Don't try to serialize half-baked cluster info (#132756)

* Don't try to serialize half-baked cluster info

x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/action/EsqlExecutionInfo.java

@@ -67,6 +67,8 @@ public class EsqlExecutionInfo implements ChunkedToXContentObject, Writeable {
     // Updates to the Cluster occur with the updateCluster method that given the key to map transforms an
     // old Cluster Object to a new Cluster Object with the remapping function.
     public final ConcurrentMap<String, Cluster> clusterInfo;
+    // Is the clusterInfo map iinitialization in progress? If so, we should not try to serialize it.
+    private transient volatile boolean clusterInfoInitializing;
     // whether the user has asked for CCS metadata to be in the JSON response (the overall took will always be present)
     private final boolean includeCCSMetadata;
 
@@ -124,10 +126,8 @@ public class EsqlExecutionInfo implements ChunkedToXContentObject, Writeable {
     @Override
     public void writeTo(StreamOutput out) throws IOException {
         out.writeOptionalTimeValue(overallTook);
-        if (clusterInfo != null) {
-            // .stream().toList() creates an immutable copy of the cluster info entries
-            // as today they might be still changing while serialization is happening
-            out.writeCollection(clusterInfo.values().stream().toList());
+        if (clusterInfo != null && clusterInfoInitializing == false) {
+            out.writeCollection(clusterInfo.values());
         } else {
             out.writeCollection(Collections.emptyList());
         }
@@ -354,6 +354,10 @@ public class EsqlExecutionInfo implements ChunkedToXContentObject, Writeable {
         return isStopped;
     }
 
+    public void clusterInfoInitializing(boolean clusterInfoInitializing) {
+        this.clusterInfoInitializing = clusterInfoInitializing;
+    }
+
     /**
      * Represents the search metadata about a particular cluster involved in a cross-cluster search.
      * The Cluster object can represent either the local cluster or a remote cluster.

x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/session/EsqlCCSUtils.java

@@ -338,15 +338,20 @@ public class EsqlCCSUtils {
                 patterns.getFirst().indexPattern()
             );
 
+            executionInfo.clusterInfoInitializing(true);
             // initialize the cluster entries in EsqlExecutionInfo before throwing the invalid license error
             // so that the CCS telemetry handler can recognize that this error is CCS-related
-            for (var entry : groupedIndices.entrySet()) {
-                final String clusterAlias = entry.getKey();
-                final String indexExpr = Strings.arrayToCommaDelimitedString(entry.getValue().indices());
-                executionInfo.swapCluster(clusterAlias, (k, v) -> {
-                    assert v == null : "No cluster for " + clusterAlias + " should have been added to ExecutionInfo yet";
-                    return new EsqlExecutionInfo.Cluster(clusterAlias, indexExpr, executionInfo.shouldSkipOnFailure(clusterAlias));
-                });
+            try {
+                for (var entry : groupedIndices.entrySet()) {
+                    final String clusterAlias = entry.getKey();
+                    final String indexExpr = Strings.arrayToCommaDelimitedString(entry.getValue().indices());
+                    executionInfo.swapCluster(clusterAlias, (k, v) -> {
+                        assert v == null : "No cluster for " + clusterAlias + " should have been added to ExecutionInfo yet";
+                        return new EsqlExecutionInfo.Cluster(clusterAlias, indexExpr, executionInfo.shouldSkipOnFailure(clusterAlias));
+                    });
+                }
+            } finally {
+                executionInfo.clusterInfoInitializing(false);
             }
 
             // check if it is a cross-cluster query