WORKDIR is not cached by legacy builder with trailing slash

[roysc]

Description

In a Dockerfile, when WORKDIR is used with a directory with a trailing slash, the resulting image is not cached if using the legacy builder.

Reproduce

1. Create the following Dockerfile:

FROM alpine:latest
WORKDIR /root/

2. Run DOCKER_BUILDKIT=0 docker build . -t test:

% DOCKER_BUILDKIT=0 docker build . -t test 
DEPRECATED: The legacy builder is deprecated and will be removed in a future release.
            BuildKit is currently disabled; enable it by removing the DOCKER_BUILDKIT=0
            environment-variable.

Sending build context to Docker daemon   5.12kB
Step 1/2 : FROM alpine:latest
 ---> f6648c04cd6c
Step 2/2 : WORKDIR /root/
 ---> Running in 34a640348602
 ---> Removed intermediate container 34a640348602
 ---> 6224b443d3a9
Successfully built 6224b443d3a9
Successfully tagged test:latest

3. Run it again. This creates a new image after the WORKDIR instruction:

% DOCKER_BUILDKIT=0 docker build . -t test
DEPRECATED: The legacy builder is deprecated and will be removed in a future release.
            BuildKit is currently disabled; enable it by removing the DOCKER_BUILDKIT=0
            environment-variable.

Sending build context to Docker daemon   5.12kB
Step 1/2 : FROM alpine:latest
 ---> f6648c04cd6c
Step 2/2 : WORKDIR /root/
 ---> Running in fa916b3ccf1c
 ---> Removed intermediate container fa916b3ccf1c
 ---> 9d1efadc4384
Successfully built 9d1efadc4384
Successfully tagged test:latest

Expected behavior

During the second run, Step 2/2 should use the cached image 6224b443d3a9.

docker version

Client:
 Cloud integration: v1.0.35+desktop.11
 Version:           25.0.3
 API version:       1.44
 Go version:        go1.21.6
 Git commit:        4debf41
 Built:             Tue Feb  6 21:13:26 2024
 OS/Arch:           darwin/arm64
 Context:           desktop-linux

Server: Docker Desktop 4.28.0 (139021)
 Engine:
  Version:          25.0.3
  API version:      1.44 (minimum version 1.24)
  Go version:       go1.21.6
  Git commit:       f417435
  Built:            Tue Feb  6 21:14:22 2024
  OS/Arch:          linux/arm64
  Experimental:     false
 containerd:
  Version:          1.6.28
  GitCommit:        ae07eda36dd25f8a1b98dfbf587313b99c0190bb
 runc:
  Version:          1.1.12
  GitCommit:        v1.1.12-0-g51d5e94
 docker-init:
  Version:          0.19.0
  GitCommit:        de40ad0

docker info

Client:
 Version:    25.0.3
 Context:    desktop-linux
 Debug Mode: false
 Plugins:
  buildx: Docker Buildx (Docker Inc.)
    Version:  v0.12.1-desktop.4
    Path:     /Users/roy/.docker/cli-plugins/docker-buildx
  compose: Docker Compose (Docker Inc.)
    Version:  v2.24.6-desktop.1
    Path:     /Users/roy/.docker/cli-plugins/docker-compose
  debug: Get a shell into any image or container. (Docker Inc.)
    Version:  0.0.24
    Path:     /Users/roy/.docker/cli-plugins/docker-debug
  dev: Docker Dev Environments (Docker Inc.)
    Version:  v0.1.0
    Path:     /Users/roy/.docker/cli-plugins/docker-dev
  extension: Manages Docker extensions (Docker Inc.)
    Version:  v0.2.22
    Path:     /Users/roy/.docker/cli-plugins/docker-extension
  feedback: Provide feedback, right in your terminal! (Docker Inc.)
    Version:  v1.0.4
    Path:     /Users/roy/.docker/cli-plugins/docker-feedback
  init: Creates Docker-related starter files for your project (Docker Inc.)
    Version:  v1.0.1
    Path:     /Users/roy/.docker/cli-plugins/docker-init
  sbom: View the packaged-based Software Bill Of Materials (SBOM) for an image (Anchore Inc.)
    Version:  0.6.0
    Path:     /Users/roy/.docker/cli-plugins/docker-sbom
  scout: Docker Scout (Docker Inc.)
    Version:  v1.5.0
    Path:     /Users/roy/.docker/cli-plugins/docker-scout
WARNING: Plugin "/Users/roy/.docker/cli-plugins/docker-scan" is not valid: failed to fetch metadata: fork/exec /Users/roy/.docker/cli-plugins/docker-scan: no such file or directory

Server:
 Containers: 9
  Running: 8
  Paused: 0
  Stopped: 1
 Images: 491
 Server Version: 25.0.3
 Storage Driver: overlay2
  Backing Filesystem: extfs
  Supports d_type: true
  Using metacopy: false
  Native Overlay Diff: true
  userxattr: false
 Logging Driver: json-file
 Cgroup Driver: cgroupfs
 Cgroup Version: 2
 Plugins:
  Volume: local
  Network: bridge host ipvlan macvlan null overlay
  Log: awslogs fluentd gcplogs gelf journald json-file local splunk syslog
 Swarm: inactive
 Runtimes: io.containerd.runc.v2 runc
 Default Runtime: runc
 Init Binary: docker-init
 containerd version: ae07eda36dd25f8a1b98dfbf587313b99c0190bb
 runc version: v1.1.12-0-g51d5e94
 init version: de40ad0
 Security Options:
  seccomp
   Profile: unconfined
  cgroupns
 Kernel Version: 6.6.16-linuxkit
 Operating System: Docker Desktop
 OSType: linux
 Architecture: aarch64
 CPUs: 10
 Total Memory: 11.67GiB
 Name: docker-desktop
 ID: d5458da4-6539-492c-aa92-1e4b8a63110e
 Docker Root Dir: /var/lib/docker
 Debug Mode: false
 HTTP Proxy: http.docker.internal:3128
 HTTPS Proxy: http.docker.internal:3128
 No Proxy: hubproxy.docker.internal
 Experimental: false
 Insecure Registries:
  hubproxy.docker.internal:5555
  127.0.0.0/8
 Live Restore Enabled: false

WARNING: daemon is not using the default seccomp profile

Additional Info

No response

[roysc]

Maybe related? #30081

[thaJeztah]

That's a really old issue, so not sure if related. My initial thought would be (possibly) the logic here;

moby/image/cache/compare.go

Lines 137 to 139 in e8496b1

	if a.WorkingDir != b.WorkingDir {
	return false
	}

It's possible the that path gets normalised somewhere (trailing slash removed/added as part of that), and possibly therefore the comparison not matching.

@vvoland any idea if that could be the case?

[vvoland]

I took a quick look and found out that the SetupWorkingDirectory did a filepath.Clean and stored the result in the container config directly, which caused a cleaned path to be stored.

Opened a PR to remove that side effect.