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67995db899
Consider 3 nodes in the cluster, n1, n2, n3 with gossip generation number g1, g2, g3. n1, n2, n3 running scylla version with commit0a52ecb6df(gossip: Fix max generation drift measure) One year later, user wants the upgrade n1,n2,n3 to a new version when n3 does a rolling restart with a new version, n3 will use a generation number g3'. Because g3' - g2 > MAX_GENERATION_DIFFERENCE and g3' - g1 > MAX_GENERATION_DIFFERENCE, so g1 and g2 will reject n3's gossip update and mark g3 as down. Such unnecessary marking of node down can cause availability issues. For example: DC1: n1, n2 DC2: n3, n4 When n3 and n4 restart, n1 and n2 will mark n3 and n4 as down, which causes the whole DC2 to be unavailable. To fix, we can start the node with a gossip generation within MAX_GENERATION_DIFFERENCE difference for the new node. Once all the nodes run the version with commit0a52ecb6df, the option is no logger needed. Fixes #5164 (cherry picked from commit743b529c2b)