fs/session: add debug logging to statewait handler

Logs the agent name and base value each time statewait is
read, and whether WaitChange returned ok or !ok.

fs/session/files.go

@@ -367,11 +367,14 @@ func (h *agentHelper) fileSpec(name string, mode os.FileMode) FileSpec {
 			if base == "" {
 				base = h.ag.State()
 			}
+			h.log.Debug("statewait: begin base=%q ag=%q", base, h.ag.Name())
 			v, ok := h.ag.WaitChange(ctx, agent.WatchState, base)
 			if !ok {
 				s := h.ag.State()
+				h.log.Debug("statewait: !ok returning state=%q", s)
 				return []byte(s + "\n"), s, nil
 			}
+			h.log.Debug("statewait: ok returning state=%q", v)
 			return []byte(v + "\n"), v, nil
 		}
 	}