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xarmian
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* feat(metrics): session-id-keyed TTL sweep for mcp_active_sessions (TASK-1120) Replaces the naive +1/-1 active-sessions accounting from TASK-961. The old logic bumped on JSON-RPC `initialize` and decremented on HTTP DELETE — but a client that crashed, lost network, or restarted mid-session never emitted DELETE, so the gauge drifted upward monotonically until the pad-cloud server restarted. Approach: - `internal/server/middleware_mcp_session.go` (new) — mcpSessionTracker is an in-memory map keyed by Mcp-Session-Id (the canonical header set by mcp-go's StreamableHTTPServer on initialize responses and echoed by the client on subsequent requests). Touch updates lastSeen on insert + refresh; evict removes; periodic sweep evicts entries older than the TTL. - Gauge is `Set(len(sessions))` via an onChange callback — single consistent observation per state-changing op, no risk of gauge drifting from map size on a multi-evict sweep. - Lifecycle: spawned by SetMCPTransport (alongside startMCPAuditWriter), shut down from Server.Stop. Idempotent on both sides. - Configurable via PAD_MCP_SESSION_TTL (default 30m) and PAD_MCP_SESSION_SWEEP_INTERVAL (default 5m). cmd/pad calls Server.SetMCPSessionTrackerConfig before SetMCPTransport. Other changes: - `recordMCPCallMetrics` no longer touches the active-sessions gauge. Updated comment + signature kept (callers pass the same args; the unused params are explicitly underscored). - `MCPAuditLog` middleware now calls trackMCPSession after next.ServeHTTP — single new line in the audit hot path. - `TestMCPAudit_BufferFull_DropsAndIncrementsCounter` updated to also shut down the new session tracker before bg.Wait(), since SetMCPTransport now spawns two goroutines on srv.bg. Test coverage (16 tests, all green under -race): - Tracker unit: touch insert/dedup, empty-id no-op, evict remove/non-existent, sweep eviction with single onChange, nil-onChange safety, concurrent touch/evict, run() clean shutdown. - Server-side integration: lifecycle happy path (initialize → call → DELETE leaves gauge at 0), failed initialize doesn't open, no-session-id no-op, nil tracker safety, idempotent start, DELETE evicts on any status (transient 5xx on shutdown still counts). - Regression guard: TestRecordMCPCallMetrics_DoesNotTouchSessionGauge pins that the audit-side helper has migrated off the gauge. Parent: PLAN-943. Follow-up to TASK-961 (PR #398). Closes the "sessions drift on client crashes" caveat documented in the metric's help text + the Grafana panel description. * fix(metrics): emit Mcp-Session-Id + serialize gauge updates per Codex review (round 1) Two findings from Codex review on PR #400: 1. WithStateLess(true) wired StatelessSessionIdManager whose Generate() returns "" — mcp-go never set the Mcp-Session-Id response header in production, so the new tracker no-op'd on every initialize and the active-sessions gauge stayed at 0. Fix: introduce padMCPGenerateOnlySessionIDManager in cmd/pad/main.go. Generates a UUID per initialize (so the response carries the header — tracker can observe), but Validate accepts ANY incoming value (including empty / arbitrary). Preserves the original "stateless server, every request stands alone" contract while making the session-id observable. Documented why mcp-go's two shipped stateless managers don't fit (one breaks observability, the other breaks back-compat for clients that never echo the ID). 2. touch / evict / sweep computed `len(sessions)` under the mutex then released the lock BEFORE invoking onChange. Two concurrent inserts could compute (n=1, n=2) under the lock and then race the callback writes — last writer wins on the gauge, leaving it permanently inconsistent with the map size. Fix: hold the mutex across onChange. Trade-off documented: any future onChange that re-enters the tracker would deadlock, but that's a clear failure mode rather than silent metric corruption. Added TestMCPSessionTracker_OnChangeUnderLock that asserts a strictly-monotonic observation sequence under 32-goroutine concurrent inserts; passes 5x in a row under -race.