Add number of max branches for FORK

Author: ioanatia

x-pack/plugin/esql/src/internalClusterTest/java/org/elasticsearch/xpack/esql/action/ForkIT.java

@@ -1007,7 +1007,7 @@ public void testOneSubQuery() {
                ( WHERE content:"fox" )
             """;
         var e = expectThrows(ParsingException.class, () -> run(query));
-        assertTrue(e.getMessage().contains("Fork requires at least two branches"));
+        assertTrue(e.getMessage().contains("Fork requires at least 2 branches"));
     }
 
     public void testForkWithinFork() {
@@ -1047,6 +1047,17 @@ public void testProfile() {
         }
     }
 
+    public void testWithTooManySubqueries() {
+        var query = """
+            FROM test
+            | FORK (WHERE true) (WHERE true) (WHERE true) (WHERE true) (WHERE true)
+                   (WHERE true) (WHERE true) (WHERE true) (WHERE true)
+            """;
+        var e = expectThrows(ParsingException.class, () -> run(query));
+        assertTrue(e.getMessage().contains("Fork requires less than 8 branches"));
+
+    }
+
     private void createAndPopulateIndices() {
         var indexName = "test";
         var client = client().admin().indices();

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

@@ -651,9 +651,13 @@ private void checkForRemoteClusters(LogicalPlan plan, Source source, String comm
     @SuppressWarnings("unchecked")
     public PlanFactory visitForkCommand(EsqlBaseParser.ForkCommandContext ctx) {
         List<PlanFactory> subQueries = visitForkSubQueries(ctx.forkSubQueries());
-        if (subQueries.size() < 2) {
-            throw new ParsingException(source(ctx), "Fork requires at least two branches");
+        if (subQueries.size() < Fork.MIN_BRANCHES) {
+            throw new ParsingException(source(ctx), "Fork requires at least " + Fork.MIN_BRANCHES + " branches");
         }
+        if (subQueries.size() > Fork.MAX_BRANCHES) {
+            throw new ParsingException(source(ctx), "Fork requires less than " + Fork.MAX_BRANCHES + " branches");
+        }
+
         return input -> {
             checkForRemoteClusters(input, source(ctx), "FORK");
             List<LogicalPlan> subPlans = subQueries.stream().map(planFactory -> planFactory.apply(input)).toList();

x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/plan/logical/Fork.java

@@ -36,13 +36,19 @@
 public class Fork extends LogicalPlan implements PostAnalysisPlanVerificationAware, TelemetryAware {
 
     public static final String FORK_FIELD = "_fork";
+    public static final int MAX_BRANCHES = 8;
+    public static final int MIN_BRANCHES = 2;
     private final List<Attribute> output;
 
     public Fork(Source source, List<LogicalPlan> children, List<Attribute> output) {
         super(source, children);
-        if (children.size() < 2) {
-            throw new IllegalArgumentException("requires more than two subqueries, got:" + children.size());
+        if (children.size() < MIN_BRANCHES) {
+            throw new IllegalArgumentException("FORK requires more than " + MIN_BRANCHES + " branches, got: " + children.size());
         }
+        if (children.size() > MAX_BRANCHES) {
+            throw new IllegalArgumentException("FORK requires less than " + MAX_BRANCHES + " subqueries, got: " + children.size());
+        }
+
         this.output = output;
     }
 

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

@@ -3403,11 +3403,18 @@ public void testForkAllCommands() {
     }
 
     public void testInvalidFork() {
-        expectError("FROM foo* | FORK (WHERE a:\"baz\")", "line 1:13: Fork requires at least two branches");
-        expectError("FROM foo* | FORK (LIMIT 10)", "line 1:13: Fork requires at least two branches");
-        expectError("FROM foo* | FORK (SORT a)", "line 1:13: Fork requires at least two branches");
-        expectError("FROM foo* | FORK (WHERE x>1 | LIMIT 5)", "line 1:13: Fork requires at least two branches");
-        expectError("FROM foo* | WHERE x>1 | FORK (WHERE a:\"baz\")", "Fork requires at least two branches");
+        expectError("FROM foo* | FORK (WHERE a:\"baz\")", "line 1:13: Fork requires at least 2 branches");
+        expectError("FROM foo* | FORK (LIMIT 10)", "line 1:13: Fork requires at least 2 branches");
+        expectError("FROM foo* | FORK (SORT a)", "line 1:13: Fork requires at least 2 branches");
+        expectError("FROM foo* | FORK (WHERE x>1 | LIMIT 5)", "line 1:13: Fork requires at least 2 branches");
+        expectError("FROM foo* | WHERE x>1 | FORK (WHERE a:\"baz\")", "Fork requires at least 2 branches");
+
+        expectError("""
+            FROM foo*
+            | FORK (where true) (where true) (where true) (where true)
+                   (where true) (where true) (where true) (where true)
+                   (where true)
+            """, "Fork requires less than 8 branches");
 
         expectError("FROM foo* | FORK ( x+1 ) ( WHERE y>2 )", "line 1:20: mismatched input 'x+1'");
         expectError("FROM foo* | FORK ( LIMIT 10 ) ( y+2 )", "line 1:33: mismatched input 'y+2'");