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Brad Fitzpatrick
7bb87d3614
The derper debug pages had no way to see which clients were connected. The expvar gauges only give counts, /debug/check only says whether the counts agree, and /debug/traffic only reports connections that moved bytes since its last tick, and only if ss is installed. Add /debug/clients/, which by default serves an index page with a form to pick one of four filters: ?all lists every connection, ?ip=1.2.3.4 and ?cidr=1.2.0.0/16 list connections from an address or prefix, and ?key=nodekey:... lists the connection(s) for one node key. Each row shows the connection number, key, remote address, connection age, flags (home, mesh, prober, notideal, dup/active/disabled), protocol version, app name, per-connection rx/tx packet and byte counts, and the estimated unique sender count. Big derpers have far too many connections for one page, so results are paginated with keyset cursors rather than page numbers: sort=key, ip, conn, rx, tx, rxpkts, or txpkts (with a leading - for descending) picks the walk order, limit=N the page size, and after=X resumes after that value of the sort field. The next-page links add afterconn=N so a page boundary that falls among connections sharing a value (duplicate keys, one IP with many ports, equal counters) resumes exactly. Column headers link to the other sort orders. The walk under Server.mu does only a filter match, a cursor comparison, and at most a bounded-heap operation per connection, so connections before the cursor are discarded without being copied and at most limit entries are ever kept. Only the summary counts (matching connections and keys) look at every connection. Snapshots are taken and the page rendered after the lock is released, so a slow debug client can't stall the server. A benchmark with 100k connections takes about 10ms per page. There were no per-connection traffic counters before, only the server-wide ones, so sclient gains four atomic.Uint64 counters (rx/tx packets and bytes, counting data packets like the server-wide ones) bumped alongside them. That's 32 bytes per connection. For the counter sorts, the value is loaded once per connection during the walk and used for both the cursor test and the heap order, so the order stays consistent while the counters keep changing. The sclient preferred field becomes an atomic.Bool so the page can report which connections are the client's home DERP; it was previously only touched by the run goroutine. Updates tailscale/corp#48933 Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com> Change-Id: I4e9b7c2d5a83f61b0e7d2c94a5f8b3e16d7c0a29