Disallow UI/IO thread blocking on any other thread.

base/threading/thread_restrictions.h

Index: base/threading/thread_restrictions.h
===================================================================
--- base/threading/thread_restrictions.h	(revision 133458)
+++ base/threading/thread_restrictions.h	(working copy)
@@ -8,8 +8,21 @@
 #include "base/base_export.h"
 #include "base/basictypes.h"
 
+class MetricsService;
+class RenderWidgetHelper;
+class TextInputClientMac;
+namespace chrome_browser_net {
+class Predictor;
+}
+namespace disk_cache {
+class BackendImpl;
+class InFlightIO;
+}
+
 namespace base {
 
+class Thread;
+
 // Certain behavior is disallowed on certain threads.  ThreadRestrictions helps
 // enforce these rules.  Examples of such rules:
 //
@@ -83,6 +96,13 @@
   // Check whether the current thread is allowed to use singletons (Singleton /
   // LazyInstance).  DCHECKs if not.
   static void AssertSingletonAllowed();
+
+  // Disable waiting on the current thread. Threads start out in the *allowed*
+  // state. Returns the previous value.
+  static void DisallowWaiting();
+
+  // Check whether the current thread is allowed to wait, and DCHECK if not.
+  static void AssertWaitAllowed();
 #else
   // In Release builds, inline the empty definitions of these functions so
   // that they can be compiled out.
@@ -90,9 +110,42 @@
   static void AssertIOAllowed() {}
   static bool SetSingletonAllowed(bool allowed) { return true; }
   static void AssertSingletonAllowed() {}
+  static void DisallowWaiting() {}
+  static void AssertWaitAllowed() {}
 #endif
 
  private:
+  // DO NOT ADD ANY OTHER FRIEND STATEMENTS.
+  friend class ::RenderWidgetHelper;     // This is allowed, see documentation.
+  friend class Thread;                   // This is allowed.
+  friend class chrome_browser_net::Predictor;  // http://crbug.com/78451
+  friend class disk_cache::BackendImpl;  // http://crbug.com/74623
+  friend class disk_cache::InFlightIO;   // http://crbug.com/74623
+  friend class ::TextInputClientMac;     // http://crbug.com/121917
+  friend class ::MetricsService;         // http://crbug.com/124954
+
+#ifndef NDEBUG
+  static bool SetWaitAllowed(bool allowed);
+#else
+  static bool SetWaitAllowed(bool allowed) { return true; }
+#endif
+
+  // Constructing a ScopedAllowWait temporarily allows waiting on the current
+  // thread.  Doing this is almost always incorrect, which is why we limit who
+  // can use this through friend. If you find yourself needing to use this, find
+  // another way. Talk to jam or brettw.
+  class BASE_EXPORT ScopedAllowWait {
+   public:
+    ScopedAllowWait() { previous_value_ = SetWaitAllowed(true); }
+    ~ScopedAllowWait() { SetWaitAllowed(previous_value_); }
+   private:
+    // Whether singleton use is allowed when the ScopedAllowWait was
+    // constructed.
+    bool previous_value_;
+
+    DISALLOW_COPY_AND_ASSIGN(ScopedAllowWait);
+  };
+
   DISALLOW_IMPLICIT_CONSTRUCTORS(ThreadRestrictions);
 };