GET aliases should 404 if aliases are missing

Previously the HEAD and GET aliases endpoints were misaigned in
behavior. The HEAD verb would 404 if any aliases are missing while the
GET verb would not if any aliases existed. When HEAD was aligned with
GET, this broke the previous usage of HEAD to serve as an existence
check for aliases. It is the behavior of GET that is problematic here
though, if any alias is missing the request should 404. This commit
addresses this by modifying the behavior of GET to behave in this
way. This fixes the behavior for HEAD to also 404 when aliases are
missing.

Relates #25043

core/src/main/java/org/elasticsearch/common/util/set/Sets.java

@@ -21,11 +21,19 @@ package org.elasticsearch.common.util.set;
 
 import java.util.Collection;
 import java.util.Collections;
+import java.util.EnumSet;
 import java.util.HashSet;
 import java.util.Iterator;
 import java.util.Objects;
 import java.util.Set;
+import java.util.SortedSet;
+import java.util.TreeSet;
 import java.util.concurrent.ConcurrentHashMap;
+import java.util.function.BiConsumer;
+import java.util.function.BinaryOperator;
+import java.util.function.Function;
+import java.util.function.Supplier;
+import java.util.stream.Collector;
 import java.util.stream.Collectors;
 
 public final class Sets {
@@ -69,6 +77,47 @@ public final class Sets {
         return left.stream().filter(k -> !right.contains(k)).collect(Collectors.toSet());
     }
 
+    public static <T> SortedSet<T> sortedDifference(Set<T> left, Set<T> right) {
+        Objects.requireNonNull(left);
+        Objects.requireNonNull(right);
+        return left.stream().filter(k -> !right.contains(k)).collect(new SortedSetCollector<>());
+    }
+
+    private static class SortedSetCollector<T> implements Collector<T, SortedSet<T>, SortedSet<T>> {
+
+        @Override
+        public Supplier<SortedSet<T>> supplier() {
+            return TreeSet::new;
+        }
+
+        @Override
+        public BiConsumer<SortedSet<T>, T> accumulator() {
+            return (s, e) -> s.add(e);
+        }
+
+        @Override
+        public BinaryOperator<SortedSet<T>> combiner() {
+            return (s, t) -> {
+                s.addAll(t);
+                return s;
+            };
+        }
+
+        @Override
+        public Function<SortedSet<T>, SortedSet<T>> finisher() {
+            return Function.identity();
+        }
+
+        static final Set<Characteristics> CHARACTERISTICS =
+                Collections.unmodifiableSet(EnumSet.of(Collector.Characteristics.IDENTITY_FINISH));
+
+        @Override
+        public Set<Characteristics> characteristics() {
+            return CHARACTERISTICS;
+        }
+
+    }
+
     public static <T> Set<T> union(Set<T> left, Set<T> right) {
         Objects.requireNonNull(left);
         Objects.requireNonNull(right);

core/src/main/java/org/elasticsearch/rest/action/admin/indices/RestGetAliasesAction.java

@@ -19,15 +19,18 @@
 
 package org.elasticsearch.rest.action.admin.indices;
 
+import com.carrotsearch.hppc.cursors.ObjectCursor;
 import com.carrotsearch.hppc.cursors.ObjectObjectCursor;
-
 import org.elasticsearch.action.admin.indices.alias.get.GetAliasesRequest;
 import org.elasticsearch.action.admin.indices.alias.get.GetAliasesResponse;
 import org.elasticsearch.action.support.IndicesOptions;
 import org.elasticsearch.client.node.NodeClient;
 import org.elasticsearch.cluster.metadata.AliasMetaData;
 import org.elasticsearch.common.Strings;
+import org.elasticsearch.common.collect.ImmutableOpenMap;
+import org.elasticsearch.common.regex.Regex;
 import org.elasticsearch.common.settings.Settings;
+import org.elasticsearch.common.util.set.Sets;
 import org.elasticsearch.common.xcontent.ToXContent;
 import org.elasticsearch.common.xcontent.XContentBuilder;
 import org.elasticsearch.rest.BaseRestHandler;
@@ -39,14 +42,17 @@ import org.elasticsearch.rest.RestStatus;
 import org.elasticsearch.rest.action.RestBuilderListener;
 
 import java.io.IOException;
+import java.util.ArrayList;
 import java.util.Arrays;
+import java.util.HashSet;
 import java.util.List;
 import java.util.Locale;
+import java.util.Set;
+import java.util.SortedSet;
 import java.util.stream.Collectors;
 
 import static org.elasticsearch.rest.RestRequest.Method.GET;
 import static org.elasticsearch.rest.RestRequest.Method.HEAD;
-import static org.elasticsearch.rest.RestStatus.OK;
 
 /**
  * The REST handler for get alias and head alias APIs.
@@ -80,41 +86,68 @@ public class RestGetAliasesAction extends BaseRestHandler {
         return channel -> client.admin().indices().getAliases(getAliasesRequest, new RestBuilderListener<GetAliasesResponse>(channel) {
             @Override
             public RestResponse buildResponse(GetAliasesResponse response, XContentBuilder builder) throws Exception {
-                if (response.getAliases().isEmpty()) {
-                    // empty body if indices were specified but no matching aliases exist
-                    if (indices.length > 0) {
-                        return new BytesRestResponse(OK, builder.startObject().endObject());
+                final ImmutableOpenMap<String, List<AliasMetaData>> aliasMap = response.getAliases();
+
+                final Set<String> aliasNames = new HashSet<>();
+                for (final ObjectCursor<List<AliasMetaData>> cursor : aliasMap.values()) {
+                    for (final AliasMetaData aliasMetaData : cursor.value) {
+                        aliasNames.add(aliasMetaData.alias());
+                    }
+                }
+
+                // first remove requested aliases that are exact matches
+                final SortedSet<String> difference = Sets.sortedDifference(Arrays.stream(aliases).collect(Collectors.toSet()), aliasNames);
+
+                // now remove requested aliases that contain wildcards that are simple matches
+                final List<String> matches = new ArrayList<>();
+                outer:
+                for (final String pattern : difference) {
+                    if (pattern.contains("*")) {
+                        for (final String aliasName : aliasNames) {
+                            if (Regex.simpleMatch(pattern, aliasName)) {
+                                matches.add(pattern);
+                                continue outer;
+                            }
+                        }
+                    }
+                }
+                difference.removeAll(matches);
+
+                final RestStatus status;
+                builder.startObject();
+                {
+                    if (difference.isEmpty()) {
+                        status = RestStatus.OK;
                     } else {
-                        final String message = String.format(Locale.ROOT, "alias [%s] missing", toNamesString(getAliasesRequest.aliases()));
-                        builder.startObject();
-                        {
-                            builder.field("error", message);
-                            builder.field("status", RestStatus.NOT_FOUND.getStatus());
+                        status = RestStatus.NOT_FOUND;
+                        final String message;
+                        if (difference.size() == 1) {
+                            message = String.format(Locale.ROOT, "alias [%s] missing", toNamesString(difference.iterator().next()));
+                        } else {
+                            message = String.format(Locale.ROOT, "aliases [%s] missing", toNamesString(difference.toArray(new String[0])));
                         }
-                        builder.endObject();
-                        return new BytesRestResponse(RestStatus.NOT_FOUND, builder);
+                        builder.field("error", message);
+                        builder.field("status", status.getStatus());
                     }
-                } else {
-                    builder.startObject();
-                    {
-                        for (final ObjectObjectCursor<String, List<AliasMetaData>> entry : response.getAliases()) {
-                            builder.startObject(entry.key);
+
+                    for (final ObjectObjectCursor<String, List<AliasMetaData>> entry : response.getAliases()) {
+                        builder.startObject(entry.key);
+                        {
+                            builder.startObject("aliases");
                             {
-                                builder.startObject("aliases");
-                                {
-                                    for (final AliasMetaData alias : entry.value) {
-                                        AliasMetaData.Builder.toXContent(alias, builder, ToXContent.EMPTY_PARAMS);
-                                    }
+                                for (final AliasMetaData alias : entry.value) {
+                                    AliasMetaData.Builder.toXContent(alias, builder, ToXContent.EMPTY_PARAMS);
                                 }
-                                builder.endObject();
                             }
                             builder.endObject();
                         }
+                        builder.endObject();
                     }
-                    builder.endObject();
-                    return new BytesRestResponse(OK, builder);
                 }
+                builder.endObject();
+                return new BytesRestResponse(status, builder);
             }
+
         });
     }
 

modules/transport-netty4/src/test/java/org/elasticsearch/rest/Netty4HeadBodyIsEmptyIT.java

@@ -108,6 +108,12 @@ public class Netty4HeadBodyIsEmptyIT extends ESRestTestCase {
         }
     }
 
+    public void testAliasDoesNotExist() throws IOException {
+        createTestDoc();
+        headTestCase("/_alias/test_alias", emptyMap(), NOT_FOUND.getStatus(), greaterThan(0));
+        headTestCase("/test/_alias/test_alias", emptyMap(), NOT_FOUND.getStatus(), greaterThan(0));
+    }
+
     public void testTemplateExists() throws IOException {
         try (XContentBuilder builder = jsonBuilder()) {
             builder.startObject();

rest-api-spec/src/main/resources/rest-api-spec/test/indices.delete_alias/10_basic.yml

@@ -1,5 +1,8 @@
 ---
 "Basic test for delete alias":
+  - skip:
+      version: " - 5.99.99"
+      reason: Previous versions did not 404 on missing aliases
 
   - do:
       indices.create:
@@ -25,8 +28,10 @@
         name: testali
 
   - do:
+      catch: missing
       indices.get_alias:
         index: testind
         name:  testali
   
-  - match: { '': {}}
+  - match: { 'status': 404 }
+  - match: { 'error': 'alias [testali] missing' }

rest-api-spec/src/main/resources/rest-api-spec/test/indices.get_alias/10_basic.yml

@@ -78,7 +78,6 @@ setup:
 
 ---
 "Get aliases via /{index}/_alias/prefix*":
-
   - do:
       indices.get_alias:
         index: test_index
@@ -166,25 +165,51 @@ setup:
 
 
 ---
-"Non-existent alias on an existing index returns an empty body":
+"Non-existent alias on an existing index returns 404":
+  - skip:
+      version: " - 5.99.99"
+      reason: Previous versions did not 404 on missing aliases
 
   - do:
+      catch: missing
       indices.get_alias:
         index: test_index
         name: non-existent
 
-  - match: { '': {}}
+  - match: { 'status': 404}
+  - match: { 'error': 'alias [non-existent] missing' }
 
 ---
-"Existent and non-existent alias returns just the existing":
+"Existent and non-existent alias returns 404 and the existing alias":
+  - skip:
+      version: " - 5.99.99"
+      reason: Previous versions did not 404 on missing aliases
 
   - do:
+      catch: missing
       indices.get_alias:
         index: test_index
         name: test_alias,non-existent
 
-  - match: {test_index.aliases.test_alias: {}}
-  - is_false: test_index.aliases.non-existent
+  - match: { 'status': 404 }
+  - match: { 'error': 'alias [non-existent] missing' }
+  - match: { test_index.aliases.test_alias: { } }
+
+---
+"Existent and non-existent aliases returns 404 and the existing alias":
+  - skip:
+      version: " - 5.99.99"
+      reason: Previous versions did not 404 on missing aliases
+
+  - do:
+      catch: missing
+      indices.get_alias:
+        index: test_index
+        name: test_alias,non-existent,another-non-existent
+
+  - match: { 'status': 404 }
+  - match: { 'error': 'aliases [another-non-existent,non-existent] missing' }
+  - match: { test_index.aliases.test_alias: { } }
 
 ---
 "Getting alias on an non-existent index should return 404":