private void setExecutor(DocprocExecutor executor) {
        this.executor = executor;
    }
Returns the DocprocExecutor of this DocprocService. This can be used to
synchronously process one Processing.

@return the DocprocExecutor of this DocprocService, or null if a CallStack has not yet been set.
docproc/src/main/java/com/yahoo/docproc/impl/DocprocService.java