[event-hubs] EventHubProducerClient.sendBatch can throw TypeError "Cannot read property 'credit' of undefined"

[chradek]

Description

The EventHubProducerClient.sendBatch() method can throw an unexpected TypeError: "Cannot read property 'credit' of undefined" if both

1. a connection disconnect occurs after the sender link has been initialized and
2. one of the send attempts fails with a retryable error, the connection is disconnected, and retries have not been exhausted.

Which versions of @azure/event-hubs are impacted?

This bug was introduced in PR #13826 which was published in version 5.5.0 of event hubs.

Root cause analyses

PR #13826 separated the sendEventPromise logic so that sender initialization occurs separately from sending the message. This means if the sending of the message fails, it does not check if the sender link exists before attempting to send the message.

When a connection disconnected event is encountered, all links are closed, which removes the reference to the old sender the EventHubSender held a reference to.

So for this to occur, the connection needs to encounter a disconnected event between the time it takes to initialize a sender link, and the time it takes to successfully send an event to the service.

Potential fix

We can move the sender initialization logic into the same retried operation as sending the link. In addition, we should remove our non-null assertions so we're properly handling cases where sender may be null.

Discovery

This showed up in one of our internal scheduled CI live test runs. It has only appeared once over the last 30 days.