Stdout Buffering in Docker Containers: A Case of Debian vs. Ubuntu

When executing code in a Docker container, stdout buffering can occur in some instances but not in others. This issue is observed in a scenario where stdout is directed to both the console and a log file using a io.MultiWriter.

Root Cause: Platform Differences

The root cause of this behavior lies in the platform differences between Debian-based images and Ubuntu-based images. Debian appears to buffer stdout by default, while Ubuntu handles stdout synchronously.

Demonstration with Dockerfiles

This can be illustrated using two Dockerfiles, one based on Debian:wheezy and the other on Ubuntu:trusty:

# TRUSTY Dockerfile
FROM ubuntu:trusty
# ... (rest of the Dockerfile as provided in the problem description)

# WHEEZY Dockerfile
FROM debian:wheezy
# ... (rest of the Dockerfile as provided in the problem description)

Running procwrap in the container based on the wheeze image results in buffered stdout, while running it in the container based on the trusty image results in synchronous stdout. However, running procwrap on a Debian VM without Docker does not exhibit stdout buffering.

Resolution Options

To resolve this issue, several approaches can be considered:

• Manually Flush Output: Modify the code to flush stdout after each write operation.
• Use stdbuf: Try using the stdbuf command to force stdout to be unbuffered. However, this may not always be effective.
• Configure Container Settings: Explore if any container-specific settings can be adjusted to disable stdout buffering.

Ultimately, the best solution depends on the specific application and operating environment.