[net] Change factory methods for HostResolver and HostCache to return a scoped_ptr

chrome/browser/chrome_browser_field_trials.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 "chrome/browser/chrome_browser_field_trials.h"
 
 #include <string>
 
 #include "base/command_line.h"
 #include "base/metrics/field_trial.h"
@@ skipping 106 matching lines @@
   SpdyFieldTrial();
   ConnectBackupJobsFieldTrial();
   WarmConnectionFieldTrial();
   PredictorFieldTrial();
   DefaultAppsFieldTrial();
   AutoLaunchChromeFieldTrial();
   gpu_util::InitializeCompositingFieldTrial();
   SetupUniformityFieldTrials();
   AutocompleteFieldTrial::Activate();
   DisableNewTabFieldTrialIfNecesssary();
+  AsyncDnsFieldTrial();
 }
 
 // This is an A/B test for the maximum number of persistent connections per
 // host. Currently Chrome, Firefox, and IE8 have this value set at 6. Safari
 // uses 4, and Fasterfox (a plugin for Firefox that supposedly configures it to
 // run faster) uses 8. We would like to see how much of an effect this value has
 // on browsing. Too large a value might cause us to run into SYN flood detection
 // mechanisms.
 void ChromeBrowserFieldTrials::ConnectionFieldTrial() {
   const base::FieldTrial::Probability kConnectDivisor = 100;
@@ skipping 438 matching lines @@
   if (trial) {
     bool using_hidpi_assets = false;
 #if defined(ENABLE_HIDPI) && defined(OS_WIN)
     // Mirrors logic in resource_bundle_win.cc.
     using_hidpi_assets = ui::GetDPIScale() > 1.5;
 #endif
     if (ui::GetDisplayLayout() != ui::LAYOUT_DESKTOP || using_hidpi_assets)
       trial->Disable();
   }
 }
+
+// When --enable-async-dns is not set, users will be in A/B test for DnsClient.

[cbentzel, 2012/08/16 20:09:21]

Do you want to move this to a ternary (enabled/disabled/auto)? We did that for
prerender trials, and then didn't count metrics for sessions in the enabled +
disabled buckets.

[szym, 2012/08/16 20:38:55]

I'm okay with that. I'm not sure what this means for the DNS.TotalTime and
AsyncDNS.TotalTime histograms. I suspect they will be merged into
DNS.TotalTime_async/sync/default?

+// group A (async enabled):  Use DnsClient if DnsConfig is known. Fall back to
+//                           getaddrinfo on failures.
+// group B (async disabled): Never use DnsClient.
+void ChromeBrowserFieldTrials::AsyncDnsFieldTrial() {
+  // Use the concurrency override from the command-line, if any.
+  if (parsed_command_line_.HasSwitch(switches::kHostResolverParallelism)) {
+    std::string s = parsed_command_line_.GetSwitchValueASCII(
+        switches::kHostResolverParallelism);
+
+    // Parse the switch (it should be a positive integer formatted as decimal).
+    int n;
+    if (base::StringToInt(s, &n) && n > 0) {
+      net::HostResolver::set_default_max_concurrent_resolves(
+          static_cast<size_t>(n));
+    } else {
+      LOG(ERROR) << "Invalid switch for host resolver parallelism: " << s;
+    }
+  }
+
+  // Use the retry attempts override from the command-line, if any.
+  if (parsed_command_line_.HasSwitch(switches::kHostResolverRetryAttempts)) {
+    std::string s = parsed_command_line_.GetSwitchValueASCII(
+        switches::kHostResolverRetryAttempts);
+    // Parse the switch (it should be a non-negative integer).
+    int n;
+    if (base::StringToInt(s, &n) && n >= 0) {
+      net::HostResolver::set_default_max_retry_attempts(static_cast<size_t>(n));
+    } else {
+      LOG(ERROR) << "Invalid switch for host resolver retry attempts: " << s;
+    }
+  }
+
+  bool use_async = parsed_command_line_.HasSwitch(switches::kEnableAsyncDns);
+  if (use_async) {
+    net::HostResolver::set_default_enable_async(true);
+    return;  // Trial disabled.
+  }
+
+  const base::FieldTrial::Probability kAsyncDnsDivisor = 100;
+  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);
+  net::HostResolver::set_default_enable_async(trial->group() == enabled_group);
+}