Initialise current token to the virtual start token

Ensures that in body fragment parsing, which adds the context element to the stack before there is a real token, has a current token during the track position for that first stack insert.

Fixes #2068

CHANGES.md

@@ -7,6 +7,8 @@
 * When tracking the source position of attributes, if source attribute name was mix-cased but the parser was
   lower-case normalizing attribute names, the source position for that attribute was not tracked
   correctly. [2067](https://github.com/jhy/jsoup/issues/2067)
+* When tracking the source position of a body fragment parse, a null pointer exception was
+  thrown. [2068](https://github.com/jhy/jsoup/issues/2068)
 
 ---
 Older changes for versions 0.1.1 (2010-Jan-31) through 1.17.1 (2023-Nov-27) may be found in

src/main/java/org/jsoup/parser/TreeBuilder.java

@@ -26,7 +26,7 @@ abstract class TreeBuilder {
     Document doc; // current doc we are building into
     ArrayList<Element> stack; // the stack of open elements
     String baseUri; // current base uri, for creating new elements
-    Token currentToken; // currentToken is used only for error tracking.
+    Token currentToken; // currentToken is used for error and source position tracking. Null at start of fragment parse
     ParseSettings settings;
     Map<String, Tag> seenTags; // tags we've used in this parse; saves tag GC for custom tags.
 
@@ -48,11 +48,11 @@ void initialiseParse(Reader input, String baseUri, Parser parser) {
         reader = new CharacterReader(input);
         trackSourceRange = parser.isTrackPosition();
         reader.trackNewlines(parser.isTrackErrors() || trackSourceRange); // when tracking errors or source ranges, enable newline tracking for better legibility
-        currentToken = null;
         tokeniser = new Tokeniser(this);
         stack = new ArrayList<>(32);
         seenTags = new HashMap<>();
         start = new Token.StartTag(this);
+        currentToken = start; // init current token to the virtual start token.
         this.baseUri = baseUri;
     }
 

src/test/java/org/jsoup/parser/PositionTest.java

@@ -470,6 +470,23 @@ private void printRange(Node node) {
         assertEquals("id:3-5=6-7; ", xmlLcPos .toString());
     }
 
+    @Test void tracksFrag() {
+        // https://github.com/jhy/jsoup/issues/2068
+        String html = "<h1 id=1>One</h1>\n<h2 id=2>Two</h2><h10>Ten</h10>";
+        Document shellDoc = Document.createShell("");
+
+        List<Node> nodes = TrackingHtmlParser.parseFragmentInput(html, shellDoc.body(), shellDoc.baseUri());
+        StringBuilder track = new StringBuilder();
+
+        // nodes is the top level nodes - want to descend to check all tracked OK
+        nodes.forEach(node -> node.nodeStream().forEach(descend -> {
+            accumulatePositions(descend, track);
+            accumulateAttributePositions(descend, track);
+        }));
+
+        assertEquals("h1:0-9~12-17; id:4-6=7-8; #text:9-12; #text:17-18; h2:18-27~30-35; id:22-24=25-26; #text:27-30; h10:35-40~43-49; #text:40-43; ", track.toString());
+    }
+
     static void accumulateAttributePositions(Node node, StringBuilder sb) {
         if (node instanceof LeafNode) return; // leafnode pseudo attributes are not tracked
         for (Attribute attribute : node.attributes()) {