Legacy index version to target indices created before N-2 (#118443)

Legacy index versions can only be read via the archive indices functionality.
The supported versions are currently 5.x and 6.x. For 9.0, we are not going
to add support for indices created in 7.x as archive indices. Rather we will
allow reading from N - 2 directly from Lucene, without relying on archive indices.

IndexVersion#isLegacyIndexVersion is tied to the archive indices functionality
and identifies index versions that can only be present in the cluster as archive.

This commit aligns isLegacyIndexVersion to only match versions before N - 2.

Author: javanna

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

@@ -1116,10 +1116,10 @@ public IndexVersion getCreationVersion() {
 
     /**
      * Return the {@link IndexVersion} that this index provides compatibility for.
-     * This is typically compared to the {@link IndexVersions#MINIMUM_COMPATIBLE} to figure out whether the index can be handled
-     * by the cluster.
-     * By default, this is equal to the {@link #getCreationVersion()}, but can also be a newer version if the index has been imported as
-     * a legacy index from an older snapshot, and its metadata has been converted to be handled by newer version nodes.
+     * This is typically compared to the {@link IndexVersions#MINIMUM_COMPATIBLE} or {@link IndexVersions#MINIMUM_READONLY_COMPATIBLE}
+     * to figure out whether the index can be handled by the cluster.
+     * By default, this is equal to the {@link #getCreationVersion()}, but can also be a newer version if the index has been created by
+     * a legacy version, and imported archive, in which case its metadata has been converted to be handled by newer version nodes.
      */
     public IndexVersion getCompatibilityVersion() {
         return indexCompatibilityVersion;

server/src/main/java/org/elasticsearch/env/NodeEnvironment.java

@@ -524,8 +524,7 @@ static void checkForIndexCompatibility(Logger logger, DataPath... dataPaths) thr
 
         logger.info("oldest index version recorded in NodeMetadata {}", metadata.oldestIndexVersion());
 
-        if (metadata.oldestIndexVersion().isLegacyIndexVersion()) {
-
+        if (metadata.oldestIndexVersion().before(IndexVersions.MINIMUM_COMPATIBLE)) {
             String bestDowngradeVersion = getBestDowngradeVersion(metadata.previousNodeVersion().toString());
             throw new IllegalStateException(
                 "Cannot start this node because it holds metadata for indices with version ["

server/src/main/java/org/elasticsearch/index/IndexVersion.java

@@ -123,8 +123,13 @@ public static IndexVersion current() {
         return CurrentHolder.CURRENT;
     }
 
+    /**
+     * Returns whether this index version is supported by this node version out-of-the-box.
+     * This is used to distinguish between ordinary indices and archive indices that may be
+     * imported into the cluster in read-only mode, and with limited functionality.
+     */
     public boolean isLegacyIndexVersion() {
-        return before(IndexVersions.MINIMUM_COMPATIBLE);
+        return before(IndexVersions.MINIMUM_READONLY_COMPATIBLE);
     }
 
     public static IndexVersion getMinimumCompatibleIndexVersion(int versionId) {

server/src/main/java/org/elasticsearch/index/IndexVersions.java

@@ -192,6 +192,7 @@ private static Version parseUnchecked(String version) {
      */
 
     public static final IndexVersion MINIMUM_COMPATIBLE = V_8_0_0;
+    public static final IndexVersion MINIMUM_READONLY_COMPATIBLE = V_7_0_0;
 
     static final NavigableMap<Integer, IndexVersion> VERSION_IDS = getAllVersionIds(IndexVersions.class);
     static final IndexVersion LATEST_DEFINED;
@@ -207,7 +208,7 @@ static NavigableMap<Integer, IndexVersion> getAllVersionIds(Class<?> cls) {
         Map<Integer, String> versionIdFields = new HashMap<>();
         NavigableMap<Integer, IndexVersion> builder = new TreeMap<>();
 
-        Set<String> ignore = Set.of("ZERO", "MINIMUM_COMPATIBLE");
+        Set<String> ignore = Set.of("ZERO", "MINIMUM_COMPATIBLE", "MINIMUM_READONLY_COMPATIBLE");
 
         for (Field declaredField : cls.getFields()) {
             if (declaredField.getType().equals(IndexVersion.class)) {

server/src/main/java/org/elasticsearch/repositories/RepositoriesModule.java

@@ -102,7 +102,9 @@ public RepositoriesModule(
         }
         if (preRestoreChecks.isEmpty()) {
             preRestoreChecks.add((snapshot, version) -> {
-                if (version.isLegacyIndexVersion()) {
+                // pre-restore checks will be run against the version in which the snapshot was created as well as
+                // the version in which the restored index was created
+                if (version.before(IndexVersions.MINIMUM_COMPATIBLE)) {
                     throw new SnapshotRestoreException(
                         snapshot,
                         "the snapshot was created with Elasticsearch version ["

x-pack/plugin/old-lucene-versions/src/internalClusterTest/java/org/elasticsearch/xpack/lucene/bwc/AbstractArchiveTestCase.java

@@ -94,11 +94,7 @@ public IndexMetadata getSnapshotIndexMetaData(RepositoryData repositoryData, Sna
                             .put(
                                 IndexMetadata.SETTING_INDEX_VERSION_CREATED.getKey(),
                                 metadata.settings()
-                                    .getAsVersionId(
-                                        "version",
-                                        IndexVersion::fromId,
-                                        IndexVersion.fromId(randomFrom(5000099, 6000099, 7000099))
-                                    )
+                                    .getAsVersionId("version", IndexVersion::fromId, IndexVersion.fromId(randomFrom(5000099, 6000099)))
                             )
                     )
                     .build();