Remember if a user declines to provide a server with a client certificate

net/base/ssl_client_auth_cache.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_BASE_SSL_CLIENT_AUTH_CACHE_H_
 #define NET_BASE_SSL_CLIENT_AUTH_CACHE_H_
 #pragma once
 
 #include <string>
 #include <map>
 
 #include "base/ref_counted.h"
-#include "net/base/x509_certificate.h"
 
 namespace net {
 
+class X509Certificate;
+
 // The SSLClientAuthCache class is a simple cache structure to store SSL
 // client certificates. Provides lookup, insertion, and deletion of entries.
 // The parameter for doing lookups, insertions, and deletions is the server's
 // host and port.
 //
 // TODO(wtc): This class is based on FtpAuthCache.  We can extract the common
 // code to a template class.
 class SSLClientAuthCache {
  public:
   SSLClientAuthCache();
   ~SSLClientAuthCache();
 
-  // Check if we have a client certificate for SSL server at |server|.
-  // Returns the client certificate (if found) or NULL (if not found).
-  X509Certificate* Lookup(const std::string& server);
+  // Check if we have a client certificate preference for SSL server at
+  // |server|. Returns true if a preference is found.
+  // If found, |*certificate| will point to the client certificate
+  // if the user has selected a certificate, or NULL if the user has opted to
+  // not provide a certificate.
+  // If a certificate preference is not found, |*certificate| will be NULL.

[wtc, 2010/11/18 01:33:07]

This should say:
  If a certificate preference is not found, returns false.
It is best to leave |*certificate| unchanged when the function
fails.

+  bool Lookup(const std::string& server, X509Certificate** certificate);

[wtc, 2010/11/18 01:33:07]

Declare the output argument as
  scoped_refptr<X509Certificate>* certificate
to force the caller to store the returned X509Certificate
pointer in a scoped_refptr for proper reference counting.

Please update the comment block above accordingly when it
talks about |*certificate|.  (Perhaps the current text also
works for scoped_refptr.)

 
   // Add a client certificate for |server| to the cache. If there is already
   // a client certificate for |server|, it will be overwritten. Both parameters
   // are IN only.
   void Add(const std::string& server, X509Certificate* client_cert);
 
   // Remove the client certificate for |server| from the cache, if one exists.
   void Remove(const std::string& server);
 
  private:
   typedef std::string AuthCacheKey;
   typedef scoped_refptr<X509Certificate> AuthCacheValue;
   typedef std::map<AuthCacheKey, AuthCacheValue> AuthCacheMap;
 
   // internal representation of cache, an STL map.
   AuthCacheMap cache_;
 };
 
 }  // namespace net
 
 #endif  // NET_BASE_SSL_CLIENT_AUTH_CACHE_H_