[net] Add AsyncDns field trial.

chrome/browser/io_thread.cc

Index: chrome/browser/io_thread.cc
diff --git a/chrome/browser/io_thread.cc b/chrome/browser/io_thread.cc
index 5e3374710a7687fd4e125d9d307600799f55a5ee..d621aff6eee410207a1ecab3eb315ba396bc8d17 100644
--- a/chrome/browser/io_thread.cc
+++ b/chrome/browser/io_thread.cc
@@ -109,10 +109,13 @@ class SystemURLRequestContext : public URLRequestContextWithUserAgent {
 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);
 
@@ -129,6 +132,7 @@ net::HostResolver* CreateGlobalHostResolver(net::NetLog* net_log) {
 
   // 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).
@@ -141,14 +145,24 @@ net::HostResolver* CreateGlobalHostResolver(net::NetLog* net_log) {
   }
 
   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.

-    global_host_resolver =
-        net::CreateAsyncHostResolver(parallelism, retry_attempts, net_log);
+    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 (!global_host_resolver) {
+  if (allow_async_dns_field_trial)
+    use_async = net::ConfigureAsyncDnsFieldTrial();
+
+  if (use_async) {
+    global_host_resolver =
+        net::CreateAsyncHostResolver(parallelism, retry_attempts, net_log);
+  } 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.