[autarch]

Description

With --logDir set, each server's stdout and stderr were piped through the runner process into the log file. mongodb-runner start exits once the cluster is up, so nothing wrote to those files afterward and every log ended at startup.

This redirects the server's stdout and stderr straight to the log file instead, so the server keeps logging on its own. The runner follows the file to read the startup output (port and build info) that it previously got from the pipe.

Log files are now named <binary>-<uuid>-<timestamp>.log rather than using the pid, since the pid isn't known before the file has to exist.

stderr now shares the file the runner parses, so lines that aren't log entries (for example docker's "Unable to find image") are skipped instead of ending the log stream and failing startup.

Open Questions

• We could take this further. Right now, if a user attempts to set --logpath for the Server, the runner is broken because it can't find the logs to tail to get the Server's port and build info. Do you want me to add this to this PR? A follow-up? Not at all?
• This PR adds a CHANGELOG.md file for mongodb-runner. Is that ok?

Checklist

• I have signed the Contributor License Agreement (https://www.mongodb.com/legal/contributor-agreement)
• New tests and/or benchmarks are included
• Documentation is changed or added

[Copilot]

🟡 Changes recommended

An unref’d tail timer can let Node exit before draining final output or signaling EOF, causing failed server startup with --logDir to silently exit successfully instead of reporting an error.

1 open issue

• Keep log tail alive to report startup failures · New

What changed in this PR

This PR keeps MongoDB server logs flowing after mongodb-runner start exits by letting servers write directly to log files while the runner tails them for startup information.

Changes:

• Redirect server stdout and stderr to UUID-based log files.
• Add file tailing and skip non-log lines during parsing.
• Add tests for tailing, parsing, and continued logging after CLI exit.

File	Description
packages/​mongodb-runner/​src/​tail-file.ts	Adds polling-based file tailing until server exit.
packages/​mongodb-runner/​src/​tail-file.spec.ts	Tests appended output and draining after exit.
packages/​mongodb-runner/​src/​mongoserver.ts	Redirects server output directly to files and tails startup logs.
packages/​mongodb-runner/​src/​mongologreader.ts	Skips non-log lines instead of ending parsing.
packages/​mongodb-runner/​src/​mongologreader.spec.ts	Tests parsing past non-log output.
packages/​mongodb-runner/​src/​cli.spec.ts	Tests continued logging after start exits.

_{Review effort: Balanced}