[PageLoadMetrics] Relax invariants and log the exceptions

chrome/browser/page_load_metrics/observers/ads_page_load_metrics_observer.cc

Index: chrome/browser/page_load_metrics/observers/ads_page_load_metrics_observer.cc
diff --git a/chrome/browser/page_load_metrics/observers/ads_page_load_metrics_observer.cc b/chrome/browser/page_load_metrics/observers/ads_page_load_metrics_observer.cc
index b1b2c49c5f60fb9edf8e7bbd0a28d2fd7ce9a879..1cc087697cf4e2c4ddefb737e83e0b7390d4ecf9 100644
--- a/chrome/browser/page_load_metrics/observers/ads_page_load_metrics_observer.cc
+++ b/chrome/browser/page_load_metrics/observers/ads_page_load_metrics_observer.cc
@@ -56,6 +56,11 @@ bool FrameIsAd(content::NavigationHandle* navigation_handle) {
                           base::CompareCase::SENSITIVE);
 }
 
+void RecordParentExistsForSubFrame(bool parent_exists) {
+  UMA_HISTOGRAM_BOOLEAN("PageLoad.Clients.Ads.Google.ParentExistsForSubFrame",
+                        parent_exists);
+}
+
 }  // namespace
 
 AdsPageLoadMetricsObserver::AdFrameData::AdFrameData(
@@ -122,7 +127,15 @@ AdsPageLoadMetricsObserver::OnDidFinishSubFrameNavigation(
   // Determine who the parent frame's ad ancestor is.
   const auto& parent_id_and_data =
       ad_frames_data_.find(parent_frame_host->GetFrameTreeNodeId());
-  DCHECK(parent_id_and_data != ad_frames_data_.end());
+  if (parent_id_and_data == ad_frames_data_.end()) {
+    // We don't know who the parent for this frame is. One possibility is that
+    // it's a frame from a previous navigation.
+    RecordParentExistsForSubFrame(false /* parent_exists */);
+
+    return CONTINUE_OBSERVING;
+  }
+  RecordParentExistsForSubFrame(true /* parent_exists */);
+
   AdFrameData* ad_data = parent_id_and_data->second;
 
   if (!ad_data && FrameIsAd(navigation_handle)) {
@@ -173,20 +186,32 @@ void AdsPageLoadMetricsObserver::ProcessLoadedResource(
   const auto& id_and_data =
       ad_frames_data_.find(extra_request_info.frame_tree_node_id);
   if (id_and_data == ad_frames_data_.end()) {
-    // This resource is for a frame that hasn't ever finished a navigation. We
-    // expect it to be the frame's main resource but don't have enough
-    // confidence in that to dcheck it. For example, there might be races
-    // between doc.written resources and navigation failure.
-    // TODO(jkarlin): Add UMA to measure how often we see multiple resources
-    // for a frame before navigation finishes.
-    ongoing_navigation_resources_.emplace(
-        std::piecewise_construct,
-        std::forward_as_tuple(extra_request_info.frame_tree_node_id),
-        std::forward_as_tuple(
-            extra_request_info.url, extra_request_info.frame_tree_node_id,
-            extra_request_info.was_cached, extra_request_info.raw_body_bytes,
-            extra_request_info.original_network_content_length, nullptr,
-            extra_request_info.resource_type));
+    if (extra_request_info.resource_type == content::RESOURCE_TYPE_MAIN_FRAME ||
+        extra_request_info.resource_type == content::RESOURCE_TYPE_SUB_FRAME) {
+      // This resource request is the primary resource load for a frame that
+      // hasn't yet finished navigating. Hang onto the request info and replay
+      // it once the frame finishes navigating.
+      ongoing_navigation_resources_.emplace(
+          std::piecewise_construct,
+          std::forward_as_tuple(extra_request_info.frame_tree_node_id),
+          std::forward_as_tuple(
+              extra_request_info.url, extra_request_info.frame_tree_node_id,
+              extra_request_info.was_cached, extra_request_info.raw_body_bytes,
+              extra_request_info.original_network_content_length, nullptr,
+              extra_request_info.resource_type));
+      UMA_HISTOGRAM_ENUMERATION(
+          "PageLoad.Clients.Ads.Google.ResourceTypeWhenNoFrameFound",

[Bryan McQuade, 2017/05/16 17:28:39]

ah, sorry, didn't realize this was already varying on resource type and thus we
don't need a separate histo name. here like the bool case i think the metrics
team asks that we factor histograms into a single helper function, so we should
try to do that here.

[jkarlin, 2017/05/16 17:33:59]

Done.

+          extra_request_info.resource_type, content::RESOURCE_TYPE_LAST_TYPE);
+    } else {
+      // This is unexpected, it could be:
+      // 1. a resource from a previous navigation that started its resource
+      //    load after this page started navigation.
+      // 2. possibly a resource from a document.written frame whose frame
+      //    failure message has yet to arrive. (uncertain of this)
+      UMA_HISTOGRAM_ENUMERATION(
+          "PageLoad.Clients.Ads.Google.ResourceTypeWhenNoFrameFound",
+          extra_request_info.resource_type, content::RESOURCE_TYPE_LAST_TYPE);
+    }
     return;
   }