[BUG] Extra newline char in stderr of docker compose exec if command returns non-zero exit status

[mriemensberger]

Description

docker compose exec prints an extra newline character in stderr that is not output by the command itself if it returns a non-zero exit status. This is inconsistent with docker exec, which never does that. docker compose exec also does not show this behavior if the command returns a zero exit status.

Steps To Reproduce

On ubuntu 24.04 with CE versions of docker and docker compose:

% echo '{"services":{"debian":{"image":"debian:12","command":["sleep","infinity"]}}}' | docker compose -p debian -f - up -d 
[+] Running 2/2
 ✔ Network debian_default     Created                                                                                                                                                                 0.0s 
 ✔ Container debian-debian-1  Started                                                                                                                                                                 0.1s 
% docker exec debian-debian-1 false
% docker exec debian-debian-1 true 
% docker compose -p debian exec -T debian true
% docker compose -p debian exec -T debian false

% 

Notice the extra newline in stderr when executing false, which does neither emit any chars to stdout or stderr on its own.

Compose Version

Docker Compose version v2.36.2

Docker Environment

Client: Docker Engine - Community
 Version:    28.2.2
 Context:    default
 Debug Mode: false
 Plugins:
  buildx: Docker Buildx (Docker Inc.)
    Version:  v0.24.0
    Path:     /usr/libexec/docker/cli-plugins/docker-buildx
  compose: Docker Compose (Docker Inc.)
    Version:  v2.36.2
    Path:     /usr/libexec/docker/cli-plugins/docker-compose
  sbom: View the packaged-based Software Bill Of Materials (SBOM) for an image (Anchore Inc.)
    Version:  0.6.1
    Path:     /home/XXXX/.docker/cli-plugins/docker-sbom

Server:
 Containers: 69
  Running: 59
  Paused: 0
  Stopped: 10
 Images: 1296
 Server Version: 28.2.2
 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: systemd
 Cgroup Version: 2
 Plugins:
  Volume: local
  Network: bridge host ipvlan macvlan null overlay
  Log: awslogs fluentd gcplogs gelf journald json-file local splunk syslog
 CDI spec directories:
  /etc/cdi
  /var/run/cdi
 Swarm: inactive
 Runtimes: sysbox sysbox-runc crun io.containerd.runc.v2 runc
 Default Runtime: runc
 Init Binary: docker-init
 containerd version: 05044ec0a9a75232cad458027ca83437aae3f4da
 runc version: v1.2.5-0-g59923ef
 init version: de40ad0
 Security Options:
  apparmor
  seccomp
   Profile: builtin
  cgroupns
 Kernel Version: 6.11.0-26-generic
 Operating System: Ubuntu 24.04.2 LTS
 OSType: linux
 Architecture: x86_64
 CPUs: 16
 Total Memory: 30.01GiB
 Name: XXXX
 ID: XXXX
 Docker Root Dir: /var/lib/docker
 Debug Mode: false
 Experimental: true
 Insecure Registries:
  ::1/128
  127.0.0.0/8
 Registry Mirrors:
  https://mirror.gcr.io/
 Live Restore Enabled: false
 Default Address Pools:
   Base: 10.252.0.0/16, Size: 24

Anything else?

No response

[mriemensberger]

Better reproducer command:

% docker compose -p debian exec -T debian bash -c 'echo qewr; echo >&2 asdf; false' >/dev/null
asdf

% docker compose -p debian exec -T debian bash -c 'echo qewr; echo >&2 asdf; true' >/dev/null
asdf
% 

% docker exec debian-debian-1 bash -c 'echo qewr; echo >&2 asdf; false' >/dev/null
asdf
% docker exec debian-debian-1 bash -c 'echo qewr; echo >&2 asdf; true' >/dev/null 
asdf
% 

[mriemensberger]

I was able to narrow it down to 2.33.1 and 2.34.0 as the versions that show this kind of behavior:

% sudo apt install --allow-downgrades docker-compose-plugin=2.33.0-1~ubuntu.24.04~noble &>/dev/null; docker compose -p debian exec -T debian bash -c 'echo qewr; echo >&2 asdf; false' >/dev/null
asdf
% sudo apt install --allow-downgrades docker-compose-plugin=2.33.1-1~ubuntu.24.04~noble &>/dev/null; docker compose -p debian exec -T debian bash -c 'echo qewr; echo >&2 asdf; false' >/dev/null
asdf
exit status 1
% sudo apt install --allow-downgrades docker-compose-plugin=2.34.0-1~ubuntu.24.04~noble &>/dev/null; docker compose -p debian exec -T debian bash -c 'echo qewr; echo >&2 asdf; false' >/dev/null
asdf

% 

[mriemensberger]

@glours @ndeloof Thanks for looking at this issue. However, in my opinion the fix should do the opposite and not add anyting to stderr, regardless of command exit status. docker exec also does not add any characters to stdout and stderr regardless of the command exit status. Using docker (compose) exec for scripting gets much easier if stdout and stderr are unmodified unless there is a real reason to print something like an internal failure to even execute the command. Not modifying stderr was also the behavior of docker compose exec prior to 2.33.

[ndeloof]

@mriemensberger I think you're right, I misunderstood the issue reviewing @glours PR. docker compose exec should be strictly equivalent to docker exec <container>