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This applies the same treatment from PR #20162 (netlog) and PR #20171 (wglog) to the local Taildrive filesystem wiring, ending the per-netmap-update O(n) rebuild of the drive remotes list. This moves the O(n peers) taildrive-remote list rebuild from every peer change (which previously happened regardless of whether you were even using taildrive) to instead happen only as needed. That running on every netmap update and was a contributor to the broader quadratic behavior we want to eliminate when a single peer is added or removed. Instead, this introduces drive.RemoteSource, a small interface the Taildrive filesystem pulls from lazily on incoming WebDAV requests, and caches by a generation counter. ipn/ipnlocal installs a driveRemoteSource once at NewLocalBackend time and bumps LocalBackend.driveGen on the three events that can actually flip the drive-capable peer set: full netmap installs (domain + self caps), UpdateNetmapDelta (peer add/remove or per-peer address changes), and updatePacketFilter (since PeerCapability values are derived from the packet filter rules, not from peer.CapMap). The hook itself is kept but narrowed: it no longer takes a *netmap.NetworkMap and its only remaining job is to re-notify IPN bus listeners of the current local shares list on full installs. This is a dependency to removing the netmap.NetworkMap type from upstream callers, like wgengine.Engine in general. (Also add a bunch more tests) Updates #12542 Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com> Change-Id: I7e3d2f5b4a9c8e1d6f0a3b7c9e2d4f8a1b6c5e9d