fasl: use longer fasl prefix

Change fasl prefix from `rkt:` to `racket/fasl:`, in the hope that future civilizations will be able to decipher these binary files.

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

@@ -31,7 +31,7 @@
 
 ;; The fasl format is meant to be forward-compatible:
 (define immutables-regression-bstr
-  #"rkt:\0\200\371\0\34\"n\4\3\6\ao\r2\16\5three\23\4four\25\4five\21\3six\"u \3vwx\36yz\35\2{|\16\afifteen%\1\2\200\16\bnineteen\16\asixteen\177%\0\2\202\23\ntwenty-one\204\23\ftwenty-three%\2\2\206\207\210\211#\16\ftwenty-eight\3\213\214\23\00231\b\340\b\200\344\f\b\201\320\204\0\0\b\2010W\5\0\b\201\200\3566\0\b\201\200\300\r\26\b\202\0\374\371\330\b\0\0\0\b\202\0`v\363\263b\1\0\b\202\0\0\220\235\316\332\2027\t\0\0\0\0\0\200D@\t\315\314\314\314\314\314\20@\v\231\322\f\232\322\f\t\0\0\0\0\0\200F@\t\0\0\0\0\0\0Y@\n\0\08B")
+  #"racket/fasl:\0\200\371\0\34\"n\4\3\6\ao\r2\16\5three\23\4four\25\4five\21\3six\"u \3vwx\36yz\35\2{|\16\afifteen%\1\2\200\16\bnineteen\16\asixteen\177%\0\2\202\23\ntwenty-one\204\23\ftwenty-three%\2\2\206\207\210\211#\16\ftwenty-eight\3\213\214\23\00231\b\340\b\200\344\f\b\201\320\204\0\0\b\2010W\5\0\b\201\200\3566\0\b\201\200\300\r\26\b\202\0\374\371\330\b\0\0\0\b\202\0`v\363\263b\1\0\b\202\0\0\220\235\316\332\2027\t\0\0\0\0\0\200D@\t\315\314\314\314\314\314\20@\v\231\322\f\232\322\f\t\0\0\0\0\0\200F@\t\0\0\0\0\0\0Y@\n\0\08B")
 
 (for ([i (in-list immutables)])
   (test i fasl->s-exp (s-exp->fasl i)))

pkgs/racket-test-extra/tests/racket/fasl-prefix.rkt

@@ -0,0 +1,33 @@
+#lang racket/base
+
+;; Test that bytestrings created by "the fasl library" start with the name
+;;  of the library.
+;;
+;; In particular:
+;; - use `s-exp->fasl` from `racket/fasl` to create a bytestring
+;; - read the prefix of the bytestring, i.e. everything before the first ':'
+;; - use `dynamic-require` on the prefix to import the `s-exp->fasl` function
+;;
+;; The idea is that if someone finds a fasl-encoded file, they can view the
+;;  file in a text editor and search the internet to learn more --- instead
+;;  of being stuck with a mysterious binary.
+
+(require rackunit)
+
+(define SECRET-VALUE 'any-value)
+
+(define bstr
+  (parameterize ([current-namespace (make-base-namespace)])
+    (namespace-require 'racket/fasl)
+    (eval `(s-exp->fasl ',SECRET-VALUE))))
+
+(define fasl-prefix
+  (string->symbol
+    (apply string
+           (for/list ([b (in-bytes bstr)]
+                       #:break (eq? (integer->char b) #\:))
+              (integer->char b)))))
+
+(check-equal?
+  ((dynamic-require fasl-prefix 's-exp->fasl) SECRET-VALUE)
+  bstr)

racket/collects/racket/fasl.rkt

@@ -83,6 +83,8 @@
 
 (define fasl-lowest-small-integer -10)
 (define fasl-highest-small-integer (- 255 (- fasl-small-integer-start fasl-lowest-small-integer) 1))
+(define fasl-prefix #"racket/fasl:")
+(define fasl-prefix-length (bytes-length fasl-prefix))
 
 (define-constants
   (fasl-hash-eq-variant    0)
@@ -124,7 +126,7 @@
   (define exploded-wrt-dir 'not-ready)
   (define (treat-immutable? v) (or (not keep-mutable?) (immutable? v)))
   ;; The fasl formal prefix:
-  (write-bytes #"rkt:" o)
+  (write-bytes fasl-prefix o)
   ;; Write content to a string, so we can measure it
   (define bstr
     (let ([o (open-output-bytes)])
@@ -303,7 +305,7 @@
                    [(bytes? orig-i) (mcons orig-i 0)]
                    [(input-port? orig-i) orig-i]
                    [else (raise-argument-error 'fasl->s-exp "(or/c bytes? input-port?)" orig-i)]))
-  (unless (bytes=? (read-bytes/exactly 4 init-i) #"rkt:")
+  (unless (bytes=? (read-bytes/exactly fasl-prefix-length init-i) fasl-prefix)
     (read-error "unrecognized prefix"))
   (define shared-count (read-fasl-integer init-i))
   (define shared (make-vector shared-count))