feat(multiplayer): add degraded admission mode

server/api/admission.go

@@ -0,0 +1,70 @@
+package api
+
+import (
+	"strings"
+	"sync/atomic"
+)
+
+// AdmissionController decides whether a classified public operation may
+// start. Implementations must be safe for concurrent requests.
+type AdmissionController interface {
+	Allow(operation string) bool
+}
+
+// AdmissionGate is the operator-controlled overload gate for new matchmaking
+// work. Degraded mode is deliberately narrow: existing matches can continue
+// to report results and clients can still use read/recovery/event endpoints.
+type AdmissionGate struct {
+	degraded atomic.Bool
+}
+
+func NewAdmissionGate(degraded bool) *AdmissionGate {
+	gate := &AdmissionGate{}
+	gate.degraded.Store(degraded)
+	return gate
+}
+
+func (g *AdmissionGate) SetDegraded(value bool) {
+	if g != nil {
+		g.degraded.Store(value)
+	}
+}
+
+func (g *AdmissionGate) Degraded() bool {
+	return g != nil && g.degraded.Load()
+}
+
+func (g *AdmissionGate) Allow(operation string) bool {
+	if !g.Degraded() {
+		return true
+	}
+	switch operation {
+	case "login", "queue", "proposal", "allocation":
+		return false
+	default:
+		return true
+	}
+}
+
+func admissionOperation(path, method string) string {
+	if method == "GET" || method == "HEAD" || method == "OPTIONS" {
+		return ""
+	}
+	path = strings.TrimSuffix(path, "/")
+	switch {
+	case path == "/v1/session/steam" || path == "/api/v1/session/steam":
+		return "login"
+	case path == "/v1/queue" || path == "/api/v1/queue/tickets":
+		return "queue"
+	case underPath(path, "/v1/queue/") || underPath(path, "/api/v1/queue/tickets/"):
+		return "queue"
+	case underPath(path, "/v1/proposals/") || underPath(path, "/api/v1/proposals/"):
+		return "proposal"
+	default:
+		return ""
+	}
+}
+
+func underPath(path, prefix string) bool {
+	return strings.HasPrefix(path, prefix) && len(path) > len(prefix)
+}