Update deserialize-module-guard. (#2147)

It can now optionally return a pair to redirect the module
that is dynamic-required.

pkgs/racket-doc/scribblings/reference/serialization.scrbl

@@ -368,13 +368,21 @@ More precisely, it returns the same value that @racket[(equal?
 
 @; ----------------------------------------------------------------------
 
-@defparam[deserialize-module-guard guard (module-path? symbol? . -> . void?)]{
+@defparam[deserialize-module-guard guard (-> module-path? symbol?
+                                             (or/c void? (cons/c module-path? symbol?)))]{
 
 A parameter whose value is called by @racket[deserialize] before
 dynamically loading a module via @racket[dynamic-require]. The two
 arguments provided to the procedure are the same as the arguments to
 be passed to @racket[dynamic-require]. The procedure can raise an
-exception to disallow the @racket[dynamic-require].}
+exception to disallow the @racket[dynamic-require].
+
+The procedure can optionally return a pair containing a
+@tech{module-path} and @tech{symbol}. If returned,
+@racket[deserialize] will use them as arguments to
+@racket[dynamic-require] instead.
+
+@history[#:changed "6.90.0.30" "Adds optional return values for bindings."]}
 
 @; ----------------------------------------------------------------------
 

pkgs/racket-test-core/tests/racket/serialize.rktl

@@ -579,4 +579,30 @@
 
 ;; ----------------------------------------
 
+(module immutable-b racket/base
+  (require racket/serialize)
+  (provide (all-defined-out))
+  (define-serializable-struct immutable-b (b))
+  (define alt-immutable-b-deserial
+    (make-deserialize-info
+     (λ _ 2048)
+     (λ () (error 'alt-immutable "no cycles")))))
+(require 'immutable-b)
+
+(let ([a (immutable-b 42)])
+  (parameterize ([deserialize-module-guard
+                  (λ (mod name)
+                    (test name values 'deserialize-info:immutable-b-v0)
+                    (void))])
+    (deserialize (serialize a))))
+
+
+(let ([a (immutable-b 42)])
+  (parameterize ([deserialize-module-guard
+                  (λ (mod name)
+                    (cons ''immutable-b 'alt-immutable-b-deserial))])
+    (test 2048 values (deserialize (serialize a)))))
+
+;; ----------------------------------------
+
 (report-errs)

racket/collects/racket/private/serialize.rkt

@@ -674,9 +674,11 @@
           (unless (null? l)
             (let* ([path+name (car l)]
                    [des (if (car path+name)
-                            (let ([p (unprotect-path (car path+name))]
+                            (let ([serial-p (unprotect-path (car path+name))]
-                                  [sym (revive-symbol (cdr path+name))])
+                                  [serial-sym (revive-symbol (cdr path+name))])
-                              ((deserialize-module-guard) p sym)
+                              (define maybe-binding ((deserialize-module-guard) serial-p serial-sym))
+                              (define p (if (pair? maybe-binding) (car maybe-binding) serial-p))
+                              (define sym (if (pair? maybe-binding) (cdr maybe-binding) serial-sym))
                               (let ([sub (add-submodule p)]
                                     [fallback
                                      (lambda ()