fix writeIndex evaluation for aliases (#31562)

AliasOrIndex.Alias#writeIndex was returning a write index when
an alias was pointing to only one index, regardless whether `is_write_index` was
set to `false`. This fixes that so that there is no write index in such a case
that an alias points to only one index with `is_write_index=false`.

Author: talevy

server/src/main/java/org/elasticsearch/cluster/metadata/AliasOrIndex.java

@@ -157,9 +157,13 @@ public void computeAndValidateWriteIndex() {
             List<IndexMetaData> writeIndices = referenceIndexMetaDatas.stream()
                 .filter(idxMeta -> Boolean.TRUE.equals(idxMeta.getAliases().get(aliasName).writeIndex()))
                 .collect(Collectors.toList());
-            if (referenceIndexMetaDatas.size() == 1) {
-                writeIndex.set(referenceIndexMetaDatas.get(0));
-            } else if (writeIndices.size() == 1) {
+
+            if (writeIndices.isEmpty() && referenceIndexMetaDatas.size() == 1
+                    && referenceIndexMetaDatas.get(0).getAliases().get(aliasName).writeIndex() == null) {
+                writeIndices.add(referenceIndexMetaDatas.get(0));
+            }
+
+            if (writeIndices.size() == 1) {
                 writeIndex.set(writeIndices.get(0));
             } else if (writeIndices.size() > 1) {
                 List<String> writeIndicesStrings = writeIndices.stream()

server/src/test/java/org/elasticsearch/cluster/metadata/MetaDataIndexAliasesServiceTests.java

@@ -136,8 +136,7 @@ public void testAddWriteOnlyWithNoExistingAliases() {
         ClusterState after = service.innerExecute(before, Arrays.asList(
             new AliasAction.Add("test", "alias", null, null, null, false)));
         assertFalse(after.metaData().index("test").getAliases().get("alias").writeIndex());
-        assertThat(((AliasOrIndex.Alias) after.metaData().getAliasAndIndexLookup().get("alias")).getWriteIndex(),
-            equalTo(after.metaData().index("test")));
+        assertNull(((AliasOrIndex.Alias) after.metaData().getAliasAndIndexLookup().get("alias")).getWriteIndex());
 
         after = service.innerExecute(before, Arrays.asList(
             new AliasAction.Add("test", "alias", null, null, null, null)));