make-{iso9660-image,squashfs,system-tarball}: drop references to build closure (#425700)

nixos/lib/make-iso9660-image.nix

···
   name = isoName;
   __structuredAttrs = true;
 
+  # the image will be self-contained so we can drop references
+  # to the closure that was used to build it
+  unsafeDiscardReferences.out = true;
+
   buildCommandPath = ./make-iso9660-image.sh;
   nativeBuildInputs = [
     xorriso

nixos/lib/make-squashfs.nix

···
   name = "${fileName}${lib.optionalString (!hydraBuildProduct) ".img"}";
   __structuredAttrs = true;
 
+  # the image will be self-contained so we can drop references
+  # to the closure that was used to build it
+  unsafeDiscardReferences.out = true;
+
   nativeBuildInputs = [ squashfsTools ];
 
   buildCommand = ''

nixos/lib/make-system-tarball.nix

···
 
 stdenv.mkDerivation {
   name = "tarball";
-  builder = ./make-system-tarball.sh;
+  __structuredAttrs = true;
+
+  # the tarball will be self-contained so we can drop references
+  # to the closure that was used to build it
+  unsafeDiscardReferences.out = true;
+
+  buildCommandPath = ./make-system-tarball.sh;
   nativeBuildInputs = extraInputs;
 
   inherit
···
     compressCommand
     ;
 
-  # !!! should use XML.
   sources = map (x: x.source) contents;
   targets = map (x: x.target) contents;
 
-  # !!! should use XML.
   inherit symlinks objects;
 
   closureInfo = closureInfo {

nixos/lib/make-system-tarball.sh

···
-sources_=($sources)
-targets_=($targets)
-
-objects=($objects)
-symlinks=($symlinks)
-
-
 # Remove the initial slash from a path, since genisofs likes it that way.
 stripSlash() {
     res="$1"
···
 }
 
 # Add the individual files.
-for ((i = 0; i < ${#targets_[@]}; i++)); do
-    stripSlash "${targets_[$i]}"
+for ((i = 0; i < ${#targets[@]}; i++)); do
+    stripSlash "${targets[$i]}"
     mkdir -p "$(dirname "$res")"
-    cp -a "${sources_[$i]}" "$res"
+    cp -a "${sources[$i]}" "$res"
 done