net/git-checkout: adjust ADLER32 check to work with streaming

Don't for an inflated stream to be held completely in memory.

racket/collects/net/git-checkout.rkt

@@ -1083,17 +1083,24 @@
 
 ;; ADLER32 implementation
 ;; https://www.ietf.org/rfc/rfc1950.txt
-(define (bytes-adler32 bstr)
+(define (adler32-through-ports in out)
   (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))
+  (define bstr (make-bytes 4096))
+  (let loop ([s1 1] [s2 0])
+    (define n (read-bytes! bstr in))
+    (cond
+      [(eof-object? n)
+       (bitwise-ior (arithmetic-shift s2 16) s1)]
+      [else
+       (write-bytes bstr out 0 n)
+       (define-values (new-s1 new-s2)
+         (for/fold ([s1 s1]
+                    [s2 s2])
+                   ([bits (in-bytes bstr 0 n)])
+           (define a (modulo (+ s1 bits) ADLER))
+           (define b (modulo (+ s2 a) ADLER))
+           (values a b)))
+       (loop new-s1 new-s2)])))
 
 ;; zlib-inflate : input-port output-port
 ;;  Reads compressed data from `i`, writes uncompressed to `o`
@@ -1105,17 +1112,20 @@
   (when (bitwise-bit-set? flg 5)
     ;; read dictid
     (read-bytes-exactly 'dictid 4 i))
-  ;; 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?
+  ;; Include adler32 checksum in the pipeline, writing to `o`:
+  (define-values (checksum-in checksum-out) (make-pipe 4096))
+  (define uncompressed-adler #f)
+  (define checksum-thread
+    (thread
+     (lambda () (set! uncompressed-adler (adler32-through-ports checksum-in o)))))
+  ;; Inflate, sending output to checksum (and then to `o`):
+  (inflate i checksum-out)
+  (close-output-port checksum-out)
+  (sync checksum-thread)
+  ;; Verify checksum
   (define adler (read-bytes-exactly 'adler-checksum 4 i))
   (unless (= (integer-bytes->integer adler #f #t)
-             (bytes-adler32 uncompressed))
+             uncompressed-adler)
     (raise-git-error 'git-checkout "adler32 checksum failed"))
   (void))