dockerTools.buildImage: support using a layered image in fromImage

Docker images used to be, essentially, a linked list of layers. Each
layer would have a tarball and a json document pointing to its parent,
and the image pointed to the top layer:

    imageA  ----> layerA
                    |
                    v
                  layerB
                    |
                    v
                  layerC

The current image spec changed this format to where the Image defined
the order and set of layers:

    imageA  ---> layerA
            |--> layerB
            `--> layerC

For backwards compatibility, docker produces images which follow both
specs: layers point to parents, and images also point to the entire
list:

    imageA  ---> layerA
            |      |
            |      v
            |--> layerB
            |      |
            |      v
            `--> layerC

This is nice for tooling which supported the older version and never
updated to support the newer format.

Our `buildImage` code only supported the old version, so in order for
`buildImage` to properly generate an image based on another image
with `fromImage`, the parent image's layers must fully support the old
mechanism.

This is not a problem in general, but is a problem with
`buildLayeredImage`.

`buildLayeredImage` creates images with newer image spec, because
individual store paths don't have a guaranteed parent layer. Including
a specific parent ID in the layer's json makes the output less likely
to cache hit when published or pulled.

This means until now, `buildLayeredImage` could not be the input to
`buildImage`.

The changes in this PR change `buildImage` to only use the layer's
manifest when locating parent IDs. This does break buildImage on
extremely old Docker images, though I do wonder how many of these
exist.

This work has been sponsored by Target.
This commit is contained in:
Graham Christensen 2018-12-04 12:18:06 -05:00
parent 33b9aa4f35
commit c88337c9ac
No known key found for this signature in database
GPG key ID: ACA1C1D120C83D5C
3 changed files with 54 additions and 15 deletions

View file

@ -62,5 +62,9 @@ import ./make-test.nix ({ pkgs, ... }: {
# Ensure Layered Docker images work
$docker->succeed("docker load --input='${pkgs.dockerTools.examples.layered-image}'");
$docker->succeed("docker run --rm ${pkgs.dockerTools.examples.layered-image.imageName}");
# Ensure building an image on top of a layered Docker images work
$docker->succeed("docker load --input='${pkgs.dockerTools.examples.layered-on-top}'");
$docker->succeed("docker run --rm ${pkgs.dockerTools.examples.layered-on-top.imageName}");
'';
})

View file

@ -188,22 +188,27 @@ rec {
# Use the name and tag to get the parent ID field.
parentID=$(jshon -e $fromImageName -e $fromImageTag -u \
< image/repositories)
cat ./image/manifest.json | jq -r '.[0].Layers | .[]' > layer-list
else
touch layer-list
fi
# Unpack all of the parent layers into the image.
lowerdir=""
while [[ -n "$parentID" ]]; do
echo "Unpacking layer $parentID"
mkdir -p image/$parentID/layer
tar -C image/$parentID/layer -xpf image/$parentID/layer.tar
rm image/$parentID/layer.tar
extractionID=0
for layerTar in $(cat layer-list); do
echo "Unpacking layer $layerTar"
extractionID=$((extractionID + 1))
find image/$parentID/layer -name ".wh.*" -exec bash -c 'name="$(basename {}|sed "s/^.wh.//")"; mknod "$(dirname {})/$name" c 0 0; rm {}' \;
mkdir -p image/$extractionID/layer
tar -C image/$extractionID/layer -xpf $layerTar
rm $layerTar
find image/$extractionID/layer -name ".wh.*" -exec bash -c 'name="$(basename {}|sed "s/^.wh.//")"; mknod "$(dirname {})/$name" c 0 0; rm {}' \;
# Get the next lower directory and continue the loop.
lowerdir=$lowerdir''${lowerdir:+:}image/$parentID/layer
parentID=$(cat image/$parentID/json \
| (jshon -e parent -u 2>/dev/null || true))
lowerdir=$lowerdir''${lowerdir:+:}image/$extractionID/layer
done
mkdir work
@ -673,6 +678,9 @@ rec {
if [[ -n "$fromImage" ]]; then
echo "Unpacking base image..."
tar -C image -xpf "$fromImage"
cat ./image/manifest.json | jq -r '.[0].Layers | .[]' > layer-list
# Do not import the base image configuration and manifest
chmod a+w image image/*.json
rm -f image/*.json
@ -690,6 +698,8 @@ rec {
for l in image/*/layer.tar; do
ls_tar $l >> baseFiles
done
else
touch layer-list
fi
chmod -R ug+rw image
@ -742,17 +752,23 @@ rec {
# Use the temp folder we've been working on to create a new image.
mv temp image/$layerID
# Add the new layer ID to the beginning of the layer list
(
# originally this used `sed -i "1i$layerID" layer-list`, but
# would fail if layer-list was completely empty.
echo "$layerID/layer.tar"
cat layer-list
) | ${pkgs.moreutils}/bin/sponge layer-list
# Create image json and image manifest
imageJson=$(cat ${baseJson} | jq ". + {\"rootfs\": {\"diff_ids\": [], \"type\": \"layers\"}}")
manifestJson=$(jq -n "[{\"RepoTags\":[\"$imageName:$imageTag\"]}]")
currentID=$layerID
while [[ -n "$currentID" ]]; do
layerChecksum=$(sha256sum image/$currentID/layer.tar | cut -d ' ' -f1)
for layerTar in $(cat ./layer-list); do
layerChecksum=$(sha256sum image/$layerTar | cut -d ' ' -f1)
imageJson=$(echo "$imageJson" | jq ".history |= [{\"created\": \"$(jq -r .created ${baseJson})\"}] + .")
imageJson=$(echo "$imageJson" | jq ".rootfs.diff_ids |= [\"sha256:$layerChecksum\"] + .")
manifestJson=$(echo "$manifestJson" | jq ".[0].Layers |= [\"$currentID/layer.tar\"] + .")
currentID=$(cat image/$currentID/json | (jshon -e parent -u 2>/dev/null || true))
manifestJson=$(echo "$manifestJson" | jq ".[0].Layers |= [\"$layerTar\"] + .")
done
imageJsonChecksum=$(echo "$imageJson" | sha256sum | cut -d ' ' -f1)

View file

@ -156,5 +156,24 @@ rec {
name = "layered-image";
tag = "latest";
config.Cmd = [ "${pkgs.hello}/bin/hello" ];
contents = [ pkgs.hello pkgs.bash pkgs.coreutils ];
};
# 11. Create an image on top of a layered image
layered-on-top = pkgs.dockerTools.buildImage {
name = "layered-on-top";
tag = "latest";
fromImage = layered-image;
extraCommands = ''
mkdir ./example-output
chmod 777 ./example-output
'';
config = {
Env = [ "PATH=${pkgs.coreutils}/bin/" ];
WorkingDir = "/example-output";
Cmd = [
"${pkgs.bash}/bin/bash" "-c" "echo hello > foo; cat foo"
];
};
};
}