Relax DCHECKs regarding Threads and RefCounts in iOS' HttpServer.

ios/web/public/test/http_server.mm

 // 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.
 
 #import "ios/web/public/test/http_server.h"
 
 #import <Foundation/Foundation.h>
 
 #include <string>
 
@@ skipping 62 matching lines @@
   for (std::unique_ptr<ResponseProvider>& provider : response_providers)
     server.AddResponseProvider(std::move(provider));
   return server;
 }
 
 void HttpServer::InitHttpServer() {
   DCHECK(gcd_web_server_);
   // Note: This block is called from an arbitrary GCD thread.
   id process_request =
       ^GCDWebServerResponse*(GCDWebServerDataRequest* request) {
+
+      // TODO(crbug.com/707764): Verify that the relaxation of the cross-thread

[sdefresne, 2017/04/03 14:18:16]

I think that a correct fix would be to:

1. split GetResponseProviderForRequest in two
  - GetResponseProviderForRequestImpl that does all the work except the locking,
  - GetResponseProviderForRequest that take the lock then call
GetResponseProviderForRequestImpl
2. changing here to acquire the lock, then call
GetResponseProviderForRequestImpl
3. changing the comment to the following

  // Relax the cross-thread access restriction to non-thread-safe RefCount.
  // |provider_list_lock_| protects the access to the
  // RefCountedResponseProviderWrapper instances.

+      // access restriction is correct.
+      base::ScopedAllowCrossThreadRefCountAccess
+          allow_cross_thread_ref_count_access;
+
       ResponseProvider::Request provider_request =
           ResponseProviderRequestFromGCDWebServerRequest(request);
       scoped_refptr<RefCountedResponseProviderWrapper>
           ref_counted_response_provider = GetResponseProviderForRequest(
               provider_request);
 
       if (!ref_counted_response_provider) {
         return [GCDWebServerResponse response];
       }
       ResponseProvider* response_provider =
@@ skipping 125 matching lines @@
       break;
     }
   }
   if (found_iter != providers_.end()) {
     providers_.erase(found_iter);
   }
 }
 
 void HttpServer::RemoveAllResponseProviders() {
   DCHECK([NSThread isMainThread]);
+  // TODO(crbug.com/707764): Verify that the relaxation of the cross-thread

[sdefresne, 2017/04/03 14:18:16]

I think you can change this to:

  // Relax the cross-thread access restriction to non-thread-safe RefCount.
  // |provider_list_lock_| protects the access to the
  // RefCountedResponseProviderWrapper instances.

+  // access restriction is correct.
+  base::ScopedAllowCrossThreadRefCountAccess
+      allow_cross_thread_ref_count_access;
   base::AutoLock autolock(provider_list_lock_);
   providers_.clear();
 }
 
 void HttpServer::SetPort(NSUInteger port) {
   base::AutoLock autolock(port_lock_);
   port_ = port;
 }
 
 } // namespace test
 } // namespace web