Fix eager evaluation of fallback routing in resolveRouting

gateway-ha/src/main/java/io/trino/gateway/ha/handler/RoutingTargetHandler.java

@@ -80,7 +80,7 @@ public RoutingTargetResponse resolveRouting(HttpServletRequest request)
             return new RoutingTargetResponse(
                     new RoutingDestination(routingGroup, cluster, buildUriWithNewCluster(cluster, request), externalUrl),
                     request);
-        }).orElse(getRoutingTargetResponse(request));
+        }).orElseGet(() -> getRoutingTargetResponse(request));
 
         logRewrite(routingTargetResponse.routingDestination().clusterHost(), request);
         return routingTargetResponse;

gateway-ha/src/test/java/io/trino/gateway/ha/handler/TestRoutingTargetHandler.java

@@ -29,6 +29,7 @@
 import jakarta.ws.rs.HttpMethod;
 import jakarta.ws.rs.WebApplicationException;
 import org.junit.jupiter.api.BeforeAll;
+import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
 import org.junit.jupiter.api.TestInstance;
 import org.junit.jupiter.api.extension.ExtendWith;
@@ -108,7 +109,6 @@ void setUp()
         config = provideGatewayConfiguration();
         httpClient = Mockito.mock(HttpClient.class);
         routingManager = Mockito.mock(RoutingManager.class);
-        when(routingManager.provideBackendConfiguration(any(), any())).thenReturn(new ProxyBackendConfiguration());
         request = prepareMockRequest();
 
         // Initialize the handler with the configuration
@@ -117,6 +117,14 @@ void setUp()
                 RoutingGroupSelector.byRoutingExternal(httpClient, config.getRoutingRules().getRulesExternalConfiguration(), config.getRequestAnalyzerConfig()), config);
     }
 
+    @BeforeEach
+    void resetMocks()
+    {
+        Mockito.reset(routingManager);
+        when(routingManager.provideBackendConfiguration(any(), any())).thenReturn(new ProxyBackendConfiguration());
+        config.getRoutingRules().getRulesExternalConfiguration().setPropagateErrors(false);
+    }
+
     @Test
     void testBasicHeaderModification()
             throws Exception
@@ -300,6 +308,38 @@ void testPropagateErrorsTrueResponseGroupAndErrorsSet()
                 .isInstanceOf(WebApplicationException.class);
     }
 
+    @Test
+    void testResolveRoutingWithKnownQueryIdAndFailingFallback()
+    {
+        // Simulate a request to /ui/query.html?queryId where the query ID is known
+        // but the fallback routing (getRoutingTargetResponse) would fail because
+        // there are no backends for the resolved routing group.
+        // This tests that the eagerly-evaluated fallback does not throw when
+        // previousCluster is present.
+        String queryId = "20240101_000000_00001_aaaaa";
+        String backendUrl = "https://trino-backend.example.com";
+
+        HttpServletRequest uiRequest = Mockito.mock(HttpServletRequest.class);
+        when(uiRequest.getMethod()).thenReturn(HttpMethod.GET);
+        when(uiRequest.getRequestURI()).thenReturn("/ui/query.html");
+        when(uiRequest.getQueryString()).thenReturn(queryId);
+
+        // Query ID is known — cache returns the backend
+        when(routingManager.findBackendForQueryId(queryId)).thenReturn(backendUrl);
+        when(routingManager.findRoutingGroupForQueryId(queryId)).thenReturn("test-group");
+        when(routingManager.findExternalUrlForQueryId(queryId)).thenReturn(backendUrl);
+
+        // Fallback routing would throw (no backends for the routing group)
+        when(routingManager.provideBackendConfiguration(any(), any()))
+                .thenThrow(new IllegalStateException("Number of active backends found zero"));
+
+        // With orElse(), this throws. With orElseGet(), this succeeds.
+        // ref: https://github.com/trinodb/trino-gateway/issues/920
+        RoutingTargetResponse response = handler.resolveRouting(uiRequest);
+
+        assertThat(response.routingDestination().clusterHost()).isEqualTo(backendUrl);
+    }
+
     private RoutingTargetHandler createHandlerWithPropagateErrorsTrue()
     {
         config.getRoutingRules().getRulesExternalConfiguration().setPropagateErrors(true);