1 files changed, 88 insertions, 0 deletions
diff --git a/nixos/lib/make-ext4-fs.nix b/nixos/lib/make-ext4-fs.nix
new file mode 100644
index 00000000000..23839ea487d
--- /dev/null
+++ b/nixos/lib/make-ext4-fs.nix
@@ -0,0 +1,88 @@
+# Builds an ext4 image containing a populated /nix/store with the closure
+# of store paths passed in the storePaths parameter. The generated image
+# is sized to only fit its contents, with the expectation that a script
+# resizes the filesystem at boot time.
+{ pkgs
+, storePaths
+, volumeLabel
+}:
+
+pkgs.stdenv.mkDerivation {
+  name = "ext4-fs.img";
+
+  buildInputs = with pkgs; [e2fsprogs libfaketime perl];
+
+  # For obtaining the closure of `storePaths'.
+  exportReferencesGraph =
+    map (x: [("closure-" + baseNameOf x) x]) storePaths;
+
+  buildCommand =
+    ''
+      # Add the closures of the top-level store objects.
+      storePaths=$(perl ${pkgs.pathsFromGraph} closure-*)
+
+      # Also include a manifest of the closures in a format suitable
+      # for nix-store --load-db.
+      printRegistration=1 perl ${pkgs.pathsFromGraph} closure-* > nix-path-registration
+
+      # Make a crude approximation of the size of the target image.
+      # If the script starts failing, increase the fudge factors here.
+      numInodes=$(find $storePaths | wc -l)
+      numDataBlocks=$(du -c -B 4096 --apparent-size $storePaths | awk '$2 == "total" { print int($1 * 1.03) }')
+      bytes=$((2 * 4096 * $numInodes + 4096 * $numDataBlocks))
+      echo "Creating an EXT4 image of $bytes bytes (numInodes=$numInodes, numDataBlocks=$numDataBlocks)"
+
+      truncate -s $bytes $out
+      faketime "1970-01-01 00:00:00" mkfs.ext4 -L ${volumeLabel} -U 44444444-4444-4444-8888-888888888888 $out
+
+      # Populate the image contents by piping a bunch of commands to the `debugfs` tool from e2fsprogs.
+      # For example, to copy /nix/store/abcd...efg-coreutils-8.23/bin/sleep:
+      #   cd /nix/store/abcd...efg-coreutils-8.23/bin
+      #   write /nix/store/abcd...efg-coreutils-8.23/bin/sleep sleep
+      #   sif sleep mode 040555
+      #   sif sleep gid 30000
+      # In particular, debugfs doesn't handle absolute target paths; you have to 'cd' in the virtual
+      # filesystem first. Likewise the intermediate directories must already exist (using `find`
+      # handles that for us). And when setting the file's permissions, the inode type flags (__S_IFDIR,
+      # __S_IFREG) need to be set as well.
+      (
+        echo write nix-path-registration nix-path-registration
+        echo mkdir nix
+        echo cd /nix
+        echo mkdir store
+
+        # XXX: This explodes in exciting ways if anything in /nix/store has a space in it.
+        find $storePaths -printf '%y %f %h %m\n'| while read -r type file dir perms; do
+          # echo "TYPE=$type DIR=$dir FILE=$file PERMS=$perms" >&2
+
+          echo "cd $dir"
+          case $type in
+            d)
+              echo "mkdir $file"
+              echo sif $file mode $((040000 | 0$perms)) # magic constant is __S_IFDIR
+              ;;
+            f)
+              echo "write $dir/$file $file"
+              echo sif $file mode $((0100000 | 0$perms)) # magic constant is __S_IFREG
+              ;;
+            l)
+              echo "symlink $file $(readlink "$dir/$file")"
+              ;;
+            *)
+              echo "Unknown entry: $type $dir $file $perms" >&2
+              exit 1
+              ;;
+          esac
+
+          echo sif $file gid 30000 # chgrp to nixbld
+        done
+      ) | faketime "1970-01-01 00:00:00" debugfs -w $out -f /dev/stdin > errorlog 2>&1
+
+      # The debugfs tool doesn't terminate on error nor exit with a non-zero status. Check manually.
+      if egrep -q 'Could not allocate|File not found' errorlog; then
+        cat errorlog
+        echo "--- Failed to create EXT4 image of $bytes bytes (numInodes=$numInodes, numDataBlocks=$numDataBlocks) ---"
+        return 1
+      fi
+    '';
+}