content::ResourceDispatcherHostImpl changes for stale-while-revalidate

content/browser/loader/async_revalidation_manager.h

Index: content/browser/loader/async_revalidation_manager.h
diff --git a/content/browser/loader/async_revalidation_manager.h b/content/browser/loader/async_revalidation_manager.h
new file mode 100644
index 0000000000000000000000000000000000000000..a13e8feafb3d35828800f15ab353f29d4aa6a640
--- /dev/null
+++ b/content/browser/loader/async_revalidation_manager.h
@@ -0,0 +1,110 @@
+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef CONTENT_BROWSER_LOADER_ASYNC_REVALIDATION_MANAGER_H_
+#define CONTENT_BROWSER_LOADER_ASYNC_REVALIDATION_MANAGER_H_
+
+#include <map>
+#include <string>
+
+#include "base/macros.h"
+
+class GURL;
+struct ResourceHostMsg_Request;
+
+namespace net {
+class URLRequest;
+class HttpCache;
+}
+
+namespace content {
+
+class AsyncRevalidationDriver;
+class ResourceContext;
+class ResourceScheduler;
+
+// This class manages all active AsyncRevalidationDriver objects for the whole
+// process. It also implements the creation of a new net::URLRequest and
+// AsyncRevalidationDriver from an existing net::URLRequest that has had the
+// stale-while-revalidate algorithm applied to it.
+class AsyncRevalidationManager {
+ public:
+  AsyncRevalidationManager();
+  ~AsyncRevalidationManager();
+
+  // Starts an async revalidation by copying |for_request|. |scheduler| must
+  // remain valid until this object is destroyed.
+  void BeginAsyncRevalidation(net::URLRequest* for_request,
+                              ResourceScheduler* scheduler);
+
+  // Cancel all pending async revalidations that use ResourceContext.
+  void CancelAsyncRevalidationsForResourceContext(
+      ResourceContext* resource_context);
+
+  static bool QualifiesForAsyncRevalidation(
+      const ResourceHostMsg_Request& request);
+
+ private:
+  // The key used to lookup an AsyncRevalidationDriver in the map of pending

[Bence, 2015/11/17 13:12:23]

Optional: feel free to remove "used to lookup".  It is okay to say that this is
"the key of the map".

[Adam Rice, 2015/11/17 17:45:53]

Done.

+  // async revalidations. This key has a
+  // distinct value for every in-progress async revalidation. It is used to
+  // avoid
+  // duplicate async revalidations, and also to cancel affected async
+  // revalidations when a ResourceContext is removed.
+  // Request headers are intentionally not included in the key as they usually
+  // don't affect caching, and could cause effectively identical revalidations
+  // to be considered different because they have a different "Referer"
+  // header. In rare cases where a "Vary" header leads to us to access different
+  // resources at the same URL, they fight a single cache slot and so performing
+  // additional async revalidations will not improve matters.

[Bence, 2015/11/17 13:12:23]

Please reflow this paragraph.

[Adam Rice, 2015/11/17 17:45:53]

Done.

+  struct AsyncRevalidationKey {
+    AsyncRevalidationKey(const ResourceContext* resource_context,
+                         const net::HttpCache* http_cache,
+                         const GURL& url);
+
+    // Create a prefix key that is used to match all of the
+    // AsyncRevalidationDrivers using |resource_context| in the map.
+    explicit AsyncRevalidationKey(const ResourceContext* resource_context);
+
+    // The key for a map needs to be copyable.
+    AsyncRevalidationKey(const AsyncRevalidationKey& rhs) = default;
+    ~AsyncRevalidationKey();
+
+    // No operator= is generated because the struct members are immutable.
+
+    // |resource_context| and |http_cache| are never dereferenced; they are only
+    // compared to other values. In order to efficiently find and delete all
+    // in-progress async revalidations using a particular ResourceContext, it
+    // forms the first part of the key.

[Bence, 2015/11/17 13:12:23]

This last sentence might be more appropriate as a comment for LessThan, because
the order of keys is really decided there.  The order of members in the struct
is irrelevant.

Optional: this is an implementation detail, maybe no need to mention the order
in which keys are used in the lexicographic order at all in the header file.

[Adam Rice, 2015/11/17 17:45:52]

I think you're right. I have removed the implementation detail from this
comment.

+    const ResourceContext* const resource_context;
+
+    // Each ResourceContext owns one or more HttpCache objects.

[Bence, 2015/11/17 13:12:23]

Optional: write "There might be multiple HttpCache objects per ResourceContext."
or something similar instead.  It took me a very long time to understand why
this comment was relevant at, and this alternate wording might have been
helpful.

[Adam Rice, 2015/11/17 17:45:53]

Sorry about that. I changed it to "There are multiple independent HttpCache
objects per ResourceContext." since at the moment there are always at least two.

[Bence, 2015/11/17 21:34:28]

Splendid.

+    const net::HttpCache* const http_cache;
+
+    // Derived from the url via net::HttpUtil::SpecForRequest().
+    const std::string url_key;
+
+    struct LessThan {
+      bool operator()(const AsyncRevalidationKey& lhs,
+                      const AsyncRevalidationKey& rhs) const;
+    };
+  };
+
+  // Map of AsyncRevalidationDriver object.

[Bence, 2015/11/17 13:12:23]

Optional: remove this comment, fold into the one below.

[Adam Rice, 2015/11/17 17:45:53]

Done.

+  typedef std::map<AsyncRevalidationKey,

[Bence, 2015/11/17 13:12:23]

Use "using" instead of "typedef", see https://chromium-cpp.appspot.com.

[Adam Rice, 2015/11/17 17:45:53]

Thanks. Old habits die hard.

+                   AsyncRevalidationDriver*,
+                   AsyncRevalidationKey::LessThan> AsyncRevalidationMap;
+
+  void OnAsyncRevalidationComplete(AsyncRevalidationMap::iterator it);
+
+  // Async revalidations that are currently in-flight: either waiting to be

[Bence, 2015/11/17 13:12:23]

Optional: "AsyncRevalidationDriver instances that are currently in-flight".

[Adam Rice, 2015/11/17 17:45:53]

Done.

+  // scheduled or active on the network.
+  AsyncRevalidationMap in_progress_;
+
+  DISALLOW_COPY_AND_ASSIGN(AsyncRevalidationManager);
+};
+
+}  // namespace content
+
+#endif  // CONTENT_BROWSER_LOADER_ASYNC_REVALIDATION_MANAGER_H_