Document confusing threading requirements in ServiceWorkerContext(Wrapper).

content/public/browser/service_worker_context.h

 // Copyright 2014 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 CONTENT_PUBLIC_BROWSER_SERVICE_WORKER_CONTEXT_H_
 #define CONTENT_PUBLIC_BROWSER_SERVICE_WORKER_CONTEXT_H_
 
 #include <set>
 #include <string>
 
@@ skipping 31 matching lines @@
   // pattern}) from a renderer, except that |pattern| is an absolute URL instead
   // of relative to some current origin.  |callback| is passed true when the JS
   // promise is fulfilled or false when the JS promise is rejected.
   //
   // The registration can fail if:
   //  * |script_url| is on a different origin from |pattern|
   //  * Fetching |script_url| fails.
   //  * |script_url| fails to parse or its top-level execution fails.
   //    TODO: The error message for this needs to be available to developers.
   //  * Something unexpected goes wrong, like a renderer crash or a full disk.
+  //
+  // This function can be called from any thread, but the callback will always
+  // be called on the UI thread.
   virtual void RegisterServiceWorker(const Scope& pattern,
                                      const GURL& script_url,
                                      const ResultCallback& callback) = 0;
 
   // Equivalent to calling navigator.serviceWorker.unregister(pattern) from a
   // renderer, except that |pattern| is an absolute URL instead of relative to
   // some current origin.  |callback| is passed true when the JS promise is
   // fulfilled or false when the JS promise is rejected.
   //
   // Unregistration can fail if:
   //  * No Service Worker was registered for |pattern|.
   //  * Something unexpected goes wrong, like a renderer crash.
+  //
+  // This function can be called from any thread, but the callback will always
+  // be called on the UI thread.
   virtual void UnregisterServiceWorker(const Scope& pattern,
                                        const ResultCallback& callback) = 0;
 
   // Methods used in response to browsing data and quota manager requests.
+
+  // Must be called from the IO thread.
   virtual void GetAllOriginsInfo(const GetUsageInfoCallback& callback) = 0;
+
+  // This function can be called from any thread, but the callback will always
+  // be called on the IO thread.
   virtual void DeleteForOrigin(const GURL& origin_url,
                                const ResultCallback& callback) = 0;
 
   // Returns true if a Service Worker registration exists that matches |url|,
   // and if |other_url| falls inside the scope of the same registration. Note
   // this still returns true even if there is a Service Worker registration
   // which has a longer match for |other_url|.
+  //
   // This function can be called from any thread, but the callback will always
   // be called on the UI thread.
   virtual void CheckHasServiceWorker(
       const GURL& url,
       const GURL& other_url,
       const CheckHasServiceWorkerCallback& callback) = 0;
 
   // Stops all running workers on the given |origin|.
+  //
+  // This function can be called from any thread.
   virtual void StopAllServiceWorkersForOrigin(const GURL& origin) = 0;
 
   // Stops all running service workers and unregisters all service worker
   // registrations. This method is used in LayoutTests to make sure that the
   // existing service worker will not affect the succeeding tests.
+  //
+  // This function can be called from any thread, but the callback will always
+  // be called on the UI thread.
   virtual void ClearAllServiceWorkersForTest(const base::Closure& callback) = 0;
 
  protected:
   ServiceWorkerContext() {}
   virtual ~ServiceWorkerContext() {}
 };
 
 }  // namespace content
 
 #endif  // CONTENT_PUBLIC_BROWSER_SERVICE_WORKER_CONTEXT_H_