Preserve multi-value request params through PathTrie iteration

server/src/main/java/org/elasticsearch/rest/RequestParams.java

@@ -118,10 +118,31 @@ public static RequestParams fromSingleValues(Map<String, String> singleValues) {
         return new RequestParams(wrapped);
     }
 
+    /**
+     * Returns an independent copy of {@code source}, preserving all multi-values.
+     *
+     * @param source the {@code RequestParams} to copy
+     * @return a new mutable {@code RequestParams} with the same contents as {@code source}
+     */
+    public static RequestParams copyOf(RequestParams source) {
+        LinkedHashMap<String, List<String>> copy = Maps.newLinkedHashMapWithExpectedSize(source.map.size());
+        source.map.forEach((k, v) -> copy.put(k, List.copyOf(v)));
+        return new RequestParams(copy);
+    }
+
     private RequestParams(Map<String, List<String>> map) {
         this.map = map;
     }
 
+    /**
+     * Copies all entries from {@code source} into this map, preserving all multi-values.
+     * Unlike the inherited {@link #putAll(Map)}, which only sees the last value per key,
+     * this overload copies every value in each key's list.
+     */
+    public void putAll(RequestParams source) {
+        source.map.forEach(map::put);
+    }
+
     /**
      * Appends {@code value} to the list of values for {@code key}.
      * If the key is not yet present, a new entry is created.

server/src/main/java/org/elasticsearch/rest/RestController.java

@@ -650,14 +650,14 @@ private static void validateErrorTrace(RestRequest request, RestChannel channel)
         }
     }
 
-    Iterator<MethodHandlers> getAllHandlers(@Nullable Map<String, String> requestParamsRef, String rawPath) {
+    Iterator<MethodHandlers> getAllHandlers(@Nullable RequestParams requestParamsRef, String rawPath) {
         final Supplier<Map<String, String>> paramsSupplier;
         if (requestParamsRef == null) {
             paramsSupplier = () -> null;
         } else {
             // Between retrieving the correct path, we need to reset the parameters,
             // otherwise parameters are parsed out of the URI that aren't actually handled.
-            final Map<String, String> originalParams = Map.copyOf(requestParamsRef);
+            final RequestParams originalParams = RequestParams.copyOf(requestParamsRef);
             paramsSupplier = () -> {
                 // PathTrie modifies the request, so reset the params between each iteration
                 requestParamsRef.clear();

server/src/test/java/org/elasticsearch/rest/RequestParamsTests.java

@@ -178,4 +178,43 @@ public void testFrom() throws Exception {
         assertThat(map.get("b"), equalTo("2"));
         assertThat(RequestParams.from(new URI("http://example.com/path")).isEmpty(), is(true));
     }
+
+    public void testCopyOfPreservesMultiValues() {
+        var original = RequestParams.of(Map.of("format", List.of("json", "yaml"), "size", List.of("10")));
+        var copy = RequestParams.copyOf(original);
+        assertThat(copy.getAll("format"), equalTo(List.of("json", "yaml")));
+        assertThat(copy.get("format"), equalTo("yaml"));
+        assertThat(copy.getAll("size"), equalTo(List.of("10")));
+        assertThat(copy.size(), equalTo(2));
+    }
+
+    public void testCopyOfIsIndependentOfSource() {
+        var original = RequestParams.of(Map.of("format", List.of("json", "yaml")));
+        var copy = RequestParams.copyOf(original);
+        original.clear();
+        assertThat(copy.getAll("format"), equalTo(List.of("json", "yaml")));
+        assertThat(original.isEmpty(), is(true));
+    }
+
+    public void testCopyOfSourceDoesNotReflectMutationsToTheCopy() {
+        var original = RequestParams.of(Map.of("format", List.of("json", "yaml")));
+        var copy = RequestParams.copyOf(original);
+        copy.put("format", "cbor");
+        assertThat(original.getAll("format"), equalTo(List.of("json", "yaml")));
+    }
+
+    public void testPutAllRequestParamsPreservesMultiValues() {
+        var source = RequestParams.of(Map.of("format", List.of("json", "yaml")));
+        var target = RequestParams.empty();
+        target.putAll(source);
+        assertThat(target.getAll("format"), equalTo(List.of("json", "yaml")));
+    }
+
+    public void testPutAllRequestParamsMergesIntoExistingEntries() {
+        var source = RequestParams.of(Map.of("format", List.of("json", "yaml")));
+        var target = RequestParams.of(Map.of("size", List.of("10")));
+        target.putAll(source);
+        assertThat(target.getAll("format"), equalTo(List.of("json", "yaml")));
+        assertThat(target.getAll("size"), equalTo(List.of("10")));
+    }
 }

server/src/test/java/org/elasticsearch/rest/RestControllerTests.java

@@ -1188,6 +1188,19 @@ public void testApiProtectionWithServerlessEnabledAsEndUser() {
         checkUnprotected.accept(inaccessiblePaths);
     }
 
+    public void testGetAllHandlersPreservesMultiValueParams() {
+        restController.registerHandler(new Route(GET, "/{index}"), (request, channel, client) -> {});
+
+        var params = RequestParams.of(Map.of("format", List.of("json", "yaml")));
+        var it = restController.getAllHandlers(params, "/my-index");
+        while (it.hasNext()) {
+            it.next();
+        }
+
+        // Multi-values must survive the per-iteration reset that PathTrie triggers
+        assertThat(params.getAll("format"), equalTo(List.of("json", "yaml")));
+    }
+
     @ServerlessScope(Scope.PUBLIC)
     private static final class PublicRestHandler extends BaseRestHandler {
         @Override

x-pack/plugin/prometheus/src/main/java/org/elasticsearch/xpack/prometheus/rest/PrometheusLabelValuesRestAction.java

@@ -9,7 +9,6 @@
 
 import org.elasticsearch.client.internal.node.NodeClient;
 import org.elasticsearch.rest.BaseRestHandler;
-import org.elasticsearch.rest.RequestParams;
 import org.elasticsearch.rest.RestRequest;
 import org.elasticsearch.rest.Scope;
 import org.elasticsearch.rest.ServerlessScope;
@@ -74,10 +73,7 @@ protected RestChannelConsumer prepareRequest(RestRequest request, NodeClient cli
         String labelName = PrometheusLabelNameUtils.decodeLabelName(rawName);
         String index = request.param(INDEX_PARAM, "*");
 
-        // Consume the parameter; re-parse from the raw URI to handle repeated match[] params,
-        // since request processing currently keeps only the last value for repeated parameters.
-        request.repeatedParamAsList(MATCH_PARAM);
-        List<String> matchSelectors = RequestParams.fromUri(request.uri()).getAll(MATCH_PARAM);
+        List<String> matchSelectors = request.repeatedParamAsList(MATCH_PARAM);
 
         // Time range
         String endParam = request.param(END_PARAM);

x-pack/plugin/prometheus/src/main/java/org/elasticsearch/xpack/prometheus/rest/PrometheusLabelsRestAction.java

@@ -9,7 +9,6 @@
 
 import org.elasticsearch.client.internal.node.NodeClient;
 import org.elasticsearch.rest.BaseRestHandler;
-import org.elasticsearch.rest.RequestParams;
 import org.elasticsearch.rest.RestRequest;
 import org.elasticsearch.rest.Scope;
 import org.elasticsearch.rest.ServerlessScope;
@@ -60,10 +59,7 @@ public List<Route> routes() {
 
     @Override
     protected RestChannelConsumer prepareRequest(RestRequest request, NodeClient client) throws IOException {
-        // Consume the parameter; re-parse from the raw URI to handle repeated match[] params,
-        // since request processing currently keeps only the last value for repeated parameters.
-        request.repeatedParamAsList(MATCH_PARAM);
-        List<String> matchSelectors = RequestParams.fromUri(request.uri()).getAll(MATCH_PARAM);
+        List<String> matchSelectors = request.repeatedParamAsList(MATCH_PARAM);
 
         // Time range
         String endParam = request.param(END_PARAM);

x-pack/plugin/prometheus/src/main/java/org/elasticsearch/xpack/prometheus/rest/PrometheusSeriesRestAction.java

@@ -9,7 +9,6 @@
 
 import org.elasticsearch.client.internal.node.NodeClient;
 import org.elasticsearch.rest.BaseRestHandler;
-import org.elasticsearch.rest.RequestParams;
 import org.elasticsearch.rest.RestRequest;
 import org.elasticsearch.rest.Scope;
 import org.elasticsearch.rest.ServerlessScope;
@@ -58,10 +57,7 @@ public List<Route> routes() {
 
     @Override
     protected RestChannelConsumer prepareRequest(RestRequest request, NodeClient client) throws IOException {
-        // Consume the parameter; re-parse from the raw URI to handle repeated match[] params,
-        // since request processing currently keeps only the last value for repeated parameters.
-        request.repeatedParamAsList(MATCH_PARAM);
-        List<String> matchSelectors = RequestParams.fromUri(request.uri()).getAll(MATCH_PARAM);
+        List<String> matchSelectors = request.repeatedParamAsList(MATCH_PARAM);
         if (matchSelectors.isEmpty()) {
             throw new IllegalArgumentException("At least one [match[]] selector is required");
         }