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

src/ha/registry/device/index.ts

@@ -1,4 +1,5 @@
 import { errorToConsole } from '../../../utils/basic';
+import { OnceRunner } from '../../../utils/concurrency/once-runner';
 import type { HomeAssistant } from '../../types';
 import { homeAssistantWSRequest } from '../../ws-request';
 import {
@@ -10,7 +11,7 @@ import {
 
 export class DeviceRegistryManager {
   private _cache: DeviceCache;
-  private _fetchedDeviceList = false;
+  private _deviceListFetch = new OnceRunner();
 
   constructor(cache: DeviceCache) {
     this._cache = cache;
@@ -35,22 +36,21 @@ export class DeviceRegistryManager {
   }
 
   private async _fetchDeviceList(hass: HomeAssistant): Promise<void> {
-    if (this._fetchedDeviceList) {
-      return;
-    }
-
-    let deviceList: DeviceList | null = null;
     try {
-      deviceList = await homeAssistantWSRequest<DeviceList>(hass, deviceListSchema, {
-        type: 'config/device_registry/list',
+      await this._deviceListFetch.run(async () => {
+        const deviceList = await homeAssistantWSRequest<DeviceList>(
+          hass,
+          deviceListSchema,
+          {
+            type: 'config/device_registry/list',
+          },
+        );
+        deviceList.forEach((device) => {
+          this._cache.set(device.id, device);
+        });
       });
     } catch (e) {
       errorToConsole(e);
-      return;
     }
-    deviceList.forEach((device) => {
-      this._cache.set(device.id, device);
-    });
-    this._fetchedDeviceList = true;
   }
 }

src/ha/registry/entity/index.ts

@@ -1,4 +1,5 @@
 import { errorToConsole } from '../../../utils/basic.js';
+import { OnceRunner } from '../../../utils/concurrency/once-runner.js';
 import type { HomeAssistant } from '../../types.js';
 import { homeAssistantWSRequest } from '../../ws-request.js';
 import {
@@ -16,7 +17,7 @@ import {
 
 export class EntityRegistryManagerLive implements EntityRegistryManager {
   private _cache: EntityCache;
-  private _fetchedEntityList = false;
+  private _entityListFetch = new OnceRunner();
 
   constructor(cache: EntityCache) {
     this._cache = cache;
@@ -69,22 +70,21 @@ export class EntityRegistryManagerLive implements EntityRegistryManager {
   }
 
   public async fetchEntityList(hass: HomeAssistant): Promise<void> {
-    if (this._fetchedEntityList) {
-      return;
-    }
-
-    let entityList: EntityList | null = null;
     try {
-      entityList = await homeAssistantWSRequest<EntityList>(hass, entityListSchema, {
-        type: 'config/entity_registry/list',
+      await this._entityListFetch.run(async () => {
+        const entityList = await homeAssistantWSRequest<EntityList>(
+          hass,
+          entityListSchema,
+          {
+            type: 'config/entity_registry/list',
+          },
+        );
+        entityList.forEach((entity) => {
+          this._cache.set(entity.entity_id, entity);
+        });
       });
     } catch (e) {
       errorToConsole(e);
-      return;
     }
-    entityList.forEach((entity) => {
-      this._cache.set(entity.entity_id, entity);
-    });
-    this._fetchedEntityList = true;
   }
 }

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;
+  }
+}