Add batch mutation drain gate

lib/batch-mutation-gate.js

@@ -0,0 +1,55 @@
+function createBatchMutationGate() {
+  let activeLeaseCount = 0;
+  let sealed = false;
+  let activeAtSeal = false;
+  let drainPromise = null;
+  let resolveDrain = null;
+
+  function acquire() {
+    if (sealed) return null;
+
+    activeLeaseCount += 1;
+    let open = true;
+
+    return Object.freeze({
+      finish() {
+        if (!open) return false;
+
+        open = false;
+        activeLeaseCount -= 1;
+
+        if (sealed && activeLeaseCount === 0 && resolveDrain) {
+          const resolve = resolveDrain;
+          resolveDrain = null;
+          resolve(activeAtSeal);
+        }
+
+        return true;
+      },
+      isOpen() {
+        return open;
+      }
+    });
+  }
+
+  function sealAndDrain() {
+    if (drainPromise) return drainPromise;
+
+    sealed = true;
+    activeAtSeal = activeLeaseCount > 0;
+
+    if (!activeAtSeal) {
+      drainPromise = Promise.resolve(false);
+      return drainPromise;
+    }
+
+    drainPromise = new Promise((resolve) => {
+      resolveDrain = resolve;
+    });
+    return drainPromise;
+  }
+
+  return Object.freeze({ acquire, sealAndDrain });
+}
+
+module.exports = { createBatchMutationGate };