escape comment trailing dash only on the final chunk in speed saver

src/main/java/org/apache/xmlbeans/impl/store/Saver.java

@@ -2106,10 +2106,11 @@ protected void emitCommentText(SaveCur c) {
             int cch = c._cchSrc;
             int off = c._offSrc;
             int index = 0;
+            boolean lastWasDash = false;
             while (index < cch) {
                 int indexLimit = Math.min(index + 512, cch);
                 CharUtil.getChars(_buf, 0, src, off + index, indexLimit - index);
-                entitizeAndWriteCommentText(indexLimit - index);
+                lastWasDash = entitizeAndWriteCommentText(indexLimit - index, lastWasDash, indexLimit == cch);
                 index = indexLimit;
             }
         }
@@ -2156,9 +2157,7 @@ private int entitizeAndWriteText(int bufLimit, int trailingBrackets) {
             return trailingBrackets;
         }
 
-        private void entitizeAndWriteCommentText(int bufLimit) {
-            boolean lastWasDash = false;
-
+        private boolean entitizeAndWriteCommentText(int bufLimit, boolean lastWasDash, boolean lastChunk) {
             for (int i = 0; i < bufLimit; i++) {
                 char ch = _buf[i];
 
@@ -2181,11 +2180,15 @@ private void entitizeAndWriteCommentText(int bufLimit) {
                 }
             }
 
-            if (_buf[bufLimit - 1] == '-') {
+            // A trailing '-' would form "--->" with the closing delimiter, so it
+            // is escaped only at the real end of the comment, not on every chunk.
+            if (lastChunk && _buf[bufLimit - 1] == '-') {
                 _buf[bufLimit - 1] = ' ';
+                lastWasDash = false;
             }
 
             emit(_buf, 0, bufLimit);
+            return lastWasDash;
         }
 
         private boolean entitizeAndWritePIText(int bufLimit, boolean lastWasQuestion) {

src/test/java/misc/checkin/SaveOptimizeForSpeedTest.java

@@ -115,6 +115,19 @@ void testBadCharInText() throws Exception {
         XmlObject.Factory.parse(out);
     }
 
+    @Test
+    void testCommentDashAtChunkBoundary() throws Exception {
+        // a lone '-' sitting on the 512-char chunk boundary. It is legal in
+        // comment content, but the speed path applied its trailing-dash fixup
+        // at the end of every chunk, so the boundary '-' was silently rewritten
+        // to a space and the saved comment no longer matched the source.
+        String data = repeat('a', 511) + "-" + repeat('b', 600);
+        XmlObject o = XmlObject.Factory.parse("<root><!--" + data + "--></root>");
+        String out = saveForSpeed(o);
+        XmlObject.Factory.parse(out);
+        assertTrue(out.contains(repeat('a', 511) + "-"));
+    }
+
     @Test
     void testProcInstTerminatorAcrossChunkBoundary() throws Exception {
         // a '?>' that straddles the 512-char chunk boundary: '?' is the last char