perf: Deduplicate concurrent Home Assistant registry list fetches (#2650)

src/utils/concurrency/once-runner.ts

@@ -0,0 +1,45 @@
+export type Work = () => Promise<void>;
+
+/**
+ * Runs asynchronous work at most once, and shares it while it is in flight.
+ *
+ * The first call starts the work. Callers that arrive while it is still running
+ * wait on that same run rather than starting their own, so ten concurrent
+ * callers make one request instead of ten identical ones. Once the work has
+ * succeeded, later calls return immediately without running it again.
+ *
+ * A failure is *not* remembered: every caller waiting on the failed run sees
+ * the rejection, and the next call starts a fresh attempt.
+ *
+ * Only the first caller's `work` ever runs. Callers that join an in-flight run
+ * have their own `work` discarded, so every caller must pass work that is
+ * interchangeable with the others'.
+ */
+export class OnceRunner {
+  private _succeeded = false;
+  private _inFlight: Promise<void> | null = null;
+
+  public async run(work: Work): Promise<void> {
+    if (this._succeeded) {
+      return;
+    }
+    this._inFlight ??= this._runOnce(work);
+
+    const inFlight = this._inFlight;
+    try {
+      await inFlight;
+    } finally {
+      // Clear only the run this caller waited on. A caller that joined the same
+      // failed run may resume after a later caller has already started a fresh
+      // one, which must not be discarded.
+      if (this._inFlight === inFlight) {
+        this._inFlight = null;
+      }
+    }
+  }
+
+  private async _runOnce(work: Work): Promise<void> {
+    await work();
+    this._succeeded = true;
+  }
+}