Introduce the NotificationIdGenerator in //content.

content/browser/notifications/notification_id_generator.h

+// 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_NOTIFICATIONS_NOTIFICATION_ID_GENERATOR_H_
+#define CONTENT_BROWSER_NOTIFICATIONS_NOTIFICATION_ID_GENERATOR_H_
+
+#include <stdint.h>
+#include <string>
+
+#include "content/common/content_export.h"
+
+class GURL;
+
+namespace content {
+
+class BrowserContext;
+
+// Generates deterministic notification ids for Web Notifications.
+//
+// The notification id must be deterministic for a given browser context, origin
+// and tag, when the tag is non-empty, or unique for the given notification when
+// the tag is empty. For non-persistent notifications, the uniqueness will be
+// based on the render process id. For persistent notifications, the generated

[johnme, 2015/05/08 15:35:25]

Can render process ids ever get reused (without the browser restarting)?

[Peter Beverloo, 2015/05/20 18:43:33]

No. See ChildProcessHostImpl::GenerateChildProcessUniqueId().

+// id will be globally unique for on the lifetime of the notification database.

[johnme, 2015/05/08 15:35:25]

s/for on/for/

[Peter Beverloo, 2015/05/20 18:43:33]

Done.

+//
+// The notification id will be used by the notification service for determining
+// when to replace notifications, and as the unique identifier when a
+// notification has to be closed programmatically.
+//
+// It is important to note that, for persistent notifications, the generated
+// notification id can outlive the browser process responsible for creating it.
+class CONTENT_EXPORT NotificationIdGenerator {

[johnme, 2015/05/08 15:35:25]

Since multiple distinct notifications may share a generated id, it feels like
this is really a NotificationTagGenerator (i.e. it generates a unique tag for
notifications with empty tag, so that replacement based on tag matching can
work). Especially since you're actually passing in an ID to the Generate
methods!

[Peter Beverloo, 2015/05/20 18:43:33]

The technical term for this form of ID would be a notification "delegate id",
but that's a concept that lives outside of //content. It's also a concept I
would very much like to generalize to a generic kind of id.

The tag of a notification is what the user (e.g. the Web API or the extensions
API) can use to identify uniqueness within its own constraints. This is just a
further representation of that concept within the delegate id.

+ public:
+  NotificationIdGenerator(BrowserContext* browser_context,
+                          int render_process_id);
+  ~NotificationIdGenerator();
+
+  // Generates an id for a persistent notification given the notification's
+  // origin, tag and persistent notification id.
+  std::string GenerateForPersistentNotification(
+      const GURL& origin,
+      const std::string& tag,
+      int64_t persistent_notification_id) const;
+
+  // Generates an id for a non-persistent notification given the notification's
+  // origin, tag and non-persistent notification id.

[johnme, 2015/05/08 15:35:26]

Where do these non_persistent_notification_ids come from? It would be nice to
add "generated by ClassNameHere" or at least "generated by Blink" or whatever so
someone reading this can reason about what they mean.

Ditto above for persistent_notification_ids - whilst I happen to know that those
are generated by the notifications DB, future readers likely won't yet know
that.

[Peter Beverloo, 2015/05/20 18:43:34]

Done.

+  std::string GenerateForNonPersistentNotification(
+      const GURL& origin,
+      const std::string& tag,
+      int non_persistent_notification_id) const;
+
+ private:
+  BrowserContext* browser_context_;

[johnme, 2015/05/08 15:35:25]

Perhaps add a comment like "Required to outlive this"? or "Should own this"?

[Peter Beverloo, 2015/05/20 18:43:34]

Done.

+  int render_process_id_;
+};
+
+}  // namespace context
+
+#endif  // CONTENT_BROWSER_NOTIFICATIONS_NOTIFICATION_ID_GENERATOR_H_