* Maintain integrity of the substitute and successor mappings when

deleting a path in the store.
* Allow absolute paths in Nix expressions.
* Get nix-prefetch-url to work again.
* Various other fixes.

corepkgs/fetchurl/builder.sh.in

@@ -0,0 +1,19 @@
+#! /bin/sh
+
+export PATH=/bin:/usr/bin
+
+echo "downloading $url into $out..."
+
+prefetch=@prefix@/store/nix-prefetch-url-$md5
+if test -f "$prefetch"; then
+    echo "using prefetched $prefetch";
+    mv $prefetch $out || exit 1
+else
+    @wget@ --passive-ftp "$url" -O "$out" || exit 1
+fi
+
+actual=$(@bindir@/nix-hash --flat $out)
+if test "$actual" != "$md5"; then
+    echo "hash is $actual, expected $md5"
+    exit 1
+fi