add some comments and unit test

internal/controller/nginx/config/maps.go

@@ -185,14 +185,18 @@ func createAddHeadersMap(name string) shared.Map {
 
 // buildInferenceMaps creates maps for InferencePool Backends.
 func buildInferenceMaps(groups []dataplane.BackendGroup) []shared.Map {
-	inferenceMaps := make([]shared.Map, 0)
+	inferenceMaps := make([]shared.Map, 0, len(groups))
 	for _, group := range groups {
 		for _, backend := range group.Backends {
 			if backend.EndpointPickerConfig != nil {
 				var defaultResult string
 				switch backend.EndpointPickerConfig.FailureMode {
+				// in FailClose mode, if the EPP is unavailable or returns an error,
+				// we return an invalid backend to ensure the request fails
 				case inference.EndpointPickerFailClose:
 					defaultResult = invalidBackendRef
+				// in FailOpen mode, if the EPP is unavailable or returns an error,
+				// we fall back to the upstream
 				case inference.EndpointPickerFailOpen:
 					defaultResult = backend.UpstreamName
 				}

internal/controller/nginx/modules/test/epp.test.js

@@ -1,12 +1,19 @@
 import { default as epp } from '../src/epp.js';
 import { expect, describe, it, beforeEach, afterEach, vi } from 'vitest';
 
-function makeRequest({ method = 'POST', headersIn = {}, requestText = '', variables = {} } = {}) {
+function makeRequest({
+	method = 'POST',
+	headersIn = {},
+	args = {},
+	requestText = '',
+	variables = {},
+} = {}) {
 	return {
 		method,
 		headersIn,
 		requestText,
 		variables,
+		args,
 		error: vi.fn(),
 		log: vi.fn(),
 		internalRedirect: vi.fn(),
@@ -79,4 +86,21 @@ describe('getEndpoint', () => {
 		expect(r.error).toHaveBeenCalledWith(expect.stringContaining('Error in ngx.fetch'));
 		expect(r.internalRedirect).toHaveBeenCalledWith('/foo');
 	});
+
+	it('preserves args in internal redirect when args are present', async () => {
+		const endpoint = 'http://endpoint';
+		globalThis.ngx = {
+			fetch: vi.fn().mockResolvedValue({
+				status: 200,
+				headers: { get: () => endpoint },
+				text: vi.fn(),
+			}),
+		};
+		const r = makeRequest({
+			variables: { epp_host: 'host', epp_port: '1234', epp_internal_path: '/foo' },
+			args: { a: '1', b: '2' },
+		});
+		await epp.getEndpoint(r);
+		expect(r.internalRedirect).toHaveBeenCalledWith('/foo?a=1&b=2');
+	});
 });