reject lone low surrogate in StreamingMarkupWriter.write

subprojects/groovy-xml/src/main/java/groovy/xml/streamingmarkupsupport/StreamingMarkupWriter.java

@@ -188,6 +188,10 @@ public void flush() throws IOException {
     public void write(final int c) throws IOException {
         if (c >= 0XDC00 && c <= 0XDFFF) {
             // Low surrogate
+            if (!this.haveHighSurrogate) {
+                this.surrogatePair.setLength(0);
+                throw new IOException("Low Surrogate not preceded by High Surrogate");
+            }
             this.surrogatePair.append((char) c);
 
             if (this.encoder.canEncode(this.surrogatePair)) {

subprojects/groovy-xml/src/test/groovy/groovy/xml/StreamingMarkupWriterTest.groovy

@@ -0,0 +1,53 @@
+/*
+ *  Licensed to the Apache Software Foundation (ASF) under one
+ *  or more contributor license agreements.  See the NOTICE file
+ *  distributed with this work for additional information
+ *  regarding copyright ownership.  The ASF licenses this file
+ *  to you under the Apache License, Version 2.0 (the
+ *  "License"); you may not use this file except in compliance
+ *  with the License.  You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ *  Unless required by applicable law or agreed to in writing,
+ *  software distributed under the License is distributed on an
+ *  "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ *  KIND, either express or implied.  See the License for the
+ *  specific language governing permissions and limitations
+ *  under the License.
+ */
+package groovy.xml
+
+import groovy.xml.streamingmarkupsupport.StreamingMarkupWriter
+import org.junit.jupiter.api.Test
+
+import static org.junit.jupiter.api.Assertions.assertEquals
+import static org.junit.jupiter.api.Assertions.assertThrows
+
+class StreamingMarkupWriterTest {
+
+    // A well-formed surrogate pair (U+1D400) is encoded as a single numeric reference.
+    @Test
+    void validSurrogatePairIsEncoded() {
+        def sw = new StringWriter()
+        def w = new StreamingMarkupWriter(sw)
+        w.write('𝐀')
+        w.flush()
+        assertEquals('𝐀', sw.toString())
+    }
+
+    // A low surrogate with no preceding high surrogate is malformed UTF-16 and must be
+    // rejected, mirroring the existing rejection of a high surrogate not followed by a low one.
+    @Test
+    void loneLowSurrogateIsRejected() {
+        def w = new StreamingMarkupWriter(new StringWriter())
+        assertThrows(IOException) { w.write('\uDC00') }
+    }
+
+    // Existing behavior: a high surrogate not followed by a low surrogate is rejected.
+    @Test
+    void loneHighSurrogateIsRejected() {
+        def w = new StreamingMarkupWriter(new StringWriter())
+        assertThrows(IOException) { w.write('\uD835' + 'A') }
+    }
+}