Bug 302115 "Try again" on XUL error page loads wrong thing after going back

r=bryner sr=bz

docshell/base/nsDocShell.cpp

@@ -3002,6 +3002,17 @@ nsDocShell::LoadErrorPage(nsIURI *aURI, const PRUnichar *aURL,
         mURIResultedInDocument = PR_TRUE;
         OnNewURI(aURI, nsnull, mLoadType, PR_TRUE, PR_FALSE);
     }
+    // Be sure to have a correct mLSHE, it may have been cleared by
+    // EndPageLoad. See bug 302115.
+    if (mSessionHistory && !mLSHE) {
+        PRInt32 idx;
+        mSessionHistory->GetRequestedIndex(&idx);
+        nsCOMPtr<nsIHistoryEntry> entry;
+        mSessionHistory->GetEntryAtIndex(idx, PR_FALSE,
+                                         getter_AddRefs(entry));
+        mLSHE = do_QueryInterface(entry);
+
+    }
 
     nsCAutoString url;
     nsCAutoString charset;

docshell/shistory/public/nsISHistory.idl

@@ -62,7 +62,7 @@ interface nsISimpleEnumerator;
 #define NS_SHISTORY_CONTRACTID "@mozilla.org/browser/shistory;1"
 %}
 
-[scriptable, uuid(7294FE9B-14D8-11D5-9882-00C04FA02F40)]
+[scriptable, uuid(9883609F-CDD8-4d83-9B55-868FF08AD433)]
 interface nsISHistory: nsISupports
 {
   /**
@@ -78,6 +78,12 @@ interface nsISHistory: nsISupports
    */
    readonly attribute long index;
 
+  /**
+   * A readonly property of the interface that returns 
+   * the index of the last document that started to load.
+   */
+   readonly attribute long requestedIndex;
+
   /**
    * A read/write property of the interface, used to Get/Set
    * the maximum number of toplevel documents, session history 

docshell/shistory/src/nsSHistory.cpp

@@ -343,11 +343,20 @@ nsSHistory::GetCount(PRInt32 * aResult)
 NS_IMETHODIMP
 nsSHistory::GetIndex(PRInt32 * aResult)
 {
-    NS_ENSURE_ARG_POINTER(aResult);
+    NS_PRECONDITION(aResult, "null out param?");
 	*aResult = mIndex;
 	return NS_OK;
 }
 
+/* Get the requestedIndex */
+NS_IMETHODIMP
+nsSHistory::GetRequestedIndex(PRInt32 * aResult)
+{
+    NS_PRECONDITION(aResult, "null out param?");
+    *aResult = mRequestedIndex;
+    return NS_OK;
+}
+
 NS_IMETHODIMP
 nsSHistory::GetEntryAtIndex(PRInt32 aIndex, PRBool aModifyIndex, nsISHEntry** aResult)
 {