[Transform] Return 408 when start api times out (#89774)

This changes the response status code returned when the
start api times out from `500` to `408`.

docs/changelog/89774.yaml

@@ -0,0 +1,5 @@
+pr: 89774
+summary: Return 408 instead of 500 when start api times out
+area: Transform
+type: bug
+issues: []

x-pack/plugin/transform/qa/multi-node-tests/src/javaRestTest/java/org/elasticsearch/xpack/transform/integration/TransformIT.java

@@ -13,9 +13,11 @@ import org.elasticsearch.Version;
 import org.elasticsearch.client.Request;
 import org.elasticsearch.client.RequestOptions;
 import org.elasticsearch.client.Response;
+import org.elasticsearch.client.ResponseException;
 import org.elasticsearch.common.Strings;
 import org.elasticsearch.common.xcontent.support.XContentMapValues;
 import org.elasticsearch.core.TimeValue;
+import org.elasticsearch.rest.RestStatus;
 import org.elasticsearch.search.aggregations.AggregationBuilders;
 import org.elasticsearch.search.aggregations.AggregatorFactories;
 import org.elasticsearch.search.aggregations.bucket.histogram.DateHistogramInterval;
@@ -448,6 +450,35 @@ public class TransformIT extends TransformRestTestCase {
         deleteTransform(config.getId());
     }
 
+    public void testStartTransform_GivenTimeout_Returns408() throws Exception {
+        String indexName = "start-transform-timeout-index";
+        String transformId = "start-transform-timeout";
+        createReviewsIndex(indexName, 10, NUM_USERS, TransformIT::getUserIdForRow, TransformIT::getDateStringForRow);
+
+        Map<String, SingleGroupSource> groups = new HashMap<>();
+        groups.put("by-day", createDateHistogramGroupSourceWithCalendarInterval("timestamp", DateHistogramInterval.DAY, null));
+        groups.put("by-user", new TermsGroupSource("user_id", null, false));
+        groups.put("by-business", new TermsGroupSource("business_id", null, false));
+
+        AggregatorFactories.Builder aggs = AggregatorFactories.builder()
+            .addAggregator(AggregationBuilders.avg("review_score").field("stars"))
+            .addAggregator(AggregationBuilders.max("timestamp").field("timestamp"));
+
+        TransformConfig config = createTransformConfigBuilder(transformId, indexName + "-dest", QueryConfig.matchAll(), indexName)
+            .setPivotConfig(createPivotConfig(groups, aggs))
+            .setSyncConfig(new TimeSyncConfig("timestamp", TimeValue.timeValueSeconds(1)))
+            .setSettings(new SettingsConfig.Builder().setAlignCheckpoints(false).build())
+            .build();
+
+        putTransform(transformId, Strings.toString(config), RequestOptions.DEFAULT);
+        ResponseException e = expectThrows(
+            ResponseException.class,
+            () -> startTransform(config.getId(), RequestOptions.DEFAULT.toBuilder().addParameter("timeout", "1nanos").build())
+        );
+
+        assertThat(e.getResponse().getStatusLine().getStatusCode(), equalTo(RestStatus.REQUEST_TIMEOUT.getStatus()));
+    }
+
     private void indexMoreDocs(long timestamp, long userId, String index) throws Exception {
         StringBuilder bulkBuilder = new StringBuilder();
         for (int i = 0; i < 25; i++) {

x-pack/plugin/transform/src/main/java/org/elasticsearch/xpack/transform/action/TransportStartTransformAction.java

@@ -9,7 +9,6 @@ package org.elasticsearch.xpack.transform.action;
 
 import org.apache.logging.log4j.LogManager;
 import org.apache.logging.log4j.Logger;
-import org.elasticsearch.ElasticsearchException;
 import org.elasticsearch.ElasticsearchStatusException;
 import org.elasticsearch.action.ActionListener;
 import org.elasticsearch.action.admin.indices.stats.IndicesStatsResponse;
@@ -328,7 +327,12 @@ public class TransportStartTransformAction extends TransportMasterNodeAction<Sta
                 @Override
                 public void onTimeout(TimeValue timeout) {
                     listener.onFailure(
-                        new ElasticsearchException("Starting transform [" + params.getId() + "] timed out after [" + timeout + "]")
+                        new ElasticsearchStatusException(
+                            "Starting transform [{}] timed out after [{}]",
+                            RestStatus.REQUEST_TIMEOUT,
+                            params.getId(),
+                            timeout
+                        )
                     );
                 }
             }