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