BUG: meshviewer.json sometimes empty

[T0biii]

We(Freifunk MUC) got a couple of meshviewer.json files:
https://github.com/freifunkMUC/ffmuc-salt-public/blob/main/yanic/yanic.conf.tmpl#L45-L89
Sometimes some of them are empty the only way to get content back is to restart yanic (sometimes several times).
In the Logs there is no hint why the output is empty

[maurerle]

I never experienced such a behavior in FFAC.
We are directly serving the meshviewer.json from the yanic running on the map server, and it never has been empty..?

[T0biii]

we think it might be a problem with more than one meshviewer output
But atm we have no idea why this is happening sometimes

[genofire]

without any setup / code / logs which make it reproducible we also have no idea how to solve this issue.

The code is designed, that we normaly write everything in a .tmp file and the move that to the correct place. On this way, the should to no time be a empty file.
See here for the code:

yanic/runtime/nodes.go

Lines 272 to 289 in ef3ee35

	func SaveJSON(input interface{}, outputFile string) {
	tmpFile := outputFile + ".tmp"
	f, err := os.OpenFile(tmpFile, os.O_WRONLY|os.O_CREATE|os.O_TRUNC, 0644)
	if err != nil {
	log.Panic(err)
	}
	err = json.NewEncoder(f).Encode(input)
	if err != nil {
	log.Panic(err)
	}
	f.Close()
	if err := os.Rename(tmpFile, outputFile); err != nil {
	log.Panic(err)
	}
	}

[T0biii]

in the logs there seems no reasone why this is happening. After an restart it is fixed.
Can we add somewhere some more logging?