Transmit a X-Chrome-UMA-Enabled bit to Google domains from clients that have UMA enabled.

chrome/browser/net/chrome_network_delegate.cc

Index: chrome/browser/net/chrome_network_delegate.cc
diff --git a/chrome/browser/net/chrome_network_delegate.cc b/chrome/browser/net/chrome_network_delegate.cc
index 545d127b789df21079f0aeae4a5142900d437290..f29f82cf0f47d9ab1281c565ec9bf4dd56c414c4 100644
--- a/chrome/browser/net/chrome_network_delegate.cc
+++ b/chrome/browser/net/chrome_network_delegate.cc
@@ -14,6 +14,8 @@
 #include "chrome/browser/extensions/extension_event_router_forwarder.h"
 #include "chrome/browser/extensions/extension_info_map.h"
 #include "chrome/browser/extensions/extension_process_manager.h"
+#include "chrome/browser/google/google_util.h"
+#include "chrome/browser/metrics/metrics_service.h"
 #include "chrome/browser/prefs/pref_member.h"
 #include "chrome/browser/profiles/profile_manager.h"
 #include "chrome/browser/task_manager/task_manager.h"
@@ -111,10 +113,12 @@ ChromeNetworkDelegate::ChromeNetworkDelegate(
     const policy::URLBlacklistManager* url_blacklist_manager,
     void* profile,
     CookieSettings* cookie_settings,
+    bool is_incognito,
     BooleanPrefMember* enable_referrers)
     : event_router_(event_router),
       profile_(profile),
       cookie_settings_(cookie_settings),
+      is_incognito_(is_incognito),
       extension_info_map_(extension_info_map),
       enable_referrers_(enable_referrers),
       url_blacklist_manager_(url_blacklist_manager) {
@@ -165,6 +169,10 @@ int ChromeNetworkDelegate::OnBeforeSendHeaders(
     net::URLRequest* request,
     const net::CompletionCallback& callback,
     net::HttpRequestHeaders* headers) {
+  // Attach any applicable Chrome metrics headers. This is a best-effort attempt
+  // where failure will not cause OnBeforeSendHeaders to back out.
+  AddChromeMetricsStateHeader(request, headers);
+
   return ExtensionWebRequestEventRouter::GetInstance()->OnBeforeSendHeaders(
       profile_, extension_info_map_.get(), request, callback, headers);
 }
@@ -304,3 +312,25 @@ bool ChromeNetworkDelegate::CanSetCookie(
 
   return allow;
 }
+
+void ChromeNetworkDelegate::AddChromeMetricsStateHeader(
+    net::URLRequest* request,
+    net::HttpRequestHeaders* headers) {
+  // Note our criteria for attaching Chrome experiment headers:
+  // 1. We only transmit to *.google.<TLD> domains. NOTE that this use of
+  //    google_util helpers to check this does not guarantee that the URL is
+  //    Google-owned, only that it is of the form *.google.<TLD>. In the future
+  //    we may choose to reinforce this check.
+  // 2. We must verify that we are transmitting for a profile network context
+  //    and not the system context. We do this by checking |profile_|, as it is
+  //    NULL when we're in the system context.
+  // 3. We only transmit for non Off The Record profiles.

[Ilya Sherman, 2012/04/17 20:55:39]

nit: s/Off The Record/Incognito

[SteveT, 2012/04/19 18:38:01]

Done.

+  // 4. For the X-Chrome-UMA-Enabled bit, we only set it if UMA is in fact
+  //    enabled for this install of Chrome.
+  if (google_util::IsGoogleDomainUrl(request->url().spec(),
+                                     google_util::ALLOW_SUBDOMAIN) &&
+      profile_ && !is_incognito_ &&
+      MetricsServiceHelper::IsMetricsReportingEnabled()) {

[battre, 2012/04/17 20:33:02]

Here we still have a threading problem. The MetricServiceHelper accesses the
PrefService which lives on the UI thread. You can use a PrefMember.

[SteveT, 2012/04/17 23:26:44]

Ah, yes. I see that we do something similar with the enable_referrers PrefMember
here. You're suggesting the same approach, but for the Metrics-Enabled pref?

We're sort of circumventing MetricServiceHelper::IsMetricsReportingEnabled...
but it looks like we have no choice since we're on the IO thread here.  Ilya can
let us know if using the PrefMember is okay or not.

[Ilya Sherman, 2012/04/17 23:33:13]

I think it's probably ok to use a PrefMember, and add a comment to the
MetricsService file documenting that this class implicitly relies on reading
that pref.  There's a slightly higher chance that the code will drift to be out
of sync, but I think it's still an ok compromise.

[battre, 2012/04/18 17:05:05]

We could probably also put the PrefMember in MetricsServiceHelper. I am not sure
which approach wold be cleaner.

[SteveT, 2012/04/19 18:38:01]

Turns out letting the PrefMember live in ProfileIOData makes the most sense. It
can be appropriately initialized on the UI thread like the enable_referrers
memeber.

willchan@ will be on the review to make sure our changes to Profile*IOData are
satisfactory.

+    headers->SetHeader("X-Chrome-UMA-Enabled", "1");
+  }
+}