Logsdb and source only snapshots.

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.
* Avoid initializing _source.mode, because otherwise an empty _source:{} gets serialized in the restored mapping.

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

Author: martijnvg

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;
@@ -190,14 +192,24 @@ public static SegmentInfos pruneUnreferencedFiles(String segmentsFileName, Direc
                 throw new IllegalStateException("no commit found in the directory");
             }
         }
+        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()

server/src/main/java/org/elasticsearch/index/mapper/SourceFieldMapper.java

@@ -176,7 +176,8 @@ public Builder(
                 true,
                 () -> null,
                 (n, c, o) -> Mode.valueOf(o.toString().toUpperCase(Locale.ROOT)),
-                m -> toType(m).enabled.explicit() ? null : toType(m).mode,
+                // Avoid initializing _source.mode if it doesn't need to be serialized:
+                m -> toType(m).enabled.explicit() ? null : toType(m).serializeMode ? toType(m).mode : null,
                 (b, n, v) -> b.field(n, v.toString().toLowerCase(Locale.ROOT)),
                 v -> v.toString().toLowerCase(Locale.ROOT)
             ).setMergeValidator((previous, current, conflicts) -> (previous == current) || current != Mode.STORED)

x-pack/plugin/core/src/main/java/org/elasticsearch/snapshots/sourceonly/SourceOnlySnapshot.java

@@ -9,6 +9,7 @@
 import org.apache.lucene.codecs.Codec;
 import org.apache.lucene.index.CheckIndex;
 import org.apache.lucene.index.DirectoryReader;
+import org.apache.lucene.index.DocValuesSkipIndexType;
 import org.apache.lucene.index.DocValuesType;
 import org.apache.lucene.index.FieldInfo;
 import org.apache.lucene.index.FieldInfos;
@@ -252,7 +253,7 @@ private SegmentCommitInfo syncSegment(
                             false,
                             IndexOptions.NONE,
                             DocValuesType.NONE,
-                            fieldInfo.docValuesSkipIndexType(),
+                            DocValuesSkipIndexType.NONE,
                             -1,
                             fieldInfo.attributes(),
                             0,