1. A computer-implemented method executed by one or more processors, the method comprising: analyzing, in an untrusted processing environment, an untrusted application that includes a data parallel pipeline to produce a data flow graph for the untrusted application; receiving, in a trusted processing environment associated with the untrusted processing environment, the data flow graph for the untrusted application; transforming, in the trusted processing environment, the data flow graph into a revised data flow graph for the untrusted application, the revised data flow graph including one or more deferred data objects and one or more deferred parallel operations each corresponding to one or more untrusted functions called by the untrusted application; and executing, in one or more untrusted worker environments, the untrusted functions corresponding to the deferred parallel operations to produce materialized parallel data objects corresponding to the deferred data objects.