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diff --git a/net/base/host_resolver_proc.h b/net/base/host_resolver_proc.h
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+++ b/net/base/host_resolver_proc.h
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+// 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_BASE_HOST_RESOLVER_PROC_H_
+#define NET_BASE_HOST_RESOLVER_PROC_H_
+
+#include <string>
+
+#include "base/ref_counted.h"
+
+namespace net {
+
+class AddressList;
+
+// Interface for a getaddrinfo()-like procedure. This is used by unit-tests
+// to control the underlying resolutions in HostResolverImpl. HostResolverProcs
+// can be chained together; they fallback to the next procedure in the chain
+// by calling ResolveUsingPrevious().
+//
+// Note that implementations of HostResolverProc *MUST BE THREADSAFE*, since
+// the HostResolver implementation using them can be multi-threaded.
+class HostResolverProc : public base::RefCountedThreadSafe<HostResolverProc> {
+ public:
+  explicit HostResolverProc(HostResolverProc* previous);
+  virtual ~HostResolverProc() {}
+
+  // Resolves |host| to an address list. If successful returns OK and fills
+  // |addrlist| with a list of socket addresses. Otherwise returns a
+  // network error code.
+  virtual int Resolve(const std::string& host, AddressList* addrlist) = 0;
+
+ protected:
+  // Asks the fallback procedure (if set) to do the resolve.
+  int ResolveUsingPrevious(const std::string& host, AddressList* addrlist);
+
+ private:
+  friend class HostResolverImpl;
+  friend class MockHostResolver;
+  friend class ScopedDefaultHostResolverProc;
+
+  // Sets the previous procedure in the chain.
+  void set_previous_proc(HostResolverProc* proc) {
+    previous_proc_ = proc;
+  }
+
+  // Sets the default host resolver procedure that is used by HostResolverImpl.
+  // This can be used through ScopedDefaultHostResolverProc to set a catch-all
+  // DNS block in unit-tests (individual tests should use MockHostResolver to
+  // prevent hitting the network).
+  static HostResolverProc* SetDefault(HostResolverProc* proc);
+  static HostResolverProc* GetDefault();
+
+ private:
+  scoped_refptr<HostResolverProc> previous_proc_;
+  static HostResolverProc* default_proc_;
+
+  DISALLOW_COPY_AND_ASSIGN(HostResolverProc);
+};
+
+// Resolves |host| to an address list, using the system's default host resolver.
+// (i.e. this calls out to getaddrinfo()). If successful returns OK and fills
+// |addrlist| with a list of socket addresses. Otherwise returns a
+// network error code.
+int SystemHostResolverProc(const std::string& host, AddressList* addrlist);
+
+}  // namespace net
+
+#endif  // NET_BASE_HOST_RESOLVER_PROC_H_