Make the PrintDlgEx() call in the UI thread.

chrome/browser/printing/printer_query.h

Index: chrome/browser/printing/printer_query.h
===================================================================
--- chrome/browser/printing/printer_query.h	(revision 13415)
+++ chrome/browser/printing/printer_query.h	(working copy)
@@ -6,7 +6,7 @@
 #define CHROME_BROWSER_PRINTING_PRINTER_QUERY_H_
 
 #include "base/scoped_ptr.h"
-#include "base/ref_counted.h"
+#include "base/task.h"
 #include "chrome/browser/printing/print_job_worker_owner.h"
 
 class CancelableTask;
@@ -20,7 +20,8 @@
 
 class PrintJobWorker;
 
-// Query the printer for settings.
+// Query the printer for settings in a worker thread for default settings and
+// in |ui_message_loop| for ASK_USER request.
 class PrinterQuery : public base::RefCountedThreadSafe<PrinterQuery>,
                      public PrintJobWorkerOwner {
  public:
@@ -30,7 +31,7 @@
     ASK_USER,
   };
 
-  PrinterQuery();
+  PrinterQuery(MessageLoop* ui_message_loop);
   virtual ~PrinterQuery();
 
   // PrintJobWorkerOwner
@@ -44,7 +45,7 @@
                                PrintingContext::Result result);
   virtual PrintJobWorker* DetachWorker(PrintJobWorkerOwner* new_owner);
   virtual MessageLoop* message_loop() {
-    return ui_message_loop_;
+    return message_loop_;
   }
   virtual const PrintSettings& settings() const { return settings_; }
 
@@ -74,9 +75,12 @@
   bool is_valid() const;
 
  private:
+  // Main UI thread for displaying the Print... Dialog.
+  MessageLoop* ui_message_loop_;
+
   // Main message loop reference. Used to send notifications in the right
   // thread.
-  MessageLoop* const ui_message_loop_;
+  MessageLoop* const message_loop_;
 
   // All the UI is done in a worker thread because many Win32 print functions
   // are blocking and enters a message loop without your consent. There is one
@@ -101,6 +105,64 @@
   DISALLOW_COPY_AND_ASSIGN(PrinterQuery);
 };
 
+// A single-use printer query task for default settings. Makes it easy to defer
+// the task to a worker thread (like WorkerPool) and executes the callbacks in
+// the caller's thread..
+// TODO(maruel): Extract common functionality in a template?
+class DefaultSettingsPrinterQueryTask : public Task {
+ public:
+  // Information about the printer setting.
+  struct Result {
+    Result();
+
+    void Run();
+
+    PrintingContext printing_context;
+    PrintingContext::Result result;
+   private:
+    DISALLOW_COPY_AND_ASSIGN(Result);
+  };
+
+  // |reply_message_loop| is optional and is equivalent to
+  // MessageLoop::current() if NULL.
+  DefaultSettingsPrinterQueryTask(MessageLoop* reply_message_loop);
+  virtual ~DefaultSettingsPrinterQueryTask() { }
+
+  virtual void Run();
+  virtual void Cancel();
+
+  // The reply task will be posted to reply_message_loop specified in the
+  // constructor. Takes ownership.
+  // TODO(maruel): We could get away by using UnboundMethod<> instead.
+  void set_reply_task(CancelableTask* reply_task) {
+    reply_task_->reply_task_.reset(reply_task);
+  }
+  // Retrieves the results. Mainly useful while creating the NewRunnableMethod()
+  // to specify to set_reply_task().
+  Result* result() { return &reply_task_->result_; }
+
+ private:
+  class ReplyTask : public CancelableTask {
+   public:
+    ReplyTask();
+    virtual ~ReplyTask() { }
+
+    virtual void Run();
+    virtual void Cancel();
+
+    scoped_ptr<CancelableTask> reply_task_;
+    Result result_;
+
+   private:
+    DISALLOW_COPY_AND_ASSIGN(ReplyTask);
+  };
+
+  ReplyTask* reply_task_;
+  MessageLoop* reply_message_loop_;
+
+  DISALLOW_COPY_AND_ASSIGN(DefaultSettingsPrinterQueryTask);
+};
+
 }  // namespace printing
 
 #endif  // CHROME_BROWSER_PRINTING_PRINTER_QUERY_H_