Update PpdProvider threading model.

chrome/browser/ui/webui/settings/chromeos/cups_printers_handler.cc

 // Copyright 2016 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.
 
 #include "chrome/browser/ui/webui/settings/chromeos/cups_printers_handler.h"
 
 #include <utility>
 
 #include "base/bind.h"
 #include "base/bind_helpers.h"
 #include "base/memory/ptr_util.h"
 #include "base/path_service.h"
 #include "base/strings/string_util.h"
+#include "base/threading/sequenced_task_runner_handle.h"
 #include "base/values.h"
 #include "chrome/browser/browser_process.h"
 #include "chrome/browser/chromeos/printing/printer_pref_manager_factory.h"
 #include "chrome/browser/download/download_prefs.h"
 #include "chrome/browser/profiles/profile.h"
 #include "chrome/browser/ui/browser_finder.h"
 #include "chrome/browser/ui/browser_window.h"
 #include "chrome/browser/ui/chrome_select_file_policy.h"
 #include "chrome/common/chrome_paths.h"
 #include "chromeos/dbus/dbus_thread_manager.h"
@@ skipping 52 matching lines @@
 
 CupsPrintersHandler::CupsPrintersHandler(content::WebUI* webui)
     : printer_discoverer_(nullptr),
       profile_(Profile::FromWebUI(webui)),
       weak_factory_(this) {
   base::FilePath ppd_cache_path;
   CHECK(
       base::PathService::Get(chrome::DIR_CHROMEOS_PPD_CACHE, &ppd_cache_path));
   ppd_provider_ = chromeos::printing::PpdProvider::Create(
       google_apis::GetAPIKey(), g_browser_process->system_request_context(),
+      content::BrowserThread::GetTaskRunnerForThread(
+          content::BrowserThread::FILE),
       chromeos::printing::PpdCache::Create(ppd_cache_path));
 }
 
 CupsPrintersHandler::~CupsPrintersHandler() {}
 
 void CupsPrintersHandler::RegisterMessages() {
   web_ui()->RegisterMessageCallback(
       "getCupsPrintersList",
       base::Bind(&CupsPrintersHandler::HandleGetCupsPrintersList,
                  base::Unretained(this)));
@@ skipping 165 matching lines @@
                  js_callback));
 }
 
 void CupsPrintersHandler::HandleGetCupsPrinterModels(
     const base::ListValue* args) {
   std::string js_callback;
   std::string manufacturer;
   CHECK_EQ(2U, args->GetSize());
   CHECK(args->GetString(0, &js_callback));
   CHECK(args->GetString(1, &manufacturer));
-  ppd_provider_->QueryAvailable(
-      base::Bind(&CupsPrintersHandler::QueryAvailableModelsDone,
-                 weak_factory_.GetWeakPtr(), js_callback, manufacturer));
+  // Special case the "asked with no manufacturer case" since the UI sometimes
+  // triggers this and it should yield a trivial (empty) result
+  if (manufacturer.empty()) {
+    base::SequencedTaskRunnerHandle::Get()->PostTask(
+        FROM_HERE,
+        base::Bind(&CupsPrintersHandler::QueryAvailableModelsDone,
+                   weak_factory_.GetWeakPtr(), js_callback, manufacturer,
+                   chromeos::printing::PpdProvider::SUCCESS,
+                   chromeos::printing::PpdProvider::AvailablePrintersMap()));
+  } else {
+    ppd_provider_->QueryAvailable(
+        base::Bind(&CupsPrintersHandler::QueryAvailableModelsDone,
+                   weak_factory_.GetWeakPtr(), js_callback, manufacturer));
+  }
 }
 
 void CupsPrintersHandler::HandleSelectPPDFile(const base::ListValue* args) {
   CHECK_EQ(1U, args->GetSize());
   CHECK(args->GetString(0, &webui_callback_id_));
 
   base::FilePath downloads_path = DownloadPrefs::FromDownloadManager(
       content::BrowserContext::GetDownloadManager(profile_))->DownloadPath();
 
   select_file_dialog_ = ui::SelectFileDialog::Create(
@@ skipping 88 matching lines @@
                          *printers_list);
 }
 
 void CupsPrintersHandler::OnDiscoveryDone() {
   CallJavascriptFunction("cr.webUIListenerCallback",
                          base::StringValue("on-printer-discovery-done"));
 }
 
 }  // namespace settings
 }  // namespace chromeos