PPAPI: Introduce concept of ResourceHosts "pinning" each other

ppapi/host/ppapi_host.h

Index: ppapi/host/ppapi_host.h
diff --git a/ppapi/host/ppapi_host.h b/ppapi/host/ppapi_host.h
index 4280c3cb74553efe05481c54833d7d6ff6351af1..10040fcc5da437659eb594560ebaa9713a5e5bbc 100644
--- a/ppapi/host/ppapi_host.h
+++ b/ppapi/host/ppapi_host.h
@@ -91,6 +91,16 @@ class PPAPI_HOST_EXPORT PpapiHost : public IPC::Sender, public IPC::Listener {
   // Returns null if the resource doesn't exist.
   host::ResourceHost* GetResourceHost(PP_Resource resource) const;
 
+  // Pin |owned_host| in memory so long as |owner_host| still needs it. Even
+  // if the plugin is done using |owned_host|, it will be kept alive until
+  // |owner_host| calls UnpinHost.
+  void PinHost(ResourceHost* owner_host, ResourceHost* owned_host);
+  // Called to indicate that the |owner_host| no longer requires |owned_host|.
+  // This may result in |owned_host| being deleted, if:
+  //  (a) The plugin has finished with it, and
+  //  (b) No other ResourceHosts have it pinned.
+  void UnpinHost(ResourceHost* owner_host, ResourceHost* owned_host);
+
  private:
   friend class InstanceMessageFilter;
 
@@ -116,6 +126,13 @@ class PPAPI_HOST_EXPORT PpapiHost : public IPC::Sender, public IPC::Listener {
   void OnHostMsgAttachToPendingHost(PP_Resource resource, int pending_host_id);
   void OnHostMsgResourceDestroyed(PP_Resource resource);
 
+  // Get a linked_ptr to the given ResourceHost. It's important to get at the
+  // right one; if we just do:
+  //   linked_ptr<ResourceHost> ptr(raw_host_ptr);
+  // ...we get a linked_ptr with its own separate reference tracking, and we'll
+  // eventually double-delete |*raw_host_ptr|.
+  linked_ptr<ResourceHost> RawToLinkedPtr(ResourceHost* host);
+
   // Non-owning pointer.
   IPC::Sender* sender_;
 
@@ -143,6 +160,17 @@ class PPAPI_HOST_EXPORT PpapiHost : public IPC::Sender, public IPC::Listener {
   PendingHostResourceMap pending_resource_hosts_;
   int next_pending_resource_host_id_;
 
+  // Some hosts may need to Pin another host in memory for some amount of time.
+  // The main example at the time of writing is PepperGraphics2DHost, which may
+  // have a queue of ImageDataHosts that it requires for doing paints. This
+  // needs to work even if the plugin deletes the ImageDataResource. So
+  // owner_to_owned_map_ keeps a linked_ptr to any "pinned" hosts to make sure
+  // that they live long enough, even when the plugin is done with the resource
+  // and the host gets removed from resource_hosts_ or pending_resource_hosts_.
+  typedef std::map<linked_ptr<ResourceHost>, int > PinCountMap;
+  typedef std::map<ResourceHost*, PinCountMap> OwnerToOwnedHostMap;
+  OwnerToOwnedHostMap owner_to_owned_map_;
+
   DISALLOW_COPY_AND_ASSIGN(PpapiHost);
 };