added OutputGuard sanity check to AddDestroyHandler to ensure that outputs are…

theano/compile/mode.py

@@ -74,9 +74,39 @@ def register_optimizer(name, opt):
         raise ValueError('Optimizer name already taken: %s' % name)
     predefined_optimizers[name] = opt
 
+class OutputGuard(gof.Op):
+    destroy_map = {0:[0]}
+    view_map = {0:[0]}
+    def make_node(self, x):
+        return gof.Apply(self, [x], [x.type()])
+    def perform(self, node, (x,), (z,)):
+        z[0] = x
+    def __str__(self):
+        return '%s' % self.__class__.__name__
+
 class AddDestroyHandler(gof.Optimizer):
+    """This optimizer performs two important functions:
+
+    1) it has a 'requirement' of the destroyhandler.  This means that the env will include it
+    as a feature for this optimization, and keep this feature enabled for subsequent
+    optimizations.  All optimizations that work inplace on any of their inputs must run *after*
+    this optimization to ensure that the DestroyHandler has been included in the env.
+
+    2) It tries to replace each output with an Op that purports to destroy it (but it won't I
+    promise).   If this replacement succeeds it means that there is a bug in theano.  It should
+    not be possible to destroy outputs.
+    """
     def apply(self, env):
-        pass
+        output_guard = OutputGuard()
+        for o in env.outputs:
+            try:
+                env.replace_validate(o, output_guard(o), reason='output_guard')
+                warning("Output variable %s required output_guard,"
+                        " how was this output left unprotected against destructive operations?"
+                        % o)
+            except gof.InconsistencyError:
+                #this output is already impossible to destroy. no guard necessary
+                pass
     def add_requirements(self, env):
         super(AddDestroyHandler, self).add_requirements(env)
         env.extend(gof.DestroyHandler())