diff --git a/netwerk/cache2/AppCacheStorage.cpp b/netwerk/cache2/AppCacheStorage.cpp
index 129a63669e57..7a045175f626 100644
--- a/netwerk/cache2/AppCacheStorage.cpp
+++ b/netwerk/cache2/AppCacheStorage.cpp
@@ -171,5 +171,13 @@ NS_IMETHODIMP AppCacheStorage::AsyncVisitStorage(nsICacheStorageVisitor* aVisito
   return NS_OK;
 }
 
+NS_IMETHODIMP AppCacheStorage::GetCacheIndexEntryAttrs(nsIURI *aURI,
+                                                       const nsACString &aIdExtension,
+                                                       bool *aHasAltData,
+                                                       uint32_t *aSizeInKB)
+{
+  return NS_ERROR_NOT_IMPLEMENTED;
+}
+
 } // namespace net
 } // namespace mozilla
diff --git a/netwerk/cache2/CacheStorage.cpp b/netwerk/cache2/CacheStorage.cpp
index 88853b5d7422..33c8f9e76293 100644
--- a/netwerk/cache2/CacheStorage.cpp
+++ b/netwerk/cache2/CacheStorage.cpp
@@ -171,7 +171,7 @@ NS_IMETHODIMP CacheStorage::Exists(nsIURI *aURI, const nsACString & aIdExtension
     this, asciiSpec, aIdExtension, aResult);
 }
 
-nsresult
+NS_IMETHODIMP
 CacheStorage::GetCacheIndexEntryAttrs(nsIURI *aURI,
                                       const nsACString &aIdExtension,
                                       bool *aHasAltData,
diff --git a/netwerk/cache2/CacheStorage.h b/netwerk/cache2/CacheStorage.h
index b25a4b11a755..85c5bccdbb31 100644
--- a/netwerk/cache2/CacheStorage.h
+++ b/netwerk/cache2/CacheStorage.h
@@ -73,9 +73,6 @@ public:
   bool LookupAppCache() const { return mLookupAppCache; }
   bool SkipSizeCheck() const { return mSkipSizeCheck; }
   bool Pinning() const { return mPinning; }
-  virtual nsresult GetCacheIndexEntryAttrs(
-          nsIURI *aURI, const nsACString &aIdExtension,
-          bool *aHasAltData, uint32_t *aSizeInKB) override;
 };
 
 } // namespace net
diff --git a/netwerk/cache2/OldWrappers.cpp b/netwerk/cache2/OldWrappers.cpp
index bd083439d707..543e42bbb756 100644
--- a/netwerk/cache2/OldWrappers.cpp
+++ b/netwerk/cache2/OldWrappers.cpp
@@ -1076,6 +1076,14 @@ NS_IMETHODIMP _OldStorage::AsyncVisitStorage(nsICacheStorageVisitor* aVisitor,
   return NS_OK;
 }
 
+NS_IMETHODIMP _OldStorage::GetCacheIndexEntryAttrs(nsIURI *aURI,
+                                                   const nsACString &aIdExtension,
+                                                   bool *aHasAltData,
+                                                   uint32_t *aSizeInKB)
+{
+  return NS_ERROR_NOT_IMPLEMENTED;
+}
+
 // Internal
 
 nsresult _OldStorage::AssembleCacheKey(nsIURI *aURI,
diff --git a/netwerk/cache2/nsICacheStorage.idl b/netwerk/cache2/nsICacheStorage.idl
index 0a716b1e8284..8c2a10f454ac 100644
--- a/netwerk/cache2/nsICacheStorage.idl
+++ b/netwerk/cache2/nsICacheStorage.idl
@@ -113,6 +113,16 @@ interface nsICacheStorage : nsISupports
    */
   boolean exists(in nsIURI aURI, in ACString aIdExtension);
 
+  /**
+   * Synchronously check on existance of alternative data and size of the
+   * content. When the index data are not up to date or index is still building,
+   * NS_ERROR_NOT_AVAILABLE is thrown. The same error may throw any storage
+   * implementation that cannot determine entry state without blocking the caller.
+   */
+  void getCacheIndexEntryAttrs(in nsIURI aURI,
+                               in ACString aIdExtension,
+                               out bool aHasAltData,
+                               out uint32_t aSizeInKB);
   /**
    * Asynchronously removes an entry belonging to the URI from the cache.
    */
@@ -132,11 +142,4 @@ interface nsICacheStorage : nsISupports
   void asyncVisitStorage(in nsICacheStorageVisitor aVisitor,
                          in boolean aVisitEntries);
 
-  %{C++
-    virtual nsresult GetCacheIndexEntryAttrs(
-          nsIURI *aURI, const nsACString &aIdExtension,
-          bool *aHasAltData, uint32_t *aSizeInKB) {
-      return NS_ERROR_NOT_IMPLEMENTED;
-    }
-  %}
 };
diff --git a/netwerk/test/unit/test_alt-data_simple.js b/netwerk/test/unit/test_alt-data_simple.js
index 6b7bd93a3e5c..8d5790fc4cb2 100644
--- a/netwerk/test/unit/test_alt-data_simple.js
+++ b/netwerk/test/unit/test_alt-data_simple.js
@@ -23,6 +23,12 @@ function make_channel(url, callback, ctx) {
   return NetUtil.newChannel({uri: url, loadUsingSystemPrincipal: true});
 }
 
+function inChildProcess() {
+  return Cc["@mozilla.org/xre/app-info;1"]
+           .getService(Ci.nsIXULRuntime)
+           .processType != Ci.nsIXULRuntime.PROCESS_TYPE_DEFAULT;
+}
+
 const responseContent = "response body";
 const responseContent2 = "response body 2";
 const altContent = "!@#$%^&*()";
@@ -31,6 +37,8 @@ const altContentType = "text/binary";
 var servedNotModified = false;
 var shouldPassRevalidation = true;
 
+var cache_storage = null;
+
 function contentHandler(metadata, response)
 {
   response.setHeader("Content-Type", "text/plain");
@@ -52,20 +60,40 @@ function contentHandler(metadata, response)
   }
 }
 
+function check_has_alt_data_in_index(aHasAltData)
+{
+  if (inChildProcess()) {
+    return;
+  }
+  var hasAltData = {};
+  cache_storage.getCacheIndexEntryAttrs(createURI(URL), "", hasAltData, {});
+  do_check_eq(hasAltData.value, aHasAltData);
+}
+
 function run_test()
 {
   do_get_profile();
   httpServer = new HttpServer();
   httpServer.registerPathHandler("/content", contentHandler);
   httpServer.start(-1);
+  do_test_pending();
 
+  if (!inChildProcess()) {
+    cache_storage = getCacheStorage("disk") ;
+    wait_for_cache_index(asyncOpen);
+  } else {
+    asyncOpen();
+  }
+}
+
+function asyncOpen()
+{
   var chan = make_channel(URL);
 
   var cc = chan.QueryInterface(Ci.nsICacheInfoChannel);
   cc.preferAlternativeDataType(altContentType);
 
   chan.asyncOpen2(new ChannelListener(readServerContent, null));
-  do_test_pending();
 }
 
 function readServerContent(request, buffer)
@@ -74,6 +102,7 @@ function readServerContent(request, buffer)
 
   do_check_eq(buffer, responseContent);
   do_check_eq(cc.alternativeDataType, "");
+  check_has_alt_data_in_index(false);
 
   do_execute_soon(() => {
     var os = cc.openAlternativeOutputStream(altContentType);
@@ -109,6 +138,7 @@ function readAltContent(request, buffer)
   do_check_eq(servedNotModified, true);
   do_check_eq(cc.alternativeDataType, altContentType);
   do_check_eq(buffer, altContent);
+  check_has_alt_data_in_index(true);
 
   requestAgain();
 }
@@ -129,6 +159,7 @@ function readEmptyAltContent(request, buffer)
   // the cache is overwrite and the alt-data is reset
   do_check_eq(cc.alternativeDataType, "");
   do_check_eq(buffer, responseContent2);
+  check_has_alt_data_in_index(false);
 
   httpServer.stop(do_test_finished);
 }