From ef9e9e3f7eb672988d1c3e55a1d041af6e4897c7 Mon Sep 17 00:00:00 2001
From: Matthew Flatt <mflatt@racket-lang.org>
Date: Sat, 17 Nov 2018 16:44:08 -0700
Subject: [PATCH] cs: check initialization of variables on instance linking

---
 .../racket-test-core/tests/racket/module.rktl |  8 ++++
 racket/src/cs/linklet.sls                     | 37 ++++++++++++-------
 2 files changed, 31 insertions(+), 14 deletions(-)

diff --git a/pkgs/racket-test-core/tests/racket/module.rktl b/pkgs/racket-test-core/tests/racket/module.rktl
index af438f2d76..ab755f62fe 100644
--- a/pkgs/racket-test-core/tests/racket/module.rktl
+++ b/pkgs/racket-test-core/tests/racket/module.rktl
@@ -2801,6 +2801,14 @@ case of module-leve bindings; it doesn't cover local bindings.
                               (regexp-match? #rx"boom" (exn-message x)))))
 (test #t procedure? (eval 'f (module->namespace ''fails-after-f-and-before-g)))
 
+(module uses-fails-after-f-and-before-g racket/base
+  (require 'fails-after-f-and-before-g)
+  g)
+
+(err/rt-test (dynamic-require ''uses-fails-after-f-and-before-g #f)
+             (lambda (x) (and (exn:fail? x)
+                              (regexp-match? #rx"uninitialized" (exn-message x)))))
+
 ;; ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 
 (report-errs)
diff --git a/racket/src/cs/linklet.sls b/racket/src/cs/linklet.sls
index 62f1350b3b..008bb298d1 100644
--- a/racket/src/cs/linklet.sls
+++ b/racket/src/cs/linklet.sls
@@ -565,7 +565,7 @@
                  (eval-from-bytevector (linklet-code linklet) (linklet-format linklet)))
              (make-variable-reference target-instance #f)
              (append (apply append
-                            (map extract-variables
+                            (map (make-extract-variables target-instance)
                                  import-instances
                                  (linklet-importss linklet)
                                  (linklet-importss-abi linklet)))
@@ -626,19 +626,28 @@
 
   ;; Find variables or values needed from an instance for a linklet's
   ;; imports
-  (define (extract-variables inst syms imports-abi)
-    (let ([ht (instance-hash inst)])
-      (map (lambda (sym import-abi)
-             (let ([var (or (hash-ref ht sym #f)
-                            (raise-arguments-error 'instantiate-linklet
-                                                   "variable not found in imported instance"
-                                                   "instance" inst
-                                                   "name" sym))])
-               (if import-abi
-                   (variable-val var)
-                   var)))
-           syms
-           imports-abi)))
+  (define (make-extract-variables target-inst)
+    (lambda (inst syms imports-abi)
+      (let ([ht (instance-hash inst)])
+        (map (lambda (sym import-abi)
+               (let ([var (or (hash-ref ht sym #f)
+                              (raise-linking-failure "is not exported" target-inst inst sym))])
+                 (when (eq? (variable-val var) unsafe-undefined)
+                   (raise-linking-failure "is uninitialized" target-inst inst sym))
+                 (if import-abi
+                     (variable-val var)
+                     var)))
+             syms
+             imports-abi))))
+
+  (define (raise-linking-failure why target-inst inst sym)
+    (raise-arguments-error 'instantiate-linklet
+                           (string-append "mismatch;\n"
+                                          " reference to a variable that " why ";\n"
+                                          " possibly, bytecode file needs re-compile because dependencies changed")
+                           "name" (unquoted-printing-string (symbol->string sym))
+                           "exporting instance" (unquoted-printing-string (instance-name inst))
+                           "importing instance" (unquoted-printing-string (instance-name target-inst))))
 
   (define (identify-module var)
     (let ([i (car (variable-inst-box var))])