Issue 530 - patterns.ini isn`t being written on Firefox 20 (Permission denied)

lib/io.js

Index: lib/io.js
===================================================================
--- a/lib/io.js
+++ b/lib/io.js
@@ -20,18 +20,20 @@
  */
 
 let {Services} = Cu.import("resource://gre/modules/Services.jsm", null);
 let {FileUtils} = Cu.import("resource://gre/modules/FileUtils.jsm", null);
 let {OS} = Cu.import("resource://gre/modules/osfile.jsm", null);
 let {Task} = Cu.import("resource://gre/modules/Task.jsm", null);
 
 let {TimeLine} = require("timeline");
+let {Prefs} = require("prefs");
 let {Utils} = require("utils");
 
+let firstRead = true;
 const BUFFER_SIZE = 0x8000;  // 32kB
 
 let IO = exports.IO =
 {
   /**
    * Retrieves the platform-dependent line break string.
    */
   get lineBreak()
@@ -174,16 +176,35 @@ let IO = exports.IO =
           TimeLine.asyncDone(timeLineID);
         }
       };
 
       let decoder = new TextDecoder();
       let array = new Uint8Array(BUFFER_SIZE);
       Task.spawn(function()
       {
+        if (firstRead && Services.vc.compare(Utils.platformVersion, "23.0a1") <= 0)
+        {
+          // See https://issues.adblockplus.org/ticket/530 - the first file
+          // opened cannot be closed due to Gecko bug 858723. Make sure that
+          // our patterns.ini file doesn't stay locked by opening a dummy file
+          // first.
+          try
+          {
+            let dummyPath = IO.resolveFilePath(Prefs.data_directory + "/dummy").path;
+            let dummy = yield OS.File.open(dummyPath, {write: true, truncate: true});
+            yield dummy.close();
+          }
+          catch (e)
+          {
+            // Dummy might be locked already, we don't care
+          }
+        }
+        firstRead = false;
+
         let f = yield OS.File.open(file.path, {read: true});
         let numBytes;
         do
         {
           numBytes = yield f.readTo(array);
           if (numBytes)
           {
             let data = decoder.decode(numBytes == BUFFER_SIZE ?