Get signals working in the EXTRA_DATA section of MHTML

chrome/browser/android/offline_pages/prerendering_offliner.cc

Index: chrome/browser/android/offline_pages/prerendering_offliner.cc
diff --git a/chrome/browser/android/offline_pages/prerendering_offliner.cc b/chrome/browser/android/offline_pages/prerendering_offliner.cc
index a6564ad078d662af7b164c009e039d8dd89aa383..784ff0f2e600e2e35d146993eae1be48eb774466 100644
--- a/chrome/browser/android/offline_pages/prerendering_offliner.cc
+++ b/chrome/browser/android/offline_pages/prerendering_offliner.cc
@@ -14,10 +14,19 @@
 #include "components/offline_pages/core/background/save_page_request.h"
 #include "components/offline_pages/core/client_namespace_constants.h"
 #include "components/offline_pages/core/downloads/download_ui_adapter.h"
+#include "components/offline_pages/core/offline_page_feature.h"
 #include "components/offline_pages/core/offline_page_model.h"
 #include "content/public/browser/browser_context.h"
+#include "content/public/browser/mhtml_extra_parts.h"
 #include "content/public/browser/web_contents.h"
 
+namespace {
+const char kContentType[] = "text/plain";
+const char kContentTransferEncodingBinary[] =
+    "Content-Transfer-Encoding: binary";
+const char kXHeaderForSignals[] = "X-Chrome-Loading-Metrics-Data: 1";
+}  // namespace
+
 namespace offline_pages {
 
 PrerenderingOffliner::PrerenderingOffliner(
@@ -83,6 +92,24 @@ void PrerenderingOffliner::OnLoadPageDone(
     else if (save_page_params.url != request.url())
       save_page_params.original_url = request.url();
 
+    if (IsOfflinePagesLoadSignalCollectingEnabled()) {
+      // Stash loading signals for writing when we write out the MHTML.
+      std::string headers =
+          base::StringPrintf("%s\r\n%s\r\n\r\n", kContentTransferEncodingBinary,
+                             kXHeaderForSignals);

[jochen (gone - plz use gerrit), 2017/04/06 13:02:03]

will this header actually hit the network, or is that just locally stored?

[Pete Williamson, 2017/04/06 16:10:59]

This header is just locally stored.
When parsing, we want to be able to find our load progress signal data in the
MHTML file, and this is how we plan to identify it.

Since the MHTML is for an offline version of a page, it doesn't go back out onto
the network, but just stays on the user's phone until they delete it.

+      std::string body = headers + SerializeLoadingSignalData();
+      std::string content_type = kContentType;
+      std::string content_location = base::StringPrintf(
+          "cid:signal-data-%lld@mhtml.blink", request.request_id());
+
+      content::MHTMLExtraParts* extra_parts =
+          content::MHTMLExtraParts::FromWebContents(web_contents);
+      DCHECK(extra_parts);
+      if (extra_parts != nullptr) {
+        extra_parts->AddExtraMHTMLPart(content_type, content_location, body);
+      }
+    }
+
     SavePage(save_page_params, std::move(archiver),
              base::Bind(&PrerenderingOffliner::OnSavePageDone,
                         weak_ptr_factory_.GetWeakPtr(), request));
@@ -94,6 +121,19 @@ void PrerenderingOffliner::OnLoadPageDone(
   }
 }
 
+std::string PrerenderingOffliner::SerializeLoadingSignalData() {
+  // Write the signal data into a single string.
+  std::string signal_string;
+  const std::vector<std::string>& signals = loader_->GetLoadingSignalData();
+
+  // TODO(petewil): Convert this to JSON, use json_writer.h
+  for (std::string signal : signals) {
+    signal_string += signal;
+    signal_string += "\n";
+  }
+  return signal_string;
+}
+
 void PrerenderingOffliner::OnSavePageDone(
     const SavePageRequest& request,
     SavePageResult save_result,