generate-thumbnail: `Error serializing return value: TypeError: Converting circular structure to JSON

In this sample: https://github.com/firebase/functions-samples/tree/master/generate-thumbnail

We routinely see the error Error serializing return value: TypeError: Converting circular structure to JSON in the logs during function execution.

We’ve been trying to isolate exactly the point of failure, but testing and debugging with cloud functions is a bit tricky.

Logging it here in case someone at Firebase has an idea.

[inlined]

That code is a noop from just returning ref.transaction (That it’s a noop is actually part of the acceptance test for a Promise implementation).

The problem you’re running into is that the promised value that ref.transaction returns isn’t serializeable as JSON. The easiest way to fix this (before we fix it in the Firebase SDK) is to transform the value to something like null.

export default functions.database.ref(listenToPath).onWrite(event => {
  if (event.data.changed()) {
      return ref.transaction(previousValue => previousValue + 1).then(() => null);
  }
});

That () => null transforms any success of TransactionResult to a success of typeof null

[dannycochran]

@inlined thanks for the workaround!