Add an optional extended parser subclass (YAMLAnchorReplayingFactory) able to inline anchors

release-notes/CREDITS-2.x

@@ -276,6 +276,9 @@ Heiko Boettger (@HeikoBoettger)
 
 * Contributed #482: (yaml) Allow passing `ParserImpl` by a subclass or overwrite the events
  (2.18.0)
+* Contributed #502: (yaml) Add an optional extended parser subclass (`YAMLAnchorReplayingFactory`)
+   able to inline anchors
+ (2.19.0)
 
 Burdyug Pavel (@Pavel38l)
 

release-notes/VERSION-2.x

@@ -16,7 +16,9 @@ Active Maintainers:
 
 2.19.0 (not yet released)
 
--
+#502: Add an optional extended parser subclass (`YAMLAnchorReplayingFactory`)
+  able to inline anchors
+ (contributed by Heiko B)
 
 2.18.2 (27-Nov-2024)
 

yaml/src/main/java/com/fasterxml/jackson/dataformat/yaml/YAMLAnchorReplayingFactory.java

@@ -0,0 +1,72 @@
+package com.fasterxml.jackson.dataformat.yaml;
+
+import java.io.CharArrayReader;
+import java.io.InputStream;
+import java.io.IOException;
+import java.io.Reader;
+
+import com.fasterxml.jackson.core.JsonEncoding;
+import com.fasterxml.jackson.core.ObjectCodec;
+import com.fasterxml.jackson.core.io.IOContext;
+
+/**
+ * A subclass of YAMLFactory with the only purpose to replace the YAMLParser by
+ * the YAMLAnchorReplayingParser subclass.
+ *
+ * @since 2.19
+ */
+public class YAMLAnchorReplayingFactory extends YAMLFactory {
+    private static final long serialVersionUID = 1L;
+
+    public YAMLAnchorReplayingFactory() {
+        super();
+    }
+
+    public YAMLAnchorReplayingFactory(ObjectCodec oc) {
+        super(oc);
+    }
+
+    public YAMLAnchorReplayingFactory(YAMLFactory src, ObjectCodec oc) {
+        super(src, oc);
+    }
+
+    protected YAMLAnchorReplayingFactory(YAMLFactoryBuilder b) {
+        super(b);
+    }
+
+    @Override
+    public YAMLAnchorReplayingFactory copy() {
+        _checkInvalidCopy(YAMLAnchorReplayingFactory.class);
+        return new YAMLAnchorReplayingFactory(this, (ObjectCodec) null);
+    }
+
+    @Override
+    protected Object readResolve() {
+        return new YAMLAnchorReplayingFactory(this, _objectCodec);
+    }
+
+    @Override
+    protected YAMLParser _createParser(InputStream input, IOContext ctxt) throws IOException {
+        return new YAMLAnchorReplayingParser(ctxt, _parserFeatures, _yamlParserFeatures,
+                _loaderOptions, _objectCodec,
+                _createReader(input, (JsonEncoding) null, ctxt));
+    }
+
+    @Override
+    protected YAMLParser _createParser(Reader r, IOContext ctxt) throws IOException {
+        return new YAMLAnchorReplayingParser(ctxt, _parserFeatures, _yamlParserFeatures,
+                _loaderOptions, _objectCodec, r);
+    }
+
+    @Override
+    protected YAMLParser _createParser(char[] data, int offset, int len, IOContext ctxt, boolean recyclable) throws IOException {
+        return new YAMLAnchorReplayingParser(ctxt, _parserFeatures, _yamlParserFeatures,
+                _loaderOptions, _objectCodec, new CharArrayReader(data, offset, len));
+    }
+
+    @Override
+    protected YAMLParser _createParser(byte[] data, int offset, int len, IOContext ctxt) throws IOException {
+        return new YAMLAnchorReplayingParser(ctxt, _parserFeatures, _yamlParserFeatures,
+                _loaderOptions, _objectCodec, _createReader(data, offset, len, (JsonEncoding) null, ctxt));
+    }
+}

yaml/src/main/java/com/fasterxml/jackson/dataformat/yaml/YAMLAnchorReplayingParser.java

@@ -0,0 +1,189 @@
+package com.fasterxml.jackson.dataformat.yaml;
+
+import java.io.Reader;
+import java.io.IOException;
+
+import java.util.*;
+
+import org.yaml.snakeyaml.LoaderOptions;
+import org.yaml.snakeyaml.events.*;
+
+import com.fasterxml.jackson.core.JsonParseException;
+import com.fasterxml.jackson.core.ObjectCodec;
+import com.fasterxml.jackson.core.exc.StreamConstraintsException;
+import com.fasterxml.jackson.core.io.IOContext;
+
+/**
+ * A parser that remembers the events of anchored parts in yaml and repeats them
+ * to inline these parts when an alias if found instead of only returning an alias.
+ *<p>
+ * Note: this overwrites the getEvent() since the base `super.nextToken()` manages to much state and
+ * it seems to be much simpler to re-emit the events.
+ *
+ * @since 2.19
+ */
+public class YAMLAnchorReplayingParser extends YAMLParser
+{
+    private static class AnchorContext {
+        public final String anchor;
+        public final List<Event> events = new ArrayList<>();
+        public int depth = 1;
+
+        public AnchorContext(String anchor) {
+            this.anchor = anchor;
+        }
+    }
+
+    /**
+     *  the maximum number of events that can be replayed
+     */
+    public static final int MAX_EVENTS = 9999;
+
+    /**
+     * the maximum limit of anchors to remember
+     */
+    public static final int MAX_ANCHORS = 9999;
+
+    /**
+     * the maximum limit of merges to follow
+     */
+    public static final int MAX_MERGES = 9999;
+
+    /**
+     * the maximum limit of references to remember
+     */
+    public static final int MAX_REFS = 9999;
+
+    /**
+     * Remembers when a merge has been started in order to skip the corresponding
+     * sequence end which needs to be excluded
+     */
+    private final ArrayDeque<Integer> mergeStack = new ArrayDeque<>();
+
+    /**
+     * Collects nested anchor definitions
+     */
+    private final ArrayDeque<AnchorContext> tokenStack = new ArrayDeque<>();
+
+    /**
+     * Keeps track of the last sequentially found definition of each anchor
+     */
+    private final Map<String, List<Event>> referencedObjects = new HashMap<>();
+
+    /**
+     * Keeps track of events that have been insert when processing alias
+     */
+    private final ArrayDeque<Event> refEvents = new ArrayDeque<>();
+
+    /**
+     * keeps track of the global depth of nested collections
+     */
+    private int globalDepth = 0;
+
+    public YAMLAnchorReplayingParser(IOContext ctxt, int parserFeatures, int formatFeatures, LoaderOptions loaderOptions, ObjectCodec codec, Reader reader) {
+        super(ctxt, parserFeatures, formatFeatures, loaderOptions, codec, reader);
+    }
+
+    private void finishContext(AnchorContext context) throws StreamConstraintsException {
+        if (referencedObjects.size() + 1 > MAX_REFS) throw new StreamConstraintsException("too many references in the document");
+        referencedObjects.put(context.anchor, context.events);
+        if (!tokenStack.isEmpty()) {
+            List<Event> events = tokenStack.peek().events;
+            if (events.size() + context.events.size() > MAX_EVENTS) throw new StreamConstraintsException("too many events to replay");
+            events.addAll(context.events);
+        }
+    }
+
+    protected Event trackDepth(Event event) {
+        if (event instanceof CollectionStartEvent) {
+            ++globalDepth;
+        } else if (event instanceof CollectionEndEvent) {
+            --globalDepth;
+        }
+        return event;
+    }
+
+    protected Event filterEvent(Event event) {
+        if (event instanceof MappingEndEvent) {
+            if (!mergeStack.isEmpty()) {
+                if (mergeStack.peek() > globalDepth) {
+                    mergeStack.pop();
+                    return null;
+                }
+            }
+        }
+        return event;
+    }
+
+    @Override
+    protected Event getEvent() throws IOException {
+        while(!refEvents.isEmpty()) {
+            Event event = filterEvent(trackDepth(refEvents.removeFirst()));
+            if (event != null) return event;
+        }
+
+        Event event = null;
+        while (event == null) {
+            event = trackDepth(super.getEvent());
+            if (event == null) return null;
+            event = filterEvent(event);
+        }
+
+        if (event instanceof AliasEvent) {
+            AliasEvent alias = (AliasEvent) event;
+            List<Event> events = referencedObjects.get(alias.getAnchor());
+            if (events != null) {
+                if (refEvents.size() + events.size() > MAX_EVENTS) throw new StreamConstraintsException("too many events to replay");
+                refEvents.addAll(events);
+                return refEvents.removeFirst();
+            }
+            throw new JsonParseException("invalid alias " + alias.getAnchor());
+        }
+
+        if (event instanceof NodeEvent) {
+            String anchor = ((NodeEvent) event).getAnchor();
+            if (anchor != null) {
+                AnchorContext context = new AnchorContext(anchor);
+                context.events.add(event);
+                if (event instanceof CollectionStartEvent) {
+                    if (tokenStack.size() + 1 > MAX_ANCHORS) throw new StreamConstraintsException("too many anchors in the document");
+                    tokenStack.push(context);
+                } else {
+                    // directly store it
+                    finishContext(context);
+                }
+                return event;
+            }
+        }
+
+        if (event instanceof ScalarEvent) {
+            ScalarEvent scalarEvent = (ScalarEvent) event;
+            if (scalarEvent.getValue().equals( "<<")) {
+                // expect next node to be a map
+                Event next = getEvent();
+                if (next instanceof MappingStartEvent) {
+                    if (mergeStack.size() + 1 > MAX_MERGES) throw new StreamConstraintsException("too many merges in the document");
+                    mergeStack.push(globalDepth);
+                    return getEvent();
+                }
+                throw new JsonParseException("found field '<<' but value isn't a map");
+            }
+        }
+
+        if (!tokenStack.isEmpty()) {
+            AnchorContext context = tokenStack.peek();
+            if (context.events.size() + 1 > MAX_EVENTS) throw new StreamConstraintsException("too many events to replay");
+            context.events.add(event);
+            if (event instanceof CollectionStartEvent) {
+                ++context.depth;
+            } else if (event instanceof CollectionEndEvent) {
+                --context.depth;
+                if (context.depth == 0) {
+                    tokenStack.pop();
+                    finishContext(context);
+                }
+            }
+        }
+        return event;
+    }
+}

yaml/src/main/java/com/fasterxml/jackson/dataformat/yaml/YAMLParser.java

@@ -580,13 +580,16 @@ public JsonToken nextToken() throws IOException
     /**
      * Since the parserImpl cannot be replaced allow subclasses to at least be able to
      * influence the events being consumed.
-     *
+     *<p>
      * A particular use case is working around the lack of anchor and alias support to
      * emit additional events.
+     *<p>
+     * NOTE: since 2.18, declared to throw {@link IOException} to allow sub-classes
+     * to do so.
      *
      * @since 2.18
      */
-    protected Event getEvent() {
+    protected Event getEvent() throws IOException {
         return _yamlParser.getEvent();
     }