fix(core): don't re-emit the batch-boundary record when paging with a limit that is a multiple of 500

hugegraph-server/hugegraph-core/src/main/java/org/apache/hugegraph/backend/serializer/BinaryEntryIterator.java

@@ -68,23 +68,33 @@ protected final boolean fetch() {
 
         while (this.results.hasNext()) {
             Elem elem = this.results.next();
+            if (this.current != null &&
+                this.sizeOf(this.current) >= INLINE_BATCH_SIZE) {
+                /*
+                 * The current entry already holds a full batch: start the
+                 * next entry with this record instead of appending it.
+                 * results.position() has already advanced to this record and
+                 * a page restarts inclusively from position(), so it must
+                 * never point at a record that has been emitted (a page
+                 * limit ending exactly at a batch boundary would otherwise
+                 * re-emit it as the first record of the next page).
+                 */
+                assert this.next == null;
+                this.next = this.merger.apply(null, elem);
+                break;
+            }
             BackendEntry merged = this.merger.apply(this.current, elem);
             E.checkState(merged != null, "Error when merging entry");
             if (this.current == null) {
                 // The first time to read
                 this.current = merged;
-            } else if (merged == this.current) {
-                // The next entry belongs to the current entry
-                assert this.current != null;
-                if (this.sizeOf(this.current) >= INLINE_BATCH_SIZE) {
-                    break;
-                }
-            } else {
+            } else if (merged != this.current) {
                 // New entry
                 assert this.next == null;
                 this.next = merged;
                 break;
             }
+            // Else the record belongs to the current entry
 
             // When limit exceed, stop fetching
             if (this.reachLimit(this.fetched() - 1)) {

hugegraph-server/hugegraph-test/src/main/java/org/apache/hugegraph/core/EdgeCoreTest.java

@@ -5380,6 +5380,43 @@ public void testQueryInEdgesOfVertexInPaging() {
         Assert.assertEquals(2, count);
     }
 
+    @Test
+    public void testQueryOutEdgesOfVertexInPagingAtBatchBoundary() {
+        HugeGraph graph = graph();
+        Assume.assumeTrue("Not support paging",
+                          storeFeatures().supportsQueryByPage());
+        // More edges than BackendEntryIterator.INLINE_BATCH_SIZE (500)
+        int total = 1200;
+        Vertex louise = graph.addVertex(T.label, "person", "name", "Louise",
+                                        "city", "Beijing", "age", 21);
+        Vertex java1 = graph.addVertex(T.label, "book", "name", "java-1");
+        for (int i = 0; i < total; i++) {
+            louise.addEdge("look", java1, "time", String.format("2017-%04d", i));
+        }
+        graph.tx().commit();
+
+        // A page limit ending exactly at a batch boundary (500, 1000) used to
+        // re-emit the last edge of a page as the first edge of the next page
+        for (int limit : new int[]{400, 500, 600, 1000}) {
+            Set<Object> ids = new HashSet<>();
+            int count = 0;
+            String page = PageInfo.PAGE_NONE;
+            while (page != null) {
+                GraphTraversal<Vertex, Edge> iterator = graph.traversal()
+                                                             .V(louise).outE("look")
+                                                             .has("~page", page)
+                                                             .limit(limit);
+                while (iterator.hasNext()) {
+                    ids.add(iterator.next().id());
+                    count++;
+                }
+                page = TraversalUtil.page(iterator);
+            }
+            Assert.assertEquals("limit " + limit, total, count);
+            Assert.assertEquals("limit " + limit, total, ids.size());
+        }
+    }
+
     @Test
     public void testQueryCount() {
         HugeGraph graph = graph();