Fail index creation using custom data path

Custom per-index data paths were deprecated in 7.14. This commit blocks
creation of indices in 8.0 from configuring custom data paths.

relates elastic#73168

Author: rjernst

docs/reference/migration/migrate_8_0/indices.asciidoc

@@ -116,4 +116,18 @@ index setting].
 Accept the new behaviour, or specify `?wait_for_active_shards=0` to preserve
 the old behaviour if needed.
 ====
+
+.The setting `index.data_path` is not longer allowed on new indices.
+[%collapsible]
+====
+*Details* +
+Creating an index relative to the node level `path.shared_data` setting was
+previously used with shadow replicas prior to their removal in 6.0. The
+per-index data path in `index.data_path` was deprecated in 7.14.0. In 8.0,
+creating new indices with `index.data_path` is no longer supported.
+
+*Impact* +
+Discontinue use of the `index.data_path` setting. Creating new indices with
+this setting will return an error.
+====
 //end::notable-breaking-changes[]

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

@@ -48,7 +48,6 @@
 import org.elasticsearch.common.xcontent.NamedXContentRegistry;
 import org.elasticsearch.common.xcontent.XContentHelper;
 import org.elasticsearch.core.Nullable;
-import org.elasticsearch.core.PathUtils;
 import org.elasticsearch.env.Environment;
 import org.elasticsearch.index.Index;
 import org.elasticsearch.index.IndexModule;
@@ -69,7 +68,6 @@
 
 import java.io.IOException;
 import java.io.UnsupportedEncodingException;
-import java.nio.file.Path;
 import java.time.Instant;
 import java.util.ArrayList;
 import java.util.Arrays;
@@ -1047,7 +1045,7 @@ public void validateIndexSettings(String indexName, final Settings settings, fin
     }
 
     List<String> getIndexSettingsValidationErrors(final Settings settings, final boolean forbidPrivateIndexSettings) {
-        List<String> validationErrors = validateIndexCustomPath(settings, env.sharedDataFile());
+        List<String> validationErrors = validateNoCustomPath(settings);
         if (forbidPrivateIndexSettings) {
             validationErrors.addAll(validatePrivateSettingsNotExplicitlySet(settings, indexScopedSettings));
         }
@@ -1068,27 +1066,17 @@ private static List<String> validatePrivateSettingsNotExplicitlySet(Settings set
     }
 
     /**
-     * Validates that the configured index data path (if any) is a sub-path of the configured shared data path (if any)
+     * Validates an index data path is not specified.
      *
      * @param settings the index configured settings
-     * @param sharedDataPath the configured `path.shared_data` (if any)
-     * @return a list containing validaton errors or an empty list if there aren't any errors
+     * @return a list containing validation errors or an empty list if there aren't any errors
      */
-    private static List<String> validateIndexCustomPath(Settings settings, @Nullable Path sharedDataPath) {
-        String customPath = IndexMetadata.INDEX_DATA_PATH_SETTING.get(settings);
-        List<String> validationErrors = new ArrayList<>();
-        if (Strings.isEmpty(customPath) == false) {
-            if (sharedDataPath == null) {
-                validationErrors.add("path.shared_data must be set in order to use custom data paths");
-            } else {
-                Path resolvedPath = PathUtils.get(new Path[]{sharedDataPath}, customPath);
-                if (resolvedPath == null) {
-                    validationErrors.add("custom path [" + customPath +
-                        "] is not a sub-path of path.shared_data [" + sharedDataPath + "]");
-                }
-            }
+    static List<String> validateNoCustomPath(Settings settings) {
+        if (IndexMetadata.INDEX_DATA_PATH_SETTING.exists(settings)) {
+            return List.of("per-index custom data path using setting ["
+                + IndexMetadata.INDEX_DATA_PATH_SETTING.getKey() + "] is no longer supported on new indices");
         }
-        return validationErrors;
+        return List.of();
     }
 
     /**

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

@@ -94,6 +94,9 @@
 
 import static org.elasticsearch.index.IndexSettings.INDEX_SOFT_DELETES_SETTING;
 import static org.elasticsearch.indices.ShardLimitValidatorTests.createTestShardLimitService;
+import static org.hamcrest.Matchers.contains;
+import static org.hamcrest.Matchers.containsString;
+import static org.hamcrest.Matchers.empty;
 import static org.hamcrest.Matchers.endsWith;
 import static org.hamcrest.Matchers.equalTo;
 import static org.hamcrest.Matchers.hasKey;
@@ -281,6 +284,15 @@ public void testValidateSplitIndex() {
             Settings.builder().put("index.number_of_shards", targetShards).build());
     }
 
+    public void testValidateNoCustomPath() {
+        Settings indexSettings = Settings.builder().put(IndexMetadata.INDEX_DATA_PATH_SETTING.getKey(), "some/path").build();
+        List<String> validationErrors = MetadataCreateIndexService.validateNoCustomPath(indexSettings);
+        assertThat(validationErrors, contains(containsString("per-index custom data path")));
+
+        List<String> noErrors = MetadataCreateIndexService.validateNoCustomPath(Settings.EMPTY);
+        assertThat(noErrors, empty());
+    }
+
     public void testPrepareResizeIndexSettings() {
         final List<Version> versions = Arrays.asList(VersionUtils.randomVersion(random()), VersionUtils.randomVersion(random()));
         versions.sort(Comparator.comparingLong(l -> l.id));