ES|QL: Add test query generator for FORK

x-pack/plugin/esql/qa/server/src/main/java/org/elasticsearch/xpack/esql/qa/rest/generative/EsqlQueryGenerator.java

@@ -13,6 +13,7 @@
 import org.elasticsearch.xpack.esql.qa.rest.generative.command.pipe.DropGenerator;
 import org.elasticsearch.xpack.esql.qa.rest.generative.command.pipe.EnrichGenerator;
 import org.elasticsearch.xpack.esql.qa.rest.generative.command.pipe.EvalGenerator;
+import org.elasticsearch.xpack.esql.qa.rest.generative.command.pipe.ForkGenerator;
 import org.elasticsearch.xpack.esql.qa.rest.generative.command.pipe.GrokGenerator;
 import org.elasticsearch.xpack.esql.qa.rest.generative.command.pipe.KeepGenerator;
 import org.elasticsearch.xpack.esql.qa.rest.generative.command.pipe.LimitGenerator;
@@ -53,6 +54,7 @@ public record QueryExecuted(String query, int depth, List<Column> outputSchema,
         DropGenerator.INSTANCE,
         EnrichGenerator.INSTANCE,
         EvalGenerator.INSTANCE,
+        ForkGenerator.INSTANCE,
         GrokGenerator.INSTANCE,
         KeepGenerator.INSTANCE,
         LimitGenerator.INSTANCE,

x-pack/plugin/esql/qa/server/src/main/java/org/elasticsearch/xpack/esql/qa/rest/generative/command/pipe/ForkGenerator.java

@@ -0,0 +1,55 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the Elastic License
+ * 2.0; you may not use this file except in compliance with the Elastic License
+ * 2.0.
+ */
+
+package org.elasticsearch.xpack.esql.qa.rest.generative.command.pipe;
+
+import org.elasticsearch.xpack.esql.qa.rest.generative.EsqlQueryGenerator;
+import org.elasticsearch.xpack.esql.qa.rest.generative.command.CommandGenerator;
+
+import java.util.List;
+import java.util.Map;
+
+import static org.elasticsearch.test.ESTestCase.randomIntBetween;
+
+public class ForkGenerator implements CommandGenerator {
+
+    public static final String FORK = "fork";
+    public static final CommandGenerator INSTANCE = new ForkGenerator();
+
+    @Override
+    public CommandDescription generate(
+        List<CommandDescription> previousCommands,
+        List<EsqlQueryGenerator.Column> previousOutput,
+        QuerySchema schema
+    ) {
+        // FORK can only be allowed once - so we skip adding another FORK if we already have one
+        // otherwise, most generated queries would only result in a validation error
+        for (CommandDescription command : previousCommands) {
+            if (command.commandName().equals(FORK)) {
+                return new CommandDescription(FORK, this, " ", Map.of());
+            }
+        }
+
+        int n = randomIntBetween(2, 10);
+
+        String cmd = " | FORK " + "( WHERE true ) ".repeat(n) + " | WHERE _fork == \"fork" + randomIntBetween(1, n) + "\" | DROP _fork";
+
+        return new CommandDescription(FORK, this, cmd, Map.of());
+    }
+
+    @Override
+    public ValidationResult validateOutput(
+        List<CommandDescription> previousCommands,
+        CommandDescription command,
+        List<EsqlQueryGenerator.Column> previousColumns,
+        List<List<Object>> previousOutput,
+        List<EsqlQueryGenerator.Column> columns,
+        List<List<Object>> output
+    ) {
+        return CommandGenerator.expectSameRowCount(previousCommands, previousOutput, output);
+    }
+}