Reland 66791 (change was innocent)...

base/thread_restrictions.h

Index: base/thread_restrictions.h
===================================================================
--- base/thread_restrictions.h	(revision 66806)
+++ base/thread_restrictions.h	(working copy)
@@ -9,9 +9,14 @@
 
 namespace base {
 
-// ThreadRestrictions helps protect threads that should not block from
-// making blocking calls.  It works like this:
+// 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.
@@ -45,6 +50,20 @@
     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.
@@ -55,15 +74,25 @@
   // 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:
-  ThreadRestrictions();  // class for namespacing only
+  DISALLOW_IMPLICIT_CONSTRUCTORS(ThreadRestrictions);
 };
 
 }  // namespace base