[builder-next] fix squashing base image layer by Blyschak · Pull Request #39187 · moby/moby

Blyschak · 2019-05-08T00:58:30Z

Signed-off-by: Stepan Blyshchak stepanblischak@gmail.com

- What I did
I am trying to fix "--squash" using DOCKER_BUILDKIT=1 squashes the top layers and the base image always resulting in one layer. Legacy builder behavior is to not squash the base layer.
This is quite important because multiple images that were squashed cannot share the same base image affecting the disk space (especially when dockers run on device which has a small drive)

- How I did it
Export base image config from Dockerfile2LLB, so later can pass to SquashImage.
Here I changed the input of SquashImage from base ID string to Image object, since when base image is resolved remotely it is not present in ImageStore, which is yet another behavior change comparing to legacy builder:

root@8be8a791847f:~/test# docker images
REPOSITORY          TAG                 IMAGE ID            CREATED             SIZE
root@8be8a791847f:~/test# DOCKER_BUILDKIT=1 docker build --squash -t busybox-squashed -f- . <<EOF
FROM busybox
COPY ./one /one
COPY ./two /two
COPY ./three /three
EOF

[+] Building 3.3s (9/9) FINISHED                                                                                                                                                                                    
...
root@8be8a791847f:~/test# docker images # there is no busybox:latest pulled
REPOSITORY          TAG                 IMAGE ID            CREATED             SIZE
busybox-squashed    latest              e5ef8abf1e26        4 seconds ago       1.2MB
<none>              <none>              cc706b1284cd        19 minutes ago      1.2MB

TODO: maybe it is better to pull base image from remote in exporter and so SquashImage will work as it is right now?

- How to verify it

squash with buildkit

root@8be8a791847f:~/test# DOCKER_BUILDKIT=1 docker build --squash -t busybox-squashed -f- . <<EOF
FROM busybox
COPY ./one /one
COPY ./two /two
COPY ./three /three
EOF
[+] Building 2.8s (9/9) FINISHED                                                                                                                                                                                    
 => [internal] load build definition from Dockerfile                                                                                                                                                           0.6s
 => => transferring dockerfile: 102B                                                                                                                                                                           0.0s
 => [internal] load .dockerignore                                                                                                                                                                              0.4s
 => => transferring context: 2B                                                                                                                                                                                0.0s
 => [internal] load metadata for docker.io/library/busybox:latest                                                                                                                                              0.4s
 => [1/4] FROM docker.io/library/busybox@sha256:954e1f01e80ce09d0887ff6ea10b13a812cb01932a0781d6b0cc23f743a874fd                                                                                               0.0s
 => [internal] load build context                                                                                                                                                                              0.2s
 => => transferring context: 70B                                                                                                                                                                               0.0s
 => CACHED [2/4] COPY ./one /one                                                                                                                                                                               0.0s
 => CACHED [3/4] COPY ./two /two                                                                                                                                                                               0.0s
 => CACHED [4/4] COPY ./three /three                                                                                                                                                                           0.0s
 => exporting to image                                                                                                                                                                                         0.3s
 => => exporting layers                                                                                                                                                                                        0.0s
 => => writing image sha256:cc706b1284cd4e0ff50fb9338f9202e4a769c9a435a139ecc3b0f4a34c7ac977                                                                                                                   0.0s
 => => naming to docker.io/library/busybox-squashed                                                                                                                                                            0.0s
root@8be8a791847f:~/test# docker image inspect --format '{{json .RootFS.Layers}}'  busybox-squashed 
["sha256:0b97b1c81a3200e9eeb87f17a5d25a50791a16fa08fc41eb94ad15f26516ccea","sha256:0486fd605761a4fafd88bcac8f3c99df5abf1cda0160f71548ef730e03ab4449"]

squash with legacy builder

root@8be8a791847f:~/test# docker build --squash -t busybox-squashed -f- . <<EOF
FROM busybox
COPY ./one /one
COPY ./two /two
COPY ./three /three
EOF

Sending build context to Docker daemon  7.215kB
Step 1/4 : FROM busybox
latest: Pulling from library/busybox
fc1a6b909f82: Already exists 
Digest: sha256:954e1f01e80ce09d0887ff6ea10b13a812cb01932a0781d6b0cc23f743a874fd
Status: Downloaded newer image for busybox:latest
 ---> af2f74c517aa
Step 2/4 : COPY ./one /one
 ---> 82189394f4fd
Step 3/4 : COPY ./two /two
 ---> e39967575c25
Step 4/4 : COPY ./three /three
 ---> 4d46acf12033
Successfully built 9a6defb57473
Successfully tagged busybox-squashed:latest
root@8be8a791847f:~/test# docker image inspect --format '{{json .RootFS.Layers}}'  busybox-squashed 
["sha256:0b97b1c81a3200e9eeb87f17a5d25a50791a16fa08fc41eb94ad15f26516ccea","sha256:0486fd605761a4fafd88bcac8f3c99df5abf1cda0160f71548ef730e03ab4449"]

- Description for the changelog

- A picture of a cute animal (not mandatory but encouraged)

Signed-off-by: Stepan Blyshchak <stepanblischak@gmail.com>

thaJeztah · 2019-05-09T03:42:35Z

vendor/github.com/moby/buildkit/frontend/dockerfile/builder/build.go

 		func(i int, tp *specs.Platform) {
 			eg.Go(func() error {
-				st, img, err := dockerfile2llb.Dockerfile2LLB(ctx, dtDockerfile, dockerfile2llb.ConvertOpt{
+				st, img, baseImg, err := dockerfile2llb.Dockerfile2LLB(ctx, dtDockerfile, dockerfile2llb.ConvertOpt{


FWIW; the changes made in the vendor directory should be upstreamed too github.com/moby/buildkit.

I'll add a needs-vendoring label

thaJeztah · 2019-05-09T03:43:33Z

ping @tonistiigi @AkihiroSuda PTAL

thaJeztah · 2019-06-07T12:46:20Z

ping @tonistiigi @AkihiroSuda PTAL

andrewhsu · 2019-10-31T18:48:21Z

Looks like this PR needs more work to be taken out of draft mode. @Blyschak you still working on this or should it be closed?

thaJeztah · 2019-10-31T19:49:42Z

perhaps @tonistiigi could have a look what's needed; I think a fix is still desirable to get feature parity

kiorky · 2020-11-14T12:53:22Z

Another related problem trying to solve the need to keep the squashing up to the FROM layer:

I'm currently using buildkit to build the image, and then still relying on legacy builder to squash it

To reproduce, just do

echo | DOCKER_BUILDKIT=1 docker build -t a -<<EOF
FROM busybox
RUN touch foo
RUN touch foo2
RUN touch foo3
EOF
echo "FROM a"| DOCKER_BUILDKIT="" docker build --squash -t b -

I tried with to build an image with docker:20.10.0-beta1-dind and DOCKER_BUILDKIT= which fails with

docker build --squash -t *** -
Error response from daemon: experimental session with v1 builder is no longer supported,
use builder version v2 (BuildKit) instead

The same is working with current docker:dind

thaJeztah · 2020-11-16T10:28:24Z

The error about experimental session ("Error response from daemon: experimental session with v1 builder is no longer supported...") is unrelated; that error is fixed in #41648, which will be in the next beta/rc of docker 20.10

fredcooke · 2025-12-25T12:08:42Z

This needs reviving. How can I help??? Keen to see this over the line as it's critical for me to have --squash and docker claims the old engine is going to be removed with zero for. buildkit.

[builder-next] fix squashing base image layer

c22b35b

Signed-off-by: Stepan Blyshchak <stepanblischak@gmail.com>

GordonTheTurtle added the status/0-triage label May 8, 2019

Blyschak mentioned this pull request May 8, 2019

DOCKER_BUILDKIT=1 docker build --squash should not squash the base image #38903

Open

AkihiroSuda added rebuild/janky labels May 8, 2019

GordonTheTurtle removed rebuild/janky labels May 8, 2019

Blyschak force-pushed the builder-next-squash branch 2 times, most recently from 22c120e to c22b35b Compare May 8, 2019 18:05

apply gofmt on modified source code

5b61eec

Signed-off-by: Stepan Blyshchak <stepanblischak@gmail.com>

thaJeztah reviewed May 9, 2019

View reviewed changes

thaJeztah added area/builder Build status/2-code-review status/needs-vendoring and removed status/0-triage labels May 9, 2019

This was referenced May 21, 2019

image/build: print warning/error for unsupported buildkit options docker/cli#1896

Open

[WIP] imagewriter: support squash moby/buildkit#833

Closed

GordonTheTurtle assigned coolljt0725 May 23, 2019

thaJeztah added the kind/bugfix PR's that fix bugs label Oct 10, 2019

thaJeztah mentioned this pull request Jan 15, 2020

[epic] make BuildKit the default builder on Linux #40379

Open

12 tasks

Niek mentioned this pull request Aug 10, 2021

feature request: allow squash build moby/buildkit#819

Open

thaJeztah mentioned this pull request Dec 14, 2023

c8d/docker-py: Skip test_build_squash #46935

Merged

fredcooke mentioned this pull request Dec 25, 2025

Get docker with buildkit working properly - squash available and behaving itself kube-kaptain/kaptain#3

Closed

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[builder-next] fix squashing base image layer#39187

[builder-next] fix squashing base image layer#39187
Blyschak wants to merge 2 commits intomoby:masterfrom
Blyschak:builder-next-squash

Blyschak commented May 8, 2019

Uh oh!

thaJeztah May 9, 2019

Uh oh!

thaJeztah commented May 9, 2019

Uh oh!

thaJeztah commented Jun 7, 2019

Uh oh!

andrewhsu commented Oct 31, 2019

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thaJeztah commented Oct 31, 2019

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kiorky commented Nov 14, 2020

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thaJeztah commented Nov 16, 2020

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Blyschak commented May 8, 2019

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