net/git-chckout: check ADLER32 after inflate

Check ADLER32 after inflate, instead of assuming that the inflated bytes
are correct.

racket/collects/net/git-checkout.rkt

@@ -1081,6 +1081,20 @@
        (arithmetic-shift (read-number-by-bits i (arithmetic-shift n -1))
                          8))]))
 
+;; ADLER32 implementation
+;; https://www.ietf.org/rfc/rfc1950.txt
+(define (bytes-adler32 bstr)
+  (define ADLER 65521)
+  (define-values (s1 s2)
+    (for/fold ([s1 1]
+               [s2 0])
+              ([bits (in-bytes bstr)])
+      (define a (modulo (+ s1 bits) ADLER))
+      (define b (modulo (+ s2 a) ADLER))
+      (values a b)))
+  ; (s2 << 16) | s1
+  (bitwise-ior (arithmetic-shift s2 16) s1))
+
 ;; zlib-inflate : input-port output-port
 ;;  Reads compressed data from `i`, writes uncompressed to `o`
 (define (zlib-inflate i o)
@@ -1091,9 +1105,18 @@
   (when (bitwise-bit-set? flg 5)
     ;; read dictid
     (read-bytes-exactly 'dictid 4 i))
-  (inflate i o)
+  ;; obtain the uncompressed bytes even if o is a file port
+  (define intermediate-out (open-output-bytes))
+  (inflate i intermediate-out)
+  (define uncompressed (get-output-bytes intermediate-out))
+  (close-output-port intermediate-out)
+  ;; pass the bytes to o
+  (display uncompressed o)
   ;; Verify checksum?
-  (read-bytes-exactly 'adler-checksum 4 i)
+  (define adler (read-bytes-exactly 'adler-checksum 4 i))
+  (unless (= (integer-bytes->integer adler #f #t)
+             (bytes-adler32 uncompressed))
+    (raise-git-error 'git-checkout "adler32 checksum failed"))
   (void))
 
 ;; ----------------------------------------