nixpkgs/nixos/lib/make-ext4-fs.nix

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# Builds an ext4 image containing a populated /nix/store with the closure
# of store paths passed in the storePaths parameter, in addition to the
# contents of a directory that can be populated with commands. The
# generated image is sized to only fit its contents, with the expectation
# that a script resizes the filesystem at boot time.
{ pkgs
# List of derivations to be included
, storePaths
# Shell commands to populate the ./files directory.
# All files in that directory are copied to the root of the FS.
, populateImageCommands ? ""
, volumeLabel
, uuid ? "44444444-4444-4444-8888-888888888888"
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, e2fsprogs
, libfaketime
, perl
, lkl
}:
let
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sdClosureInfo = pkgs.buildPackages.closureInfo { rootPaths = storePaths; };
in
pkgs.stdenv.mkDerivation {
name = "ext4-fs.img";
nativeBuildInputs = [e2fsprogs.bin libfaketime perl lkl];
buildCommand =
''
(
mkdir -p ./files
${populateImageCommands}
)
# Add the closures of the top-level store objects.
storePaths=$(cat ${sdClosureInfo}/store-paths)
# Make a crude approximation of the size of the target image.
# If the script starts failing, increase the fudge factors here.
numInodes=$(find $storePaths ./files | wc -l)
numDataBlocks=$(du -s -c -B 4096 --apparent-size $storePaths ./files | tail -1 | awk '{ 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 -f "1970-01-01 00:00:01" mkfs.ext4 -L ${volumeLabel} -U ${uuid} $out
# Also include a manifest of the closures in a format suitable for nix-store --load-db.
cp ${sdClosureInfo}/registration nix-path-registration
cptofs -t ext4 -i $out nix-path-registration /
# Create nix/store before copying paths
faketime -f "1970-01-01 00:00:01" mkdir -p nix/store
cptofs -t ext4 -i $out nix /
echo "copying store paths to image..."
cptofs -t ext4 -i $out $storePaths /nix/store/
(
echo "copying files to image..."
cd ./files
cptofs -t ext4 -i $out ./* /
)
# I have ended up with corrupted images sometimes, I suspect that happens when the build machine's disk gets full during the build.
if ! fsck.ext4 -n -f $out; then
echo "--- Fsck failed for EXT4 image of $bytes bytes (numInodes=$numInodes, numDataBlocks=$numDataBlocks) ---"
cat errorlog
return 1
fi
(
# Resizes **snugly** to its actual limits (or closer to)
free=$(dumpe2fs $out | grep '^Free blocks:')
blocksize=$(dumpe2fs $out | grep '^Block size:')
blocks=$(dumpe2fs $out | grep '^Block count:')
blocks=$((''${blocks##*:})) # format the number.
blocksize=$((''${blocksize##*:})) # format the number.
# System can't boot with 0 blocks free.
# Add 16MiB of free space
fudge=$(( 16 * 1024 * 1024 / blocksize ))
size=$(( blocks - ''${free##*:} + fudge ))
echo "Resizing from $blocks blocks to $size blocks. (~ $((size*blocksize/1024/1024))MiB)"
EXT2FS_NO_MTAB_OK=yes resize2fs $out -f $size
)
# And a final fsck, because of the previous truncating.
fsck.ext4 -n -f $out
'';
}