Ensure cluster string could be quoted

docs/changelog/120355.yaml

@@ -0,0 +1,5 @@
+pr: 120355
+summary: Ensure cluster string could be quoted
+area: ES|QL
+type: enhancement
+issues: []

x-pack/plugin/esql/src/main/antlr/EsqlBaseParser.g4

@@ -143,6 +143,7 @@ indexPattern
 
 clusterString
     : UNQUOTED_SOURCE
+    | QUOTED_STRING
     ;
 
 indexString

x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/parser/IdentifierBuilder.java

@@ -13,6 +13,7 @@
 import org.elasticsearch.cluster.metadata.MetadataCreateIndexService;
 import org.elasticsearch.common.Strings;
 import org.elasticsearch.indices.InvalidIndexNameException;
+import org.elasticsearch.transport.RemoteClusterService;
 import org.elasticsearch.xpack.esql.core.util.Holder;
 import org.elasticsearch.xpack.esql.parser.EsqlBaseParser.IdentifierContext;
 import org.elasticsearch.xpack.esql.parser.EsqlBaseParser.IndexStringContext;
@@ -51,29 +52,51 @@ protected static String quoteIdString(String unquotedString) {
         return "`" + unquotedString.replace("`", "``") + "`";
     }
 
+    @Override
+    public String visitClusterString(EsqlBaseParser.ClusterStringContext ctx) {
+        if (ctx == null) {
+            return null;
+        } else if (ctx.UNQUOTED_SOURCE() != null) {
+            return ctx.UNQUOTED_SOURCE().getText();
+        } else {
+            return unquote(ctx.QUOTED_STRING().getText());
+        }
+    }
+
     @Override
     public String visitIndexString(IndexStringContext ctx) {
-        TerminalNode n = ctx.UNQUOTED_SOURCE();
-        return n != null ? n.getText() : unquote(ctx.QUOTED_STRING().getText());
+        if (ctx.UNQUOTED_SOURCE() != null) {
+            return ctx.UNQUOTED_SOURCE().getText();
+        } else {
+            return unquote(ctx.QUOTED_STRING().getText());
+        }
     }
 
     public String visitIndexPattern(List<EsqlBaseParser.IndexPatternContext> ctx) {
         List<String> patterns = new ArrayList<>(ctx.size());
         Holder<Boolean> hasSeenStar = new Holder<>(false);
         ctx.forEach(c -> {
             String indexPattern = visitIndexString(c.indexString());
-            String clusterString = c.clusterString() != null ? c.clusterString().getText() : null;
+            String clusterString = visitClusterString(c.clusterString());
             // skip validating index on remote cluster, because the behavior of remote cluster is not consistent with local cluster
             // For example, invalid#index is an invalid index name, however FROM *:invalid#index does not return an error
             if (clusterString == null) {
                 hasSeenStar.set(indexPattern.contains(WILDCARD) || hasSeenStar.get());
                 validateIndexPattern(indexPattern, c, hasSeenStar.get());
+            } else {
+                validateClusterString(clusterString, c);
             }
             patterns.add(clusterString != null ? clusterString + REMOTE_CLUSTER_INDEX_SEPARATOR + indexPattern : indexPattern);
         });
         return Strings.collectionToDelimitedString(patterns, ",");
     }
 
+    protected static void validateClusterString(String clusterString, EsqlBaseParser.IndexPatternContext ctx) {
+        if (clusterString.indexOf(RemoteClusterService.REMOTE_CLUSTER_INDEX_SEPARATOR) != -1) {
+            throw new ParsingException(source(ctx), "cluster string [{}] must not contain ':'", clusterString);
+        }
+    }
+
     private static void validateIndexPattern(String indexPattern, EsqlBaseParser.IndexPatternContext ctx, boolean hasSeenStar) {
         // multiple index names can be in the same double quote, e.g. indexPattern = "idx1, *, -idx2"
         String[] indices = indexPattern.split(",");

x-pack/plugin/esql/src/test/java/org/elasticsearch/xpack/esql/IdentifierGenerator.java

@@ -80,7 +80,7 @@ public static String randomIndexPattern(Feature... features) {
 
         var pattern = maybeQuote(index.toString());
         if (canAdd(Features.CROSS_CLUSTER, features)) {
-            var cluster = randomIdentifier();
+            var cluster = maybeQuote(randomIdentifier());
             pattern = maybeQuote(cluster + ":" + pattern);
         }