Address SF bug #577530: del __builtins__ breaks out of rexec

Using the suggestion there: add_module() forces __builtin__ back; this
fixes r_exec, r_eval, r_execfile.  The interactive console had to be
fixed separately, because it doesn't use r_exec, but relies on the
'locals' dict having the right __builtins__.  Fixed this by
subclassing InteractiveConsole and overriding runcode(), which does
the exec.  This changes the banner output slightly: instead of
starting with *** RESTRICTED ***, a subtler (RestrictedConsole) is
printed before the first >>> prompt.

Also import readline (if it exists) when the interactive console is
used, for more convenient input editing and history.

This does not mean that rexec is now considered safe!  But for those
willing to take the risk, it's safer than before.  (Note that a safety
analysis of the code module would be wise if you plan to use the
interactive console for real -- I've only ever used it to play with
restricted mode.)

This should be backported to 2.2 and 2.1.

Lib/rexec.py

@@ -288,9 +288,9 @@ class RExec(ihooks._Verbose):
     # Add a module -- return an existing module or create one
 
     def add_module(self, mname):
-        if mname in self.modules:
-            return self.modules[mname]
-        self.modules[mname] = m = self.hooks.new_module(mname)
+        m = self.modules.get(mname)
+        if m is None:
+            self.modules[mname] = m = self.hooks.new_module(mname)
         m.__builtins__ = self.modules['__builtin__']
         return m
 
@@ -552,13 +552,17 @@ def test():
             print "%s: can't open file %s" % (sys.argv[0], `args[0]`)
             return 1
     if fp.isatty():
+        try:
+            import readline
+        except ImportError:
+            pass
         import code
+        class RestrictedConsole(code.InteractiveConsole):
+            def runcode(self, co):
+                self.locals['__builtins__'] = r.modules['__builtin__']
+                r.s_apply(code.InteractiveConsole.runcode, (self, co))
         try:
-            code.interact(
-                "*** RESTRICTED *** Python %s on %s\n"
-                'Type "help", "copyright", "credits" or "license" '
-                "for more information." % (sys.version, sys.platform),
-                local=r.modules['__main__'].__dict__)
+            RestrictedConsole(r.modules['__main__'].__dict__).interact()
         except SystemExit, n:
             return n
     else: