[8.x] Logsdb and source only snapshots. (#122572)

* [8.x] Logsdb and source only snapshots.

Backporting #122199 to 8.x branch.

Addresses a few issues with logsdb and source only snapshots:
* Avoid initializing index sorting, because sort fields will not have doc values.
* Also disable doc value skippers when doc values get disabled.
* As part of source only validation figure out what the nested parent field is.

Also added a few more tests that snapshot and restore logsdb data streams.

* fix test

Author: martijnvg

docs/changelog/122199.yaml

@@ -0,0 +1,5 @@
+pr: 122199
+summary: Fix issues that prevents using search only snapshots for indices that use index sorting. This is includes Logsdb and time series indices.
+area: Logs
+type: bug
+issues: []

server/src/main/java/org/elasticsearch/common/lucene/Lucene.java

@@ -20,6 +20,8 @@
 import org.apache.lucene.index.ConcurrentMergeScheduler;
 import org.apache.lucene.index.CorruptIndexException;
 import org.apache.lucene.index.DirectoryReader;
+import org.apache.lucene.index.FieldInfo;
+import org.apache.lucene.index.FieldInfos;
 import org.apache.lucene.index.FilterCodecReader;
 import org.apache.lucene.index.FilterDirectoryReader;
 import org.apache.lucene.index.FilterLeafReader;
@@ -189,14 +191,26 @@ public static SegmentInfos pruneUnreferencedFiles(String segmentsFileName, Direc
                 throw new IllegalStateException("no commit found in the directory");
             }
         }
+        // Need to figure out what the parent field is that, so that validation in IndexWriter doesn't fail
+        // if no parent field is configured, but FieldInfo says there is a parent field.
+        String parentField = null;
         final IndexCommit cp = getIndexCommit(si, directory);
+        try (var reader = DirectoryReader.open(cp)) {
+            var topLevelFieldInfos = FieldInfos.getMergedFieldInfos(reader);
+            for (FieldInfo fieldInfo : topLevelFieldInfos) {
+                if (fieldInfo.isParentField()) {
+                    parentField = fieldInfo.getName();
+                }
+            }
+        }
         try (
             IndexWriter writer = new IndexWriter(
                 directory,
                 indexWriterConfigWithNoMerging(Lucene.STANDARD_ANALYZER).setSoftDeletesField(Lucene.SOFT_DELETES_FIELD)
                     .setIndexCommit(cp)
                     .setCommitOnClose(false)
                     .setOpenMode(IndexWriterConfig.OpenMode.APPEND)
+                    .setParentField(parentField)
             )
         ) {
             // do nothing and close this will kick off IndexFileDeleter which will remove all pending files

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

@@ -232,7 +232,8 @@ public IndexService(
                 mapperMetrics
             );
             this.indexFieldData = new IndexFieldDataService(indexSettings, indicesFieldDataCache, circuitBreakerService);
-            if (indexSettings.getIndexSortConfig().hasIndexSort()) {
+            boolean sourceOnly = Boolean.parseBoolean(indexSettings.getSettings().get("index.source_only"));
+            if (indexSettings.getIndexSortConfig().hasIndexSort() && sourceOnly == false) {
                 // we delay the actual creation of the sort order for this index because the mapping has not been merged yet.
                 // The sort order is validated right after the merge of the mapping later in the process.
                 this.indexSortSupplier = () -> indexSettings.getIndexSortConfig()