When datanodes are being upgraded, an out-of-band ack is sent upstream and the client does a pipeline recovery. The client may hit this multiple times as more nodes get upgraded. This normally does not cause any issue, but if the client is holding the stream open without writing any data during this time, a permanent write failure can occur.
This is because there is a limit of 5 recovery trials for the same packet, which is tracked by "last acked sequence number". Since the empty heartbeat packets for an idle output stream does not increment the sequence number, the write will fail after it seeing 5 pipeline breakages by datanode upgrades.
This check/limit was added to avoid spinning until running out of nodes in the cluster due to a corruption or any other irrecoverable conditions. The datanode upgrade-restart should be excluded from the count.