SOLR-18190: Index upgrade Collection level API

solr/api/src/java/org/apache/solr/client/api/model/UpgradeCoreIndexRequestBody.java

@@ -30,4 +30,21 @@ public class UpgradeCoreIndexRequestBody {
           "updateChain to be used for reindexing during index upgrade if you don't want to use the one used by /update by default")
   @JsonProperty
   public String updateChain;
+
+  @Schema(
+      description =
+          "Internal flag set by the collection-level UPGRADECOLLECTIONINDEX command to indicate"
+              + " this upgrade is being coordinated by SolrCloud. When true, the SolrCloud guard is"
+              + " bypassed, a local-only update chain is used (no distributed forwarding), and the"
+              + " core's readOnly flag is temporarily cleared for the duration of the upgrade.")
+  @JsonProperty
+  public Boolean cloudMode;
+
+  @Schema(
+      description =
+          "When true, only checks whether the index contains old-format segments without performing"
+              + " any upgrade. Returns the number of segments needing upgrade and whether the index"
+              + " is already at the current Lucene format. No writes are performed.")
+  @JsonProperty
+  public Boolean checkOnly;
 }

solr/api/src/java/org/apache/solr/client/api/model/UpgradeCoreIndexResponse.java

@@ -35,4 +35,10 @@ public class UpgradeCoreIndexResponse extends SolrJerseyResponse {
   @Schema(description = "Status of the core index upgrade operation.")
   @JsonProperty
   public String upgradeStatus;
+
+  @Schema(
+      description =
+          "Whether the index is fully at the current Lucene format. Set when checkOnly=true.")
+  @JsonProperty
+  public Boolean indexUpgraded;
 }

solr/core/src/java/org/apache/solr/cloud/api/collections/CollApiCmds.java

@@ -65,6 +65,7 @@
 import static org.apache.solr.common.params.CollectionParams.CollectionAction.REPLACENODE;
 import static org.apache.solr.common.params.CollectionParams.CollectionAction.RESTORE;
 import static org.apache.solr.common.params.CollectionParams.CollectionAction.SPLITSHARD;
+import static org.apache.solr.common.params.CollectionParams.CollectionAction.UPGRADECOLLECTIONINDEX;
 import static org.apache.solr.common.params.CommonParams.NAME;
 
 import io.opentelemetry.api.trace.Span;
@@ -189,7 +190,8 @@ private CommandMap(OverseerNodePrioritizer overseerPrioritizer, CollectionComman
               Map.entry(ADDREPLICA, new AddReplicaCmd(ccc)),
               Map.entry(MOVEREPLICA, new MoveReplicaCmd(ccc)),
               Map.entry(REINDEXCOLLECTION, new ReindexCollectionCmd(ccc)),
-              Map.entry(RENAME, new RenameCmd(ccc)));
+              Map.entry(RENAME, new RenameCmd(ccc)),
+              Map.entry(UPGRADECOLLECTIONINDEX, new UpgradeCollectionIndexCmd(ccc)));
     }
 
     CollApiCmds.CollectionApiCommand getActionCommand(CollectionParams.CollectionAction action) {