[Query enhancements] use status 503 if search strategy throws 500 (#8876

)

* [Query enhancements] use status 503 if opensearch throws 500

Signed-off-by: Joshua Li <joshuali925@gmail.com>

* update unit tests

Signed-off-by: Joshua Li <joshuali925@gmail.com>

---------

Signed-off-by: Joshua Li <joshuali925@gmail.com>

src/plugins/query_enhancements/server/routes/index.test.ts

@@ -0,0 +1,21 @@
+/*
+ * Copyright OpenSearch Contributors
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import { coerceStatusCode } from '.';
+
+describe('coerceStatusCode', () => {
+  it('should return 503 when input is 500', () => {
+    expect(coerceStatusCode(500)).toBe(503);
+  });
+
+  it('should return the input status code when it is not 500', () => {
+    expect(coerceStatusCode(404)).toBe(404);
+  });
+
+  it('should return 503 when input is undefined or null', () => {
+    expect(coerceStatusCode((undefined as unknown) as number)).toBe(503);
+    expect(coerceStatusCode((null as unknown) as number)).toBe(503);
+  });
+});

src/plugins/query_enhancements/server/routes/index.ts

@@ -16,6 +16,15 @@ import { API } from '../../common';
 import { registerQueryAssistRoutes } from './query_assist';
 import { registerDataSourceConnectionsRoutes } from './data_source_connection';
 
+/**
+ * Coerce status code to 503 for 500 errors from dependency services. Only use
+ * this function to handle errors throw by other services, and not from OSD.
+ */
+export const coerceStatusCode = (statusCode: number) => {
+  if (statusCode === 500) return 503;
+  return statusCode || 503;
+};
+
 /**
  * @experimental
  *
@@ -92,7 +101,7 @@ export function defineSearchStrategyRouteProvider(logger: Logger, router: IRoute
             error = err;
           }
           return res.custom({
-            statusCode: error.status || err.status,
+            statusCode: coerceStatusCode(error.status || err.status),
             body: err.message,
           });
         }