Add an onError handler to getClient options

Signed-off-by: Eric Peterson <ericpeterson@spotify.com>

packages/backend-common/src/cache/CacheManager.test.ts

@@ -86,6 +86,17 @@ describe('CacheManager', () => {
       expect(client).toHaveBeenCalledTimes(1);
     });
 
+    it('attaches error handler to client', () => {
+      const pluginId = 'error-test';
+      const handler = jest.fn();
+      manager.forPlugin(pluginId).getClient({ onError: handler });
+
+      const client = DefaultCacheClient as jest.Mock;
+      const mockCalls = client.mock.calls.splice(-1);
+      const realClient = mockCalls[0][0].client as Keyv;
+      expect(realClient.on).toHaveBeenCalledWith('error', handler);
+    });
+
     it('provides different plugins different cache clients', async () => {
       const plugin1Id = 'test1';
       const plugin2Id = 'test2';

packages/backend-common/src/cache/CacheManager.ts

@@ -73,6 +73,12 @@ export class CacheManager {
     return {
       getClient: (opts = {}): CacheClient => {
         const concreteClient = this.getClientWithTtl(pluginId, opts.defaultTtl);
+
+        // Attach error handler if provided.
+        if (typeof opts?.onError === 'function') {
+          concreteClient.on('error', opts.onError);
+        }
+
         return new DefaultCacheClient({
           client: concreteClient,
         });

packages/backend-common/src/cache/types.ts

@@ -23,6 +23,12 @@ type ClientOptions = {
    * can be configured per entry at set-time).
    */
   defaultTtl?: number;
+
+  /**
+   * An optional handler for connection errors emitted from the underlying data
+   * store.
+   */
+  onError?: (err: Error) => void
 };
 
 /**