chore: import upstream snapshot with attribution

apps/sim/lib/core/admission/gate.ts

@@ -0,0 +1,62 @@
+import { createLogger } from '@sim/logger'
+import { NextResponse } from 'next/server'
+import { env } from '@/lib/core/config/env'
+
+const logger = createLogger('AdmissionGate')
+
+const MAX_INFLIGHT = Number.parseInt(env.ADMISSION_GATE_MAX_INFLIGHT ?? '') || 500
+
+let inflight = 0
+
+export interface AdmissionTicket {
+  release: () => void
+}
+
+/**
+ * Attempts to admit a request through the in-process gate.
+ * Returns a ticket with a release() handle on success, or null if at capacity.
+ * Zero external calls — purely in-process atomic counter. Each pod maintains its
+ * own counter, so the effective aggregate limit across N pods is N × MAX_INFLIGHT.
+ * Configure ADMISSION_GATE_MAX_INFLIGHT per pod based on what each pod can sustain.
+ */
+export function tryAdmit(): AdmissionTicket | null {
+  if (inflight >= MAX_INFLIGHT) {
+    return null
+  }
+
+  inflight++
+  let released = false
+
+  return {
+    release() {
+      if (released) return
+      released = true
+      inflight--
+    },
+  }
+}
+
+/**
+ * Returns a 429 response for requests rejected by the admission gate.
+ */
+export function admissionRejectedResponse(): NextResponse {
+  logger.warn('Admission gate rejecting request', { inflight, maxInflight: MAX_INFLIGHT })
+  return NextResponse.json(
+    {
+      error: 'Too many requests',
+      message: 'Server is at capacity. Please retry shortly.',
+      retryAfterSeconds: 5,
+    },
+    {
+      status: 429,
+      headers: { 'Retry-After': '5' },
+    }
+  )
+}
+
+/**
+ * Returns the current gate metrics for observability.
+ */
+export function getAdmissionGateStatus(): { inflight: number; maxInflight: number } {
+  return { inflight, maxInflight: MAX_INFLIGHT }
+}