Add UMA histogram for the number of SiteInstances per BrowsingInstance.

chrome/browser/site_details.cc

Index: chrome/browser/site_details.cc
diff --git a/chrome/browser/site_details.cc b/chrome/browser/site_details.cc
index b38c389d48a8e4ba1d3d3250f95511803f2d728f..04acae206195f0ee6c99602c3027106edb1f6454 100644
--- a/chrome/browser/site_details.cc
+++ b/chrome/browser/site_details.cc
@@ -4,6 +4,7 @@
 
 #include "chrome/browser/site_details.h"
 
+#include "base/containers/hash_tables.h"

[ncarter (slow), 2015/12/02 18:16:49]

This #include belongs in the .h file.

[nasko, 2015/12/02 19:06:56]

Done.

 #include "base/metrics/histogram.h"
 #include "content/public/browser/browser_thread.h"
 #include "content/public/browser/render_frame_host.h"
@@ -105,11 +106,36 @@ void CollectForScenario(std::map<RenderFrameHost*, GURL>* frame_urls,
   (*frame_urls)[frame] = site;
 }
 
+content::SiteInstance* DeterminePrimarySiteInstance(
+    content::SiteInstance* instance,
+    SiteData* site_data) {
+  // Find the BrowsingInstance this WebContents belongs to by iterating over
+  // the "primary" SiteInstances of each BrowsingInstance we've seen so far.
+  SiteInstance* primary = nullptr;
+  for (auto& already_collected_instance : site_data->instances) {
+    if (instance->IsRelatedSiteInstance(already_collected_instance.first)) {
+      primary = already_collected_instance.first;
+      already_collected_instance.second.insert(instance);
+      break;

[ncarter (slow), 2015/12/02 18:16:49]

Just return here. That'll let you get rid of the |primary| local var altogether,
and shave a couple lines off the function.

[nasko, 2015/12/02 19:06:56]

Done.

+    }
+  }
+  if (!primary) {
+    // Remember this as the "primary" SiteInstance of a new BrowsingInstance.
+    primary = instance;
+    site_data->instances.insert(
+        std::make_pair(instance, std::set<SiteInstance*>()));

[ncarter (slow), 2015/12/02 18:16:49]

Shouldn't the new std::set contain |instance| initially? Seems like a subsequent
calls to DeterminePrimarySiteInstance will insert it, so that seems
inconsistent. (Probably this isn't a bug in practice because
DeterminePrimarySiteInstance always happens twice on the main frame)

See my next comment, however.

[nasko, 2015/12/02 19:06:56]

Acknowledged.

+  }
+
+  return primary;
+}
+
 void CollectCurrentSnapshot(SiteData* site_data, RenderFrameHost* frame) {
   if (frame->GetParent()) {
     if (frame->GetSiteInstance() != frame->GetParent()->GetSiteInstance())
       site_data->out_of_process_frames++;
   }
+
+  DeterminePrimarySiteInstance(frame->GetSiteInstance(), site_data);

[ncarter (slow), 2015/12/02 18:16:49]

We've already determined the primary site instance for the webcontents's
browsinginstance; if we plumbed it in here, this operation could become:

site_data->instances[primary].insert(frame->GetSiteInstance())

[nasko, 2015/12/02 19:06:56]

Unfortunately not. Due to crbug.com/522302 not all SiteInstances in a FrameTree
are in the same BrowsingInstance. We should be counting reality until we fix
this.

 }
 
 }  // namespace
@@ -132,21 +158,8 @@ SiteDetails::~SiteDetails() {}
 void SiteDetails::CollectSiteInfo(WebContents* contents,
                                   SiteData* site_data) {
   DCHECK_CURRENTLY_ON(BrowserThread::UI);
-  // Find the BrowsingInstance this WebContents belongs to by iterating over
-  // the "primary" SiteInstances of each BrowsingInstance we've seen so far.
-  SiteInstance* instance = contents->GetSiteInstance();
-  SiteInstance* primary = NULL;
-  for (SiteInstance* already_collected_instance : site_data->instances) {
-    if (instance->IsRelatedSiteInstance(already_collected_instance)) {
-      primary = already_collected_instance;
-      break;
-    }
-  }
-  if (!primary) {
-    // Remember this as the "primary" SiteInstance of a new BrowsingInstance.
-    primary = instance;
-    site_data->instances.push_back(instance);
-  }
+  SiteInstance* primary =
+      DeterminePrimarySiteInstance(contents->GetSiteInstance(), site_data);
 
   // Now keep track of how many sites we have in this BrowsingInstance (and
   // overall), including sites in iframes.
@@ -185,6 +198,10 @@ void SiteDetails::UpdateHistograms(
     }
     num_browsing_instances += i->second.scenarios[ISOLATE_ALL_SITES]
                                   .browsing_instance_site_map.size();
+    for (const auto& site : i->second.instances) {
+      UMA_HISTOGRAM_COUNTS_100("SiteIsolation.SiteInstancesPerBrowsingInstance",
+                               site.second.size());
+    }
     num_oopifs += i->second.out_of_process_frames;
   }