Скачать или смотреть Resolving the Postgres FATAL: Could Not Open Log File Permission Denied Error in Docker

Discover how to fix the pesky `permission denied` issue when running Postgres in Docker. Our structured guide provides elegant solutions for troubleshooting logging errors.
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This video is based on the question https://stackoverflow.com/q/70659742/ asked by the user 'Rui Nian' ( https://stackoverflow.com/u/9191460/ ) and on the answer https://stackoverflow.com/a/70659866/ provided by the user 'Hans Kilian' ( https://stackoverflow.com/u/3924803/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Resolving the Postgres FATAL: Could Not Open Log File Permission Denied Error in Docker

Deploying Postgres in a Docker container can sometimes lead to frustrating situations, especially when error messages disrupt logging functionality. One common issue developers encounter is the "FATAL: could not open log file permission denied" message. If you've enabled logging and suddenly face this error, you might feel puzzled. This guide tackles the problem head-on and provides clear steps to resolve it.

Understanding the Problem

When running a Postgres container, the application tries to write log files to specified directories. When it encounters a permission-related issue, it results in a fatal error, effectively halting your log collection. In this specific case, the error message indicated that the Postgres process running inside the Docker container does not have the necessary permissions to create or access the specified log file.

Key Elements of the Error

Error Message: FATAL: could not open log file "/mnt/log/postgresql-2022-01-10_224118.log": Permission denied

User Identity: Postgres runs as a user called postgres, which has a User ID (UID) of 999 inside the container.

Host File Permissions: If the UID 999 doesn't have permission to the host's directory where logs are to be stored, you will see the permission denied error.

Analyzing Docker Compose Configuration

Here’s a snippet of the relevant Docker Compose configuration for context:

[[See Video to Reveal this Text or Code Snippet]]

The Log Directory

Notice the volume mapping - ./data/log_db:/mnt/log. This line binds the host's ./data/log_db directory to the container's /mnt/log directory. The error you are seeing typically occurs when the user inside the container (in this case, postgres with UID 999) does not have the right permissions to access or create files in the host directory.

The Elegant Solution

To resolve the permission issue while ensuring system security, you can follow these steps without altering the permissions of the host directory indiscriminately.

Step 1: Change Group Ownership

You will need to change the group owner of the log_db directory to match the group ID for the postgres user.

[[See Video to Reveal this Text or Code Snippet]]

Step 2: Set Directory Permissions

Next, you should set the permissions of the directory to be accessible by both the owner and the group:

[[See Video to Reveal this Text or Code Snippet]]

Step 3: Apply Setgid Bit

Finally, to ensure that any files created within the directory will inherit the group ownership, set the setgid bit on the directory:

[[See Video to Reveal this Text or Code Snippet]]

Conclusion

Once you've made these changes, you should be able to rerun your Postgres container without encountering the "permission denied" error for log files. You will find that the log files get created properly, owned by user 999, making them accessible only to the postgres user and root.

By following the steps above, you'll successfully allow Docker's Postgres container to function as intended, complete with log file generation. This solution is more elegant than simply changing permissions on all files/directories, preserving the security of your system.

If you continue to experience issues, check Docker configurations or consult the community for updates on best practices. Happy logging with Postgres in Docker!