fix file descriptor leak in backup

[emlaver]

Checklist

• Tick to sign-off your agreement to the Developer Certificate of Origin (DCO) 1.1
• Added tests for code changes or test/build only changes
• Updated the change log file (CHANGES.md|CHANGELOG.md) or test/build only changes
• Completed the PR template below:

Description

Replace tmp.fileSync().name in apiDefaults with tmp.tmpNameSync() to discard descriptor of temporary file during backup.

fixes #535

Approach

Update tmp call in includes/config.js

Schema & API Changes

Security and Privacy

Testing

Used this gist to verify that file descriptors are discarded once backup completes.

Monitoring and Logging

[eiri]

To be honest I can't see how this fixes the leaked file descriptors, consider that fileSync is using tmpNameSync under the hood. Do you have an explanation why this works?

As an alternative I'd suggest to use tmp.fileSync({ discardDescriptor: true }) instead, as described in https://github.com/raszi/node-tmp/tree/master#controlling-the-descriptor or go an extra mile and use detachDescriptor and manage the descriptor closing at backup done callback, though that sounds like a hustle.

[ricellis]

Do you have an explanation why this works?

tmpNameSync just returns a name, fileSync uses that name to open the file. We actually have no need to open the file when processing the config, we open it later by creating a read stream anyway.

[eiri]

So this is essentially using fstat vs fopen? I'd expect fd to still be allocated, tbh, so we'll win on memory and probably op speed, but not on fd count reduce, but I'm not 100% sure, maybe system takes care of that.

Ok, thanks for the clarification, no harm in using this.