assertion for path validation

palps.rkt

@@ -75,6 +75,26 @@
          path])))
   (loop))
 
+;; raised by assert-confined-to-call-dir when attemping to traverse
+;; outside of (call-dir)
+(struct exn:fail:contract:illegal-path exn:fail:contract ()
+  #:extra-constructor-name make-exn:fail:contract:illegal-path
+  #:transparent)
+
+;; check if path, once resolved, is a child of base
+(define (confined-path? base path)
+  (string-prefix?
+   (path->string (simplify-path (build-path base path)))
+   (path->string (simplify-path base))))
+
+;; asserts that path is within (call-dir), and raises
+;; exn:fail:contract:illegal-path otherwise.
+(define (assert-confined-to-call-dir path)
+  (when (not (confined-path? (call-dir) path))
+    (raise (make-exn:fail:contract:illegal-path
+            "assert-confined-to-call-dir: traversed outside of root"
+            (current-continuation-marks)))))
+
 ;; "main" procedure to be executed in an interactive context i.e. for
 ;; BPQ
 (define (session-startup)