base: add a thread-safety assertion checker

base/thread_restrictions.h

+// 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_THREAD_RESTRICTIONS_H_
+#define BASE_THREAD_RESTRICTIONS_H_
+
+namespace base {
+
+// ThreadRestrictions helps protect threads that should not block from
+// making blocking calls.  It works like this:
+//
+// 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.
+//
+class ThreadRestrictions {
+ public:
+  // Set whether the current thread to make IO calls.
+  // Threads start out in the *allowed* state.
+  static void SetIOAllowed(bool allowed);

[darin (slow to review), 2010/10/22 06:30:44]

I think it is confusing to use the term "IO" here like this.  Asynchronous IO is
fine.  The problem is blocking IO.

SetBlockingIOAllowed and AssertBlockingIOAllowed would have been better.

+
+  // Check whether the current thread is allowed to make IO calls,
+  // and DCHECK if not.
+  static void AssertIOAllowed();
+
+ private:
+  ThreadRestrictions();  // class for namespacing only

[darin (slow to review), 2010/10/22 06:30:44]

DISALLOW_IMPLICIT_CONSTRUCTORS?

+};
+
+// In Release builds, inline the empty definitions of these functions so
+// that they can be compiled out.
+#ifdef NDEBUG
+void ThreadRestrictions::SetIOAllowed(bool allowed) {}
+void ThreadRestrictions::AssertIOAllowed() {}
+#endif
+
+}  // namespace base
+
+#endif  // BASE_THREAD_RESTRICTIONS_H_