Make dense_vector fields updatable to bbq_flat/bbq_hnsw (elastic#128291)

(cherry picked from commit 629a366)

Author: tteofili

docs/changelog/128291.yaml

@@ -0,0 +1,5 @@
+pr: 128291
+summary: Make `dense_vector` fields updatable to bbq_flat/bbq_hnsw
+area: Vector Search
+type: enhancement
+issues: []

docs/reference/mapping/types/dense-vector.asciidoc

@@ -476,10 +476,10 @@ To better accommodate scaling and performance needs, updating the `type` setting
 
 [source,txt]
 ----
-flat --> int8_flat --> int4_flat --> hnsw --> int8_hnsw --> int4_hnsw
+flat --> int8_flat --> int4_flat --> bbq_flat --> hnsw --> int8_hnsw --> int4_hnsw --> bbq_hnsw
 ----
 
-For updating all HNSW types (`hnsw`, `int8_hnsw`, `int4_hnsw`) the number of connections `m` must either stay the same or increase. For scalar quantized formats  (`int8_flat`, `int4_flat`, `int8_hnsw`, `int4_hnsw`) the `confidence_interval` must always be consistent (once defined, it cannot change).
+For updating all HNSW types (`hnsw`, `int8_hnsw`, `int4_hnsw`, `bbq_hnsw`) the number of connections `m` must either stay the same or increase. For the scalar quantized formats  `int8_flat`, `int4_flat`, `int8_hnsw` and `int4_hnsw` the `confidence_interval` must always be consistent (once defined, it cannot change).
 
 Updating `type` in `index_options` will fail in all other scenarios.
 

qa/rolling-upgrade/src/javaRestTest/java/org/elasticsearch/upgrades/DenseVectorMappingUpdateIT.java

@@ -15,6 +15,7 @@
 import org.elasticsearch.client.Request;
 import org.elasticsearch.client.Response;
 import org.elasticsearch.common.Strings;
+import org.elasticsearch.index.mapper.SourceFieldMapper;
 import org.elasticsearch.xcontent.XContentBuilder;
 import org.elasticsearch.xcontent.XContentType;
 
@@ -31,6 +32,8 @@
  */
 public class DenseVectorMappingUpdateIT extends AbstractRollingUpgradeTestCase {
 
+    private static final String SYNTHETIC_SOURCE_FEATURE = "gte_v8.12.0";
+
     private static final String BULK1 = """
                     {"index": {"_id": "1"}}
                     {"embedding": [1, 1, 1, 1]}
@@ -86,10 +89,22 @@ public void testDenseVectorMappingUpdateOnOldCluster() throws IOException {
             String indexName = "test_index";
             if (isOldCluster()) {
                 Request createIndex = new Request("PUT", "/" + indexName);
-                XContentBuilder mappings = XContentBuilder.builder(XContentType.JSON.xContent())
-                    .startObject()
-                    .startObject("mappings")
-                    .startObject("properties")
+                boolean useSyntheticSource = randomBoolean() && oldClusterHasFeature(SYNTHETIC_SOURCE_FEATURE);
+
+                boolean useIndexSetting = SourceFieldMapper.onOrAfterDeprecateModeVersion(getOldClusterIndexVersion());
+                XContentBuilder payload = XContentBuilder.builder(XContentType.JSON.xContent()).startObject();
+                if (useSyntheticSource) {
+                    if (useIndexSetting) {
+                        payload.startObject("settings").field("index.mapping.source.mode", "synthetic").endObject();
+                    }
+                }
+                payload.startObject("mappings");
+                if (useIndexSetting == false) {
+                    payload.startObject("_source");
+                    payload.field("mode", "synthetic");
+                    payload.endObject();
+                }
+                payload.startObject("properties")
                     .startObject("embedding")
                     .field("type", "dense_vector")
                     .field("index", "true")
@@ -104,7 +119,7 @@ public void testDenseVectorMappingUpdateOnOldCluster() throws IOException {
                     .endObject()
                     .endObject()
                     .endObject();
-                createIndex.setJsonEntity(Strings.toString(mappings));
+                createIndex.setJsonEntity(Strings.toString(payload));
                 client().performRequest(createIndex);
                 Request index = new Request("POST", "/" + indexName + "/_bulk/");
                 index.addParameter("refresh", "true");