CancellableStream returned by docker_client.api.exec_start does not adequately release resources when closed, causing "I/O operation on closed file" errors #3345

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1. TTimo commented on Jul 7, 2025

   

   TTimo

   on Jul 7, 2025

   Author

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   More context about this problem:

   > pip freeze | grep docker && python --version && docker version
   docker==7.1.0
   Python 3.13.5
   Client:
    Version:           28.3.0
    API version:       1.51
    Go version:        go1.24.4
    Git commit:        38b7060a21
    Built:             Wed Jun 25 15:40:54 2025
    OS/Arch:           linux/amd64
    Context:           default
   
   Server:
    Engine:
     Version:          28.3.0
     API version:      1.51 (minimum version 1.24)
     Go version:       go1.24.4
     Git commit:       265f709647
     Built:            Wed Jun 25 15:40:54 2025
     OS/Arch:          linux/amd64
     Experimental:     false
    containerd:
     Version:          v2.1.3
     GitCommit:        c787fb98911740dd3ff2d0e45ce88cdf01410486.m
    runc:
     Version:          1.3.0
     GitCommit:        
    docker-init:
     Version:          0.19.0
     GitCommit:        de40ad0
   

   The exception doesn't reproduce reliably. I've seen it happen on and off for months now, but I only now got what looked like a stable reproduction. Probably just lucky.

   Slightly edited extract from our code that was triggering this:

       exec_instance = docker_client.api.exec_create(
           docker_container.id,
           conf.command,
           workdir=start_dir,
           privileged=True,
           tty=False,
           environment=environment,
           user='root' if conf.root else None
       )
       exec_output = docker_client.api.exec_start(
           exec_instance['Id'],
           tty=False,
           stream=True,
           demux=True,
       )
       try:
           [..]
           for (stdout_out, stderr_out) in exec_output:
               [..]
       finally:
           exec_output.close()
           # Workaround for https://github.com/docker/docker-py/issues/3345
           exec_output._response.close()