JRUBY-4821: Clear last error when calling #close during IO#open block

src/org/jruby/RubyIO.java

@@ -1109,6 +1109,7 @@ public static IRubyObject open(ThreadContext context, IRubyObject recv, IRubyObj
                     RubyException rubyEx = re.getException();
                     if (rubyEx.kind_of_p(context, runtime.getStandardError()).isTrue()) {
                         // MRI behavior: swallow StandardErorrs
+                        runtime.getGlobalVariables().clear("$!");
                     } else {
                         throw re;
                     }

src/org/jruby/internal/runtime/GlobalVariables.java

@@ -129,7 +129,11 @@ public IRubyObject set(String name, IRubyObject value) {
         variable.trace(value);
         return result;
     }
-
+
+    public IRubyObject clear(String name) {
+        return set(name, runtime.getNil());
+    }
+
     public void setTraceVar(String name, RubyProc proc) {
         assert name != null;
         assert name.startsWith("$");

src/org/jruby/runtime/load/LoadService.java

@@ -544,7 +544,7 @@ public void trySearch(SearchState state) {
                 state.library = new ClassExtensionLibrary(theClass);
             } catch (Exception ee) {
                 state.library = null;
-                runtime.getGlobalVariables().set("$!", runtime.getNil());
+                runtime.getGlobalVariables().clear("$!");
             }
 
             // If there was a good library before, we go back to that

test/test_io.rb

@@ -461,6 +461,14 @@ def test_tty_leak
     end
   end
 
+  # JRUBY-4821
+  def test_clear_dollar_bang_after_open_block
+    open(__FILE__) do |io|
+      io.close
+    end
+    assert_nil $!
+  end
+
   private
   def ensure_files(*files)
     files.each {|f| File.open(f, "w") {|g| g << " " } }