[net] Add AsyncDns field trial.

net/base/host_resolver_impl.cc

 // Copyright (c) 2012 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.
 
 #include "net/base/host_resolver_impl.h"
 
 #if defined(OS_WIN)
 #include <Winsock2.h>
 #elif defined(OS_POSIX)
 #include <netdb.h>
@@ skipping 449 matching lines @@
                                       size_t max_retry_attempts,
                                       NetLog* net_log) {
   return CreateHostResolver(max_concurrent_resolves,
                             max_retry_attempts,
                             HostCache::CreateDefaultCache(),
                             DnsConfigService::CreateSystemService(),
                             DnsClient::CreateClient(net_log),
                             net_log);
 }
 
+bool ConfigureAsyncDnsFieldTrial() {
+  const base::FieldTrial::Probability kAsyncDnsDivisor = 100;

[cbentzel, 2012/08/20 11:45:56]

Perhaps restrict enabled_probability to non-0 values for canary+dev channels
only to be safe.

[szym, 2012/08/20 17:54:21]

Good idea although net cannot depend on chrome/ and only chrome/ knows channels.
I could just disable the field trial in io_thread.cc for the other channels, but
to have the flexibility of per-channel probability, I'll have to move this to
chrome/browser/net/async_dns_field_trial.{h,cc}.

[cbentzel, 2012/08/21 10:50:53]

Yes. I personally think it makes sense there - you don't want to run this field
trial for non-Chrome embedders of the network stack.

+  base::FieldTrial::Probability enabled_probability = 50;
+
+  scoped_refptr<base::FieldTrial> trial(
+      base::FieldTrialList::FactoryGetFieldTrial(
+          "AsyncDns", kAsyncDnsDivisor, "disabled", 2012, 9, 30, NULL));
+
+  int enabled_group = trial->AppendGroup("enabled", enabled_probability);

[cbentzel, 2012/08/21 10:50:53]

Should this be restricted to supported platforms only? What if 50% are "enabled"
in iOS or Android?

+  return trial->group() == enabled_group;
+}
+
 //-----------------------------------------------------------------------------
 
 // Holds the data for a request that could not be completed synchronously.
 // It is owned by a Job. Canceled Requests are only marked as canceled rather
 // than removed from the Job's |requests_| list.
 class HostResolverImpl::Request {
  public:
   Request(const BoundNetLog& source_net_log,
           const BoundNetLog& request_net_log,
           const RequestInfo& info,
@@ skipping 1666 matching lines @@
   // |this| may be deleted inside AbortAllInProgressJobs().
   if (self)
     TryServingAllJobsFromHosts();
 }
 
 bool HostResolverImpl::HaveDnsConfig() const {
   return (dns_client_.get() != NULL) && (dns_client_->GetConfig() != NULL);
 }
 
 }  // namespace net