PPAPI: Fix GetBrowserInterface race conditions

ppapi/proxy/interface_list.h

 // Copyright (c) 2012 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 PPAPI_PROXY_INTERFACE_LIST_H_
 #define PPAPI_PROXY_INTERFACE_LIST_H_
 
 #include <map>
 #include <string>
 
+#include "base/atomic_sequence_num.h"
 #include "base/basictypes.h"
+#include "base/containers/scoped_ptr_hash_map.h"
 #include "ppapi/proxy/interface_proxy.h"
 #include "ppapi/proxy/ppapi_proxy_export.h"
 #include "ppapi/shared_impl/ppapi_permissions.h"
 
 namespace ppapi {
 namespace proxy {
 
 class PPAPI_PROXY_EXPORT InterfaceList {
  public:
   InterfaceList();
@@ skipping 21 matching lines @@
   // or NULL if it's not supported.
   const void* GetInterfaceForPPB(const std::string& name);
   const void* GetInterfaceForPPP(const std::string& name) const;
 
  private:
   friend class InterfaceListTest;
 
   struct InterfaceInfo {
     InterfaceInfo()
         : iface(NULL),
-          required_permission(PERMISSION_NONE),
-          interface_logged(false) {
+          required_permission(PERMISSION_NONE) {
     }
     InterfaceInfo(const void* in_interface, Permission in_perm)
         : iface(in_interface),
-          required_permission(in_perm),
-          interface_logged(false) {
+          required_permission(in_perm) {
     }
 
     const void* iface;
 
     // Permission required to return non-null for this interface. This will
     // be checked with the value set via SetProcessGlobalPermissionBits when
     // an interface is requested.
     Permission required_permission;
 
-    // Interface usage is logged just once per-interface-per-plugin-process.
-    bool interface_logged;
+    // Count how many times the interface has been requested. This helps us
+    // only log the first time the interface is requested (or in the unlikely
+    // event that the count rolls over).
+    base::AtomicSequenceNumber interface_request_count_;

[teravest, 2014/09/12 19:08:27]

Since we don't really need a sequence number, I wonder if it'd be better to use
either:
  CompareAndSwap, or
  an internal lock that's only held (briefly) when managing a flag.

Since this shouldn't be contended, now that I think about it, it seems like a
mutex+flag should be fine.

[dmichael (off chromium), 2014/09/12 20:44:00]

done with a lock-per-interface. Contention should be incredibly rare.

+   private:
+    DISALLOW_COPY_AND_ASSIGN(InterfaceInfo);
   };
 
-  typedef std::map<std::string, InterfaceInfo> NameToInterfaceInfoMap;
+  typedef base::ScopedPtrHashMap<std::string, InterfaceInfo>
+      NameToInterfaceInfoMap;

[dmichael (off chromium), 2014/09/12 18:12:07]

changed because AtomicSequenceNumber is not copyable

 
   void AddProxy(ApiID id, InterfaceProxy::Factory factory);
 
   // Permissions is the type of permission required to access the corresponding
   // interface. Currently this must be just one unique permission (rather than
   // a bitfield).
   void AddPPB(const char* name, const void* iface, Permission permission);
   void AddPPP(const char* name, const void* iface);
 
   // Hash the interface name for UMA logging.
   static int HashInterfaceName(const std::string& name);
 
   PpapiPermissions permissions_;
 
   NameToInterfaceInfoMap name_to_browser_info_;
   NameToInterfaceInfoMap name_to_plugin_info_;
 
   InterfaceProxy::Factory id_to_factory_[API_ID_COUNT];
 
   DISALLOW_COPY_AND_ASSIGN(InterfaceList);
 };
 
 }  // namespace proxy
 }  // namespace ppapi
 
 #endif  // PPAPI_PROXY_INTERFACE_LIST_H_