squashes / cpios: properly break the Nix storepaths by breaking them up in to chunks

pkgs/squashfs-recursive/default.nix

@@ -2,20 +2,28 @@
 let
   map-squash = ./map-squash.nix;
 
-  mkSquashfsManifest = { name, storeContents, reverse ? false }:
-    runCommand "${name}-squashfs-manifest" {
+  mkSquashfsManifest = { name, storeContents }:
+    runCommand "${name}-squashfs-manifests" {
       buildInputs = [ nix jq ];
       requiredSystemFeatures = [ "recursive-nix" ];
       exportReferencesGraph = [ "root" storeContents ];
       NIX_PATH = "nixpkgs=${path}";
+      outputs = [ "out" "manifest" ];
     } ''
       cat root | grep /nix/store | sort | uniq | jq -R . | jq -s . > paths.json
-      nix-build ${map-squash} --arg pathsJson ./paths.json --arg reverse ${if reverse then "true" else "false"}
+      nix-build ${map-squash} --arg pathsJson ./paths.json
 
       touch $out
       for f in $(cat result); do
         find "$f" -type f >> $out
       done
+
+      touch $manifest
+      for f in $(cat "$out"); do
+        prefix=$(echo "$f" | head -c20)
+        suffix=$(echo "$f" | tail -c+21)
+        echo "$prefix $suffix" >> $manifest
+      done
     '';
 in
 mkSquashfsManifest

pkgs/squashfs-recursive/map-squash.nix

@@ -1,4 +1,4 @@
-{ reverse ? false, pathsJson, pkgs ? import <nixpkgs> {} }:
+{ pathsJson, pkgs ? import <nixpkgs> {} }:
 let
   namePart = strPath:
     let
@@ -12,7 +12,9 @@ let
     buildInputs = [ pkgs.squashfsTools pkgs.utillinux ];
   } ''
     mkdir $out
-    revout=$(echo "$(basename ${strPath})" | rev)
+    dirname=$(echo "$(basename ${strPath})" | head -c8)
+    filename=$(echo "$(basename ${strPath})" | tail -c+9)
+
     mksquashfs \
       "${builtins.storePath strPath}" \
       ./result \
@@ -20,11 +22,8 @@ let
       -keep-as-directory \
       -all-root
 
-    if ${if reverse then "true" else "false"}; then
-      tac result > result.rev
-      mv result.rev result
-    fi
-    mv result "$out/$revout"
+    mkdir -p "$out/$dirname"
+    mv result "$out/$dirname/$filename"
   '';
 in
 pkgs.writeText "squashes" (pkgs.lib.concatMapStringsSep "\n" mksquash paths)

quickly.nix

@@ -46,8 +46,12 @@ in
       mkdir -p /mnt-root/nix/.squash
       mkdir -p /mnt-root/nix/store
 
-      for f in $(rev /nix-store-isos); do
-        dest=$(basename "$f" | rev)
+      # the manifest splits the /nix/store/.... path with a " " to
+      # prevent Nix from determining it depends on things.
+      for f in $(cat /nix-store-isos | sed 's/ //'); do
+        prefix=$(basename "$(dirname "$f")")
+        suffix=$(basename "$f")
+        dest="$prefix$suffix"
         echo "$dest"
         mkdir "/mnt-root/nix/.squash/$dest"
         mount -t squashfs -o loop "$f" "/mnt-root/nix/.squash/$dest"
@@ -80,6 +84,12 @@ in
       inherit config pkgs;
       initrds = {
         initrd = "${config.system.build.initialRamdisk}/initrd";
+
+        # squashfsStore is just a manifest file, and makeCpioRecursive
+        # will bring in all its dependencies automatically.
+        # the nix-store initrd is the actual files, and the manifest
+        # is intentionally only just the manifest file, located
+        # at /nix-store-isos
         nix-store = "${(
           netbootpkgs.makeCpioRecursive {
             name = "better-initrd";
@@ -92,9 +102,7 @@ in
           contents =
             [
               {
-                object = pkgs.runCommand "nix-store-isos-reversed" {} ''
-                  ${pkgs.utillinux}/bin/rev ${config.system.build.squashfsStore} > $out
-                '';
+                object = config.system.build.squashfsStore.manifest;
                 symlink = "/nix-store-isos";
               }
             ];