If multiple pipeline close recoveries are performed against the same block, the replica may go corrupt. Here is one case I have observed:
The client tried to close a block, but the ACK timed out. It excluded the first DN and tried pipeline recovery (recoverClose). It too failed and another recovery was attempted with only one DN. This took more than usual but the client eventually got an ACK and the file was closed successfully. Later on the one and only replica was found to be corrupt.
It turned out the DN was having transient slow disk I/O issue at that time. The first recovery was stuck until the second recovery was attempted 30 seconds later. After few seconds, they both threads started running. The second recovery finished first and then the first recovery with an older gen stamp finished, turning gen stamp backward.
There is a sanity check in recoverCheck(), but since check and modify are not synchronized, recoverClose() is not multi-thread safe.