Added a field trial (finch experiment) to disable SPDY for x% of users.

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 d2745b3148316620a4bde35bac99abf9b1ce5b5b..505cc6cb4b3c62e9bea8374aac59f72ef4f3121a 100644
--- a/chrome/browser/io_thread.cc
+++ b/chrome/browser/io_thread.cc
@@ -103,6 +103,9 @@ const char kQuicFieldTrialName[] = "QUIC";
 const char kQuicFieldTrialEnabledGroupName[] = "Enabled";
 const char kQuicFieldTrialHttpsEnabledGroupName[] = "HttpsEnabled";
 
+const char kSpdyFieldTrialName[] = "SPDY";
+const char kSpdyFieldTrialDisabledGroupName[] = "SpdyDisabled";
+
 #if defined(OS_MACOSX) && !defined(OS_IOS)
 void ObserveKeychainEvents() {
   DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
@@ -686,6 +689,8 @@ void IOThread::InitializeNetworkOptions(const CommandLine& command_line) {
   // Only handle use-spdy command line flags if "spdy.disabled" preference is
   // not disabled via policy.
   if (!is_spdy_disabled_by_policy_) {
+    std::string spdy_trial_group;
+
     if (command_line.HasSwitch(switches::kEnableIPPooling))
       globals_->enable_spdy_ip_pooling.set(true);
 
@@ -698,7 +703,11 @@ void IOThread::InitializeNetworkOptions(const CommandLine& command_line) {
     if (command_line.HasSwitch(switches::kEnableWebSocketOverSpdy)) {
       // Enable WebSocket over SPDY.
       net::WebSocketJob::set_websocket_over_spdy_enabled(true);
+    } else {
+      spdy_trial_group =
+          base::FieldTrialList::FindFullName(kSpdyFieldTrialName);

[Alexei Svitkine (slow), 2013/10/01 13:55:31]

Why is this here?

In your pending config CL, you have a group for kEnableWebSocketOverSpdy. But in
this initialization code, it looks like it doesn't actually enable spdy
explicitly... its still based on the flags below.

So it looks like you should just do this right above the if
(command_line.HasSwitch(switches::kUseSpdy)) line (since you have entries for
all of those groups). Or let me know what I'm missing here...

[ramant (doing other things), 2013/10/01 17:38:18]

If the command line kEnableWebSocketOverSpdy is selected, I didn't like to
disable SPDY. Reading the code as it is written here, I couldn't assert that
spdy_trial_group wouldn't be kSpdyFieldTrialDisabledGroupName if the above
command line is selected.

The other alternative is to check for
"command_line.HasSwitch(switches::kEnableWebSocketOverSpdy" at line# 738 and if
the command line is set, then default to Spdy3.1.

As per our discussion offline, added the check at line#738.

     }
+
     if (command_line.HasSwitch(switches::kMaxSpdyConcurrentStreams)) {
       globals_->max_spdy_concurrent_streams_limit.set(
           GetSwitchValueAsInt(command_line,
@@ -726,9 +735,17 @@ void IOThread::InitializeNetworkOptions(const CommandLine& command_line) {
     } else if (command_line.HasSwitch(switches::kEnableNpnHttpOnly)) {
       net::HttpStreamFactory::EnableNpnHttpOnly();
     } else {
-      // Use SPDY/3.1 by default.
-      net::HttpStreamFactory::EnableNpnSpdy31();
+      if (spdy_trial_group == kSpdyFieldTrialDisabledGroupName) {
+         net::HttpStreamFactory::set_spdy_enabled(false);
+      } else {
+        // Use SPDY/3.1 by default.
+        net::HttpStreamFactory::EnableNpnSpdy31();
+      }
     }
+  } else {
+    base::FieldTrial* trial = base::FieldTrialList::Find(kSpdyFieldTrialName);

[Alexei Svitkine (slow), 2013/10/01 13:55:31]

Nit: I'd negate the conditional and put this at the beginning to make this part
clearer.

Alternatively, if you can guarantee that nothing else calls FindFullName() on
this trial, then you can omit this (since it won't be marked active).

[ramant (doing other things), 2013/10/01 17:38:18]

Negated the check.

Done.

+    if (trial)
+      trial->Disable();
   }
 
   // TODO(rch): Make the client socket factory a per-network session