Add a template to handle properly issuing completion callbacks. This fixes

ppapi/proxy/enter_proxy.h

Index: ppapi/proxy/enter_proxy.h
===================================================================
--- ppapi/proxy/enter_proxy.h	(revision 94913)
+++ ppapi/proxy/enter_proxy.h	(working copy)
@@ -6,6 +6,7 @@
 #define PPAPI_PROXY_ENTER_PROXY_H_
 
 #include "base/logging.h"
+#include "ppapi/cpp/completion_callback.h"
 #include "ppapi/proxy/host_dispatcher.h"
 #include "ppapi/proxy/plugin_dispatcher.h"
 #include "ppapi/proxy/plugin_resource_tracker.h"
@@ -52,6 +53,95 @@
   }
 };
 
+// Enters a resource and forces a completion callback to be issued.
+//
+// This is used when implementing the host (renderer) side of a resource
+// function that issues a completion callback. In all cases, we need to issue
+// the callback to avoid hanging the plugin.
+//
+// This class automatically constructs a callback with the given factory
+// calling the given method. The method will generally be the one that sends
+// the message to trigger the completion callback in the plugin process.
+//
+// It will automatically issue the callback with PP_ERROR_NOINTERFACE if the
+// host resource is invalid (i.e. failed() is set). In all other cases you
+// should call SetResult(), which will issue the callback immediately if the
+// result value isn't PP_OK_COMPLETIONPENDING. In the "completion pending"
+// case, it's assumed the function the proxy is calling will take responsibility
+// of executing the callback (returned by callback()).
+//
+// Example:
+//   EnterHostFromHostResourceForceCallback<PPB_Foo_API> enter(
+//       resource, callback_factory_, &MyClass::SendResult, resource);
+//   if (enter.failed())
+//     return;  // SendResult automatically called with PP_ERROR_BADRESOURCE.
+//   enter.SetResult(enter.object()->DoFoo(enter.callback()));
+//
+// Where DoFoo's signature looks like this:
+//   int32_t DoFoo(PP_CompletionCallback callback);
+// And SendResult's implementation looks like this:
+//   void MyClass::SendResult(int32_t result, const HostResource& res) {
+//     Send(new FooMsg_FooComplete(..., result, res));
+//   }
+template<typename ResourceT>
+class EnterHostFromHostResourceForceCallback
+    : public EnterHostFromHostResource<ResourceT> {
+ public:
+  // For callbacks that take no parameters except the "int32_t result". Most
+  // implementations will use the 1-extra-argument constructor below.
+  template<class CallbackFactory, typename Method>
+  EnterHostFromHostResourceForceCallback(const HostResource& host_resource,
+                                         CallbackFactory& factory,
+                                         Method method)
+      : EnterHostFromHostResource<ResourceT>(host_resource),
+        needs_running_(true),
+        callback_(factory.NewOptionalCallback(method)) {
+    if (this->failed())
+      RunCallback(PP_ERROR_BADRESOURCE);
+  }
+
+  // For callbacks that take an extra parameter as a closure.
+  template<class CallbackFactory, typename Method, typename A>
+  EnterHostFromHostResourceForceCallback(const HostResource& host_resource,
+                                         CallbackFactory& factory,
+                                         Method method,
+                                         const A& a)
+      : EnterHostFromHostResource<ResourceT>(host_resource),
+        needs_running_(true),
+        callback_(factory.NewOptionalCallback(method, a)) {
+    if (this->failed())
+      RunCallback(PP_ERROR_BADRESOURCE);
+  }
+
+  ~EnterHostFromHostResourceForceCallback() {
+    if (needs_running_) {
+      NOTREACHED() << "Should always call SetResult except in the "
+                      "initialization failed case.";
+      RunCallback(PP_ERROR_FAILED);
+    }
+  }
+
+  void SetResult(int32_t result) {
+    DCHECK(needs_running_) << "Don't call SetResult when there already is one.";
+    if (result != PP_OK_COMPLETIONPENDING)
+      callback_.Run(result);
+  }
+
+  PP_CompletionCallback callback() {
+    return callback_.pp_completion_callback();
+  }
+
+ private:
+  void RunCallback(int32_t result) {
+    DCHECK(needs_running_);
+    needs_running_ = false;
+    callback_.Run(result);
+  }
+
+  bool needs_running_;
+  pp::CompletionCallback callback_;
+};
+
 }  // namespace proxy
 }  // namespace pp