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nerdctl attach freezes the terminal sessions randomly. #4374
Description
Description
When attaching two terminal sessions using nerdctl attach and executing commands, the terminal randomly freezes and doesn't display the commands being entered.
output.mp4
Steps to reproduce the issue
- sudo nerdctl run -it --rm --name enipla alpine
- sudo nerdctl attach enipla
Describe the results you received and expected
This is the result when running with docker.
output-expected.mp4
What version of nerdctl are you using?
Client:
Version: v2.1.2
OS/Arch: linux/amd64
Git commit: 2a2834dd1c926528aa141726a9a9132eb1665568
buildctl:
Version: v0.23.0-rc1-24-g91ff73e1c
GitCommit: 91ff73e1c26e761a64de9a05bc1e67387a7ef8d0
Server:
containerd:
Version: 1.7.27
GitCommit: 05044ec0a9a75232cad458027ca83437aae3f4da
runc:
Version: 1.3.0
GitCommit: v1.3.0-0-g4ca628d1
Are you using a variant of nerdctl? (e.g., Rancher Desktop)
Others
Host information
Client:
Namespace: default
Debug Mode: false
Server:
Server Version: 1.7.27
Storage Driver: overlayfs
Logging Driver: json-file
Cgroup Driver: systemd
Cgroup Version: 2
Plugins:
Log: fluentd journald json-file none syslog
Storage: native overlayfs
Security Options:
apparmor
seccomp
Profile: builtin
cgroupns
Kernel Version: 6.8.0-62-generic
Operating System: Linux Mint 22.1
OSType: linux
Architecture: x86_64
CPUs: 8
Total Memory: 13.57GiB
Name: anton
ID: 5616474f-8d43-470c-890a-3a1c59b1e89c
Activity
apostasie commentedon Jun 25, 2025
Yeah, this is unfortunate:
https://github.com/containerd/nerdctl/blob/v2.1.2/cmd/nerdctl/container/container_attach.go#L44-L48
Limitation of the original implementation: #2108
Duplicate of #3570 I believe.
apostasie commentedon Jun 25, 2025
Help is welcome in fixing this...
Note this is likely quite involved and requires a good grasp of IO.
ChengyuZhu6 commentedon Jul 2, 2025
@apostasie I‘d like to take it on.
ekalinin commentedon Aug 31, 2026
Same root cause as #3570, as @apostasie suspected: a FIFO has one queue, so two sessions reading the container's stdout each get a random subset of it. Nothing is dropped, it is just handed to whichever reader asked first, which is why one terminal looks frozen while the other prints characters it never asked for.
I have posted a design proposal for fixing this properly in #3570, since that is the tracking issue for it. Happy to take the discussion there.
feat(attach): support multiple concurrent attach sessions
feat(attach): support multiple concurrent attach sessions
feat(attach): support multiple concurrent attach sessions