Racket uses checksums to test if it needs to recompile its source
files to bytecode.  If Racket is updated by grafting, the source and
bytecode files get updated, but the checksum stays the same.  Since
the checksum no longer matches the source file, Racket tries to
regenerate the bytecode and write it to the store, causing errors
because the store is immutable.  This patch makes Racket ignore
checksums for files in the store.

See <https://debbugs.gnu.org/30680> for details.

diff -ruN racket-6.12/collects/compiler/cm.rkt racket-6.12-patched/collects/compiler/cm.rkt
--- racket-6.12/collects/compiler/cm.rkt	1969-12-31 19:00:00.000000000 -0500
+++ racket-6.12-patched/collects/compiler/cm.rkt	2018-08-12 06:36:46.061142149 -0400
@@ -7,6 +7,7 @@
          racket/list
          racket/path
          racket/promise
+         racket/string
          openssl/sha1
          racket/place
          setup/collects
@@ -627,6 +628,10 @@
       #f
       (list src-hash recorded-hash)))
 
+(define (store-reference? path)
+  (let ([store-prefix (or (getenv "NIX_STORE") "/gnu/store")])
+    (string-prefix? (path->string path) store-prefix)))
+
 (define (rkt->ss p)
   (if (path-has-extension? p #".rkt")
       (path-replace-extension p #".ss")
@@ -679,7 +684,8 @@
               (trace-printf "newer src... ~a > ~a" path-time path-zo-time)
               ;; If `sha1-only?', then `maybe-compile-zo' returns a #f or thunk:
               (maybe-compile-zo sha1-only? deps path->mode roots path orig-path read-src-syntax up-to-date collection-cache new-seen)]
-             [(different-source-sha1-and-dep-recorded path deps)
+             [(and (not (store-reference? path))
+                   (different-source-sha1-and-dep-recorded path deps))
               => (lambda (difference)
                    (trace-printf "different src hash... ~a" difference)
                    ;; If `sha1-only?', then `maybe-compile-zo' returns a #f or thunk: