Avoid doing concurrent DNS resolves of the same hostname

net/proxy/proxy_resolver_v8.h

 // Copyright (c) 2009 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 NET_PROXY_PROXY_RESOLVER_V8_H_
 #define NET_PROXY_PROXY_RESOLVER_V8_H_
 
 #include <string>
 
 #include "base/scoped_ptr.h"
 #include "net/proxy/proxy_resolver.h"
 
+class MessageLoop;
+
 namespace net {
 
+class HostResolver;
+
 // Implementation of ProxyResolver that uses V8 to evaluate PAC scripts.
 //
 // ----------------------------------------------------------------------------
 // !!! Important note on threading model:
 // ----------------------------------------------------------------------------
 // There can be only one instance of V8 running at a time. To enforce this
 // constraint, ProxyResolverV8 holds a v8::Locker during execution. Therefore
 // it is OK to run multiple instances of ProxyResolverV8 on different threads,
 // since only one will be running inside V8 at a time.
 //
 // It is important that *ALL* instances of V8 in the process be using
 // v8::Locker. If not there can be race conditions beween the non-locked V8
 // instances and the locked V8 instances used by ProxyResolverV8 (assuming they
 // run on different threads).
 //
 // This is the case with the V8 instance used by chromium's renderer -- it runs
 // on a different thread from ProxyResolver (renderer thread vs PAC thread),
 // and does not use locking since it expects to be alone.
 class ProxyResolverV8 : public ProxyResolver {
  public:
-  // Constructs a ProxyResolverV8 with default javascript bindings.
-  //
-  // The default javascript bindings will:
-  //   - Send script error messages to LOG(INFO)
-  //   - Send script alert()s to LOG(INFO)
-  //   - Use the default host mapper to service dnsResolve(), synchronously
-  //     on the V8 thread.
-  //
-  // For clients that need more control (for example, sending the script output
-  // to a UI widget), use the ProxyResolverV8(JSBindings*) and specify your
-  // own bindings.
-  ProxyResolverV8();
-
   class JSBindings;
 
   // Constructs a ProxyResolverV8 with custom bindings. ProxyResolverV8 takes
   // ownership of |custom_js_bindings| and deletes it when ProxyResolverV8
   // is destroyed.
   explicit ProxyResolverV8(JSBindings* custom_js_bindings);
 
   ~ProxyResolverV8();
 
   // ProxyResolver implementation:
   virtual int GetProxyForURL(const GURL& query_url,
                              const GURL& /*pac_url*/,
                              ProxyInfo* results);
   virtual void SetPacScript(const std::string& bytes);
 
   JSBindings* js_bindings() const { return js_bindings_.get(); }
 
+  // Creates a default javascript bindings implementation that will:
+  //   - Send script error messages to LOG(INFO)
+  //   - Send script alert()s to LOG(INFO)
+  //   - Use the provided host mapper to service dnsResolve().
+  //
+  // For clients that need more control (for example, sending the script output
+  // to a UI widget), use the ProxyResolverV8(JSBindings*) and specify your
+  // own bindings.
+  //
+  // |host_resolver| will be used in async mode on |host_resolver_loop|. If
+  // |host_resolver_loop| is NULL, then |host_resolver| will be used in sync
+  // mode on the PAC thread.
+  static JSBindings* CreateDefaultBindings(HostResolver* host_resolver,
+                                           MessageLoop* host_resolver_loop);
+
  private:
   // Context holds the Javascript state for the most recently loaded PAC
   // script. It corresponds with the data from the last call to
   // SetPacScript().
   class Context;
   scoped_ptr<Context> context_;
 
   scoped_ptr<JSBindings> js_bindings_;
 
   DISALLOW_COPY_AND_ASSIGN(ProxyResolverV8);
@@ skipping 14 matching lines @@
   virtual std::string DnsResolve(const std::string& host) = 0;
 
   // Handler for when an error is encountered. |line_number| may be -1
   // if a line number is not applicable to this error.
   virtual void OnError(int line_number, const std::string& error) = 0;
 };
 
 }  // namespace net
 
 #endif  // NET_PROXY_PROXY_RESOLVER_V8_H_