Go strings are not marshallers

So, you’ve built a nifty custom slog.Handler in Go. It’s dutifully processing your application’s logs, maybe even shipping them off to a service like BigQuery. Things are great, until… fatal error: stack overflow. What gives?

This was precisely the situation I found myself in. My CologHandler had a method, uploadToBigQuery, responsible for, well, uploading logs. Inside this method, I was iterating through log attributes. If an attribute wasn’t a json.Marshaler, I’d log a warning using slog.Warn. I was aware that the new slog would call itself and therefore also trigger a new upload. Since I converted the value, that wasn’t suitable for JSON marshalling, to a string, I thought everything was safe. Because strings can be marshalled to JSON, right? Well, not quite.

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r.Attrs(func(a slog.Attr) bool {
	value, ok := a.Value.Any().(json.Marshaler)
	if ok {
		params[a.Key] = value
	} else {
		slog.Warn("non-json-marshaler value in log attributes",
			"key", a.Key, "value", a.Value.String())
	}
	return true
})

Primitive types, including strings, do not implement the json.Marshaler interface directly. They can be marshalled to JSON, but they don’t explicitly declare that they can be marshalled.

This of course caused another call to slog.Warn, and another, and another… until the stack overflowed.

Go strings are not marshallers#

Go strings are not marshallers