+   private:
+    // Whether singleton use is allowed when the ScopedAllowSingleton was
+    // constructed.
+    bool previous_value_;
+
+    DISALLOW_COPY_AND_ASSIGN(ScopedAllowSingleton);
+  };
+
+#ifndef NDEBUG
+  // Set whether the current thread to make IO calls.
+  // Threads start out in the *allowed* state.
+  // Returns the previous value.
+  static bool SetIOAllowed(bool allowed);
+
+  // Check whether the current thread is allowed to make IO calls,
+  // and DCHECK if not.  See the block comment above the class for
+  // a discussion of where to add these checks.
+  static void AssertIOAllowed();
+
+  // Set whether the current thread can use singletons.  Returns the previous
+  // value.
+  static bool SetSingletonAllowed(bool allowed);
+
+  // Check whether the current thread is allowed to use singletons (Singleton /
+  // LazyInstance).  DCHECKs if not.
+  static void AssertSingletonAllowed();
+#else
+  // In Release builds, inline the empty definitions of these functions so
+  // that they can be compiled out.
+  static bool SetIOAllowed(bool allowed) { return true; }
+  static void AssertIOAllowed() {}
+  static bool SetSingletonAllowed(bool allowed) { return true; }
+  static void AssertSingletonAllowed() {}
+#endif
+
+ private:
+  DISALLOW_IMPLICIT_CONSTRUCTORS(ThreadRestrictions);
+};
+
+}  // namespace base
+
+#endif  // BASE_THREADING_THREAD_RESTRICTIONS_H_