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Previously we had two maps keyed on a direction-specific tuple, with distinct values containing the data (action) for that direction. Values pointed at each other across maps to ensure they were removed at the same time in the case of tuple overwrite, but LRU eviction was per-map. So if LRU was turned on, it was possible for one direction's data (action) to be evicted and leave the other direction dangling. NewFlow replaces the two direction-specific flow constructors, and lookups return the direction-specific PacketAction directly. Now the values in each map point to the same element, with data for both directions in the element. A linked list also points to the elements to implement LRU. The previous flowtrack.Cache is removed. The single LRU structure will allow us to implement idle time expiration by walking the list backward starting with the least recently used flow, and stopping after a fixed number of flows, or at the first non-expired flow. We add commented-out unused placeholder fields for tracking the "last seen" timestamp, and an on-removal hook, to document the intent for the follow-up expiry work. Updates tailscale/corp#38630 Signed-off-by: Michael Ben-Ami <mzb@tailscale.com>