[net] Add AsyncDns field trial.

chrome/browser/io_thread.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/io_thread.h"
 
 #include <vector>
 
 #include "base/bind.h"
 #include "base/bind_helpers.h"
@@ skipping 91 matching lines @@
   virtual ~SystemURLRequestContext() {
 #if defined(USE_NSS)
     net::SetURLRequestContextForNSSHttpIO(NULL);
 #endif  // defined(USE_NSS)
   }
 };
 
 net::HostResolver* CreateGlobalHostResolver(net::NetLog* net_log) {
   const CommandLine& command_line = *CommandLine::ForCurrentProcess();
 
+  bool allow_async_dns_field_trial = true;
+
   size_t parallelism = net::HostResolver::kDefaultParallelism;
 
   // Use the concurrency override from the command-line, if any.
   if (command_line.HasSwitch(switches::kHostResolverParallelism)) {

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

Did you see if we could just get rid of the kHostResolverParallelism and
kHostResolverRetryAttempts flags?

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

Judging from the old bugs which established our current level of parallelism at
6-8, I concluded that reducing parallelism is likely still a very useful tool
when diagnosing network issues. Conceivably, we could replace both flags with
--disable-resolver-parallelism. I asked eroman about this.

+    allow_async_dns_field_trial = false;
     std::string s =
         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) {
       parallelism = static_cast<size_t>(n);
     } else {
       LOG(ERROR) << "Invalid switch for host resolver parallelism: " << s;
     }
   }
 
   size_t retry_attempts = net::HostResolver::kDefaultRetryAttempts;
 
   // Use the retry attempts override from the command-line, if any.
   if (command_line.HasSwitch(switches::kHostResolverRetryAttempts)) {
+    allow_async_dns_field_trial = false;
     std::string s =
         command_line.GetSwitchValueASCII(switches::kHostResolverRetryAttempts);
     // Parse the switch (it should be a non-negative integer).
     int n;
     if (base::StringToInt(s, &n) && n >= 0) {
       retry_attempts = static_cast<size_t>(n);
     } else {
       LOG(ERROR) << "Invalid switch for host resolver retry attempts: " << s;
     }
   }
 
   net::HostResolver* global_host_resolver = NULL;
+  bool use_async = false;
   if (command_line.HasSwitch(switches::kEnableAsyncDns)) {

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

Why isn't this just --async-dns=[enabled|disabled|auto]?

Seems like the about:flags approach would steer towards that.

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

I followed the existing flags. There's currently 8 ternary flags in
about_flags.cc. 4/8 use --XXX=enabled|disabled while 4/8 use
--enable-XXX|--disable-XXX. There's also a number of the latter which are not in
about_flags.cc (XXX = spdy, ipv6, etc.).

Would you prefer to see --async-dns?

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

No - it sounds different from common practice.

+    allow_async_dns_field_trial = false;
+    use_async = true;
+  } else if (command_line.HasSwitch(switches::kDisableAsyncDns)) {
+    allow_async_dns_field_trial = false;
+    use_async = false;
+  }
+
+  if (allow_async_dns_field_trial)
+    use_async = net::ConfigureAsyncDnsFieldTrial();
+
+  if (use_async) {
     global_host_resolver =
         net::CreateAsyncHostResolver(parallelism, retry_attempts, net_log);
-  }
-
-  if (!global_host_resolver) {
+  } else {
     global_host_resolver =
-      net::CreateSystemHostResolver(parallelism, retry_attempts, net_log);
+        net::CreateSystemHostResolver(parallelism, retry_attempts, net_log);
   }
 
   // Determine if we should disable IPv6 support.
   if (!command_line.HasSwitch(switches::kEnableIPv6)) {
     if (command_line.HasSwitch(switches::kDisableIPv6)) {
       global_host_resolver->SetDefaultAddressFamily(net::ADDRESS_FAMILY_IPV4);
     } else {
       global_host_resolver->ProbeIPv6Support();
     }
   }
@@ skipping 468 matching lines @@
       new net::HttpNetworkLayer(
           new net::HttpNetworkSession(system_params)));
   globals_->system_ftp_transaction_factory.reset(
       new net::FtpNetworkLayer(globals_->host_resolver.get()));
   globals_->system_request_context.reset(
       ConstructSystemRequestContext(globals_, net_log_));
 
   sdch_manager_->set_sdch_fetcher(
       new SdchDictionaryFetcher(system_url_request_context_getter_.get()));
 }