Patch ERB to not reuse TOPLEVEL_BINDING when none is specified

gistfile1.diff

diff --git a/lib/erb.rb b/lib/erb.rb
index 26c34ee..a78a8cb 100644
--- a/lib/erb.rb
+++ b/lib/erb.rb
@@ -818,7 +818,7 @@ class ERB
   end
 
   # Generate results and print them. (see ERB#result)
-  def run(b=TOPLEVEL_BINDING)
+  def run(b=new_toplevel)
     print self.result(b)
   end
 
@@ -830,7 +830,7 @@ class ERB
   # _b_ accepts a Binding or Proc object which is used to set the context of
   # code evaluation.
   #
-  def result(b=TOPLEVEL_BINDING)
+  def result(b=new_toplevel)
     if @safe_level
       proc {
         $SAFE = @safe_level
@@ -841,6 +841,12 @@ class ERB
     end
   end
 
+  def new_toplevel
+    # New binding each time *near* toplevel for unspecified runs
+    eval "proc{binding}.call", TOPLEVEL_BINDING
+  end
+  private :new_toplevel
+
   # Define _methodname_ as instance method of _mod_ from compiled ruby source.
   #
   # example:
diff --git a/test/erb/test_erb.rb b/test/erb/test_erb.rb
index 7ddbc87..47caf03 100644
--- a/test/erb/test_erb.rb
+++ b/test/erb/test_erb.rb
@@ -49,6 +49,14 @@ class TestERB < Test::Unit::TestCase
     assert_equal("", ERB::Util.html_escape(nil))
     assert_equal("123", ERB::Util.html_escape(123))
   end
+
+  def test_concurrent_default_binding
+    template1 = 'one <%= ERB.new(template2).result %>'
+
+    eval 'template2 = "two"', TOPLEVEL_BINDING
+
+    assert_equal("one two", ERB.new(template1).result)
+  end
 end
 
 class TestERBCore < Test::Unit::TestCase