klarna / brod / 928

79%

master: 80%

Pull #291

travis-ci

Fix topic/group subscriber re-subscribe begin offset

Prior to this change, after brod_consumer restart,
topic/group subscribers use last-acked-offset + 1
as begin-offset to start fetching messages.

This resulted in re-deliveries of the messages fetched before
restart but not yet acked by message handler (async ack).

In this change, `last_offset` is added to track each partition's
progress, then only re-subscribe when ack_offset = last_offset
i.e. all previouly fetched messages are acked.

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