Protect shared dialect presets from external mutation

src/main/java/net/sf/jsqlparser/parser/AbstractJSqlParser.java

@@ -13,6 +13,7 @@
 import net.sf.jsqlparser.parser.feature.FeatureConfiguration;
 
 import java.util.ArrayList;
+import java.util.Collections;
 import java.util.Arrays;
 import java.util.EnumSet;
 import java.util.List;
@@ -61,8 +62,9 @@ public enum Dialect {
             this(AdjacentStringLiterals.OFF, lexerFeatures);
         }
 
+        /** Returns the immutable lexer defaults shared by this dialect preset. */
         public Set<Feature> getLexerFeatures() {
-            return lexerFeatures;
+            return Collections.unmodifiableSet(lexerFeatures);
         }
 
         public AdjacentStringLiterals getAdjacentStringLiterals() {
@@ -111,6 +113,10 @@ public P withTimeOut(long timeOutMillSeconds) {
         return withFeature(Feature.timeOut, timeOutMillSeconds);
     }
 
+    /**
+     * Applies this dialect's enabled lexer defaults to the current configuration. Existing options
+     * remain set; apply explicit overrides after this call, or use a new parser for a fresh preset.
+     */
     public P withDialect(Dialect dialect) {
         withFeature(Feature.dialect, dialect.name());
         if (dialect.getAdjacentStringLiterals() != AdjacentStringLiterals.OFF) {

src/test/java/net/sf/jsqlparser/parser/DialectPresetIsolationTest.java

@@ -0,0 +1,48 @@
+/*-
+ * #%L
+ * JSQLParser library
+ * %%
+ * Copyright (C) 2004 - 2019 JSQLParser
+ * %%
+ * Dual licensed under GNU LGPL 2.1 or Apache License 2.0
+ * #L%
+ */
+package net.sf.jsqlparser.parser;
+
+import static org.junit.jupiter.api.Assertions.*;
+import net.sf.jsqlparser.parser.AbstractJSqlParser.Dialect;
+import net.sf.jsqlparser.parser.feature.Feature;
+import org.junit.jupiter.api.Test;
+
+class DialectPresetIsolationTest {
+    @Test
+    void callersCannotMutateSharedPresets() {
+        for (Dialect dialect : Dialect.values()) {
+            assertThrows(UnsupportedOperationException.class,
+                    () -> dialect.getLexerFeatures().add(Feature.allowHashLineComments));
+            assertThrows(UnsupportedOperationException.class,
+                    () -> dialect.getLexerFeatures().clear());
+        }
+    }
+
+    @Test
+    void explicitParserOverridesAreLocalAndDoNotModifyPresetDefaults() {
+        CCJSqlParser first = CCJSqlParserUtil.newParser("SELECT 1").withDialect(Dialect.MYSQL)
+                .withHashLineComments(false);
+        CCJSqlParser second = CCJSqlParserUtil.newParser("SELECT 1").withDialect(Dialect.MYSQL);
+        assertFalse(first.getAsBoolean(Feature.allowHashLineComments));
+        assertTrue(second.getAsBoolean(Feature.allowHashLineComments));
+        assertTrue(Dialect.MYSQL.getLexerFeatures().contains(Feature.allowHashLineComments));
+    }
+
+    @Test
+    void preservesAdditivePresetApplicationAndExplicitOverrideOrder() {
+        CCJSqlParser parser = CCJSqlParserUtil.newParser("SELECT 1").withDialect(Dialect.SQLSERVER)
+                .withDialect(Dialect.POSTGRESQL);
+        assertTrue(parser.getAsBoolean(Feature.allowSquareBracketQuotation));
+        parser.withSquareBracketQuotation(false);
+        assertFalse(parser.getAsBoolean(Feature.allowSquareBracketQuotation));
+        assertFalse(CCJSqlParserUtil.newParser("SELECT 1").withDialect(Dialect.POSTGRESQL)
+                .getAsBoolean(Feature.allowSquareBracketQuotation));
+    }
+}