Fix decoding empty response for object instance

leshan-core/src/main/java/org/eclipse/leshan/core/node/codec/senml/LwM2mNodeSenMLDecoder.java

@@ -296,9 +296,14 @@ private LwM2mNode parseRecords(Collection<LwM2mResolvedSenMLRecord> records, LwM
             Map<Integer, Collection<LwM2mResolvedSenMLRecord>> recordsByInstanceId = groupRecordsByInstanceId(records);
 
             // validate we have resources for only 1 instance
-            if (recordsByInstanceId.size() != 1)
+            if (recordsByInstanceId.size() > 1)
                 throw new CodecException("One instance expected in the payload [path:%s]", path);
 
+            // handle empty object instance
+            if (recordsByInstanceId.size() == 0) {
+                return new LwM2mObjectInstance(path.getObjectInstanceId(), Collections.emptyList());
+            }
+
             // Extract resources
             Entry<Integer, Collection<LwM2mResolvedSenMLRecord>> instanceEntry = recordsByInstanceId.entrySet()
                     .iterator().next();

leshan-core/src/test/java/org/eclipse/leshan/core/node/codec/LwM2mNodeDecoderTest.java

@@ -57,12 +57,16 @@
 import org.eclipse.leshan.core.tlv.Tlv;
 import org.eclipse.leshan.core.tlv.Tlv.TlvType;
 import org.eclipse.leshan.core.tlv.TlvEncoder;
+import org.eclipse.leshan.core.util.ContentFormatArgumentConverter;
 import org.eclipse.leshan.core.util.Hex;
 import org.eclipse.leshan.core.util.TestLwM2mId;
 import org.eclipse.leshan.core.util.TestObjectLoader;
 import org.eclipse.leshan.core.util.datatype.ULong;
 import org.junit.jupiter.api.BeforeAll;
 import org.junit.jupiter.api.Test;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.converter.ConvertWith;
+import org.junit.jupiter.params.provider.ValueSource;
 
 /**
  * Unit tests for {@link LwM2mDecoder}
@@ -966,6 +970,40 @@ public void senml_json_decode_device_object_instance() {
         assertEquals("U", instance.getResource(16).getValue());
     }
 
+    @ParameterizedTest()
+    @ValueSource(strings = { "SENML_JSON", "SENML_CBOR", "TLV" })
+    public void decode_null_object(@ConvertWith(ContentFormatArgumentConverter.class) ContentFormat contentFormat) {
+        LwM2mObject object = decoder.decode(new byte[0], contentFormat, new LwM2mPath("/1"), model, LwM2mObject.class);
+
+        assertNotNull(object);
+        assertEquals(1, object.getId());
+        assertTrue(object.getInstances().isEmpty());
+    }
+
+    @ParameterizedTest()
+    @ValueSource(strings = { "SENML_JSON", "SENML_CBOR", "TLV" })
+    public void decode_null_object_instance(
+            @ConvertWith(ContentFormatArgumentConverter.class) ContentFormat contentFormat) {
+        LwM2mObjectInstance instance = decoder.decode(new byte[0], contentFormat, new LwM2mPath("/3/0"), model,
+                LwM2mObjectInstance.class);
+
+        assertNotNull(instance);
+        assertEquals(0, instance.getId());
+        assertTrue(instance.getResources().isEmpty());
+    }
+
+    @ParameterizedTest()
+    @ValueSource(strings = { "SENML_JSON", "SENML_CBOR", "TLV" })
+    public void decode_null_multi_resource(
+            @ConvertWith(ContentFormatArgumentConverter.class) ContentFormat contentFormat) {
+        LwM2mResource resource = decoder.decode(new byte[0], contentFormat, new LwM2mPath("/3/0/7"), model,
+                LwM2mResource.class);
+
+        assertNotNull(resource);
+        assertEquals(7, resource.getId());
+        assertTrue(resource.getInstances().isEmpty());
+    }
+
     @Test
     public void senml_json_decode_single_resource() {
         String payload = "[{\"bn\":\"/3/0/0\",\"vs\":\"Open Mobile Alliance\"}]";

leshan-core/src/test/java/org/eclipse/leshan/core/util/ContentFormatArgumentConverter.java

@@ -0,0 +1,30 @@
+/*******************************************************************************
+ * Copyright (c) 2024 Sierra Wireless and others.
+ *
+ * All rights reserved. This program and the accompanying materials
+ * are made available under the terms of the Eclipse Public License v2.0
+ * and Eclipse Distribution License v1.0 which accompany this distribution.
+ *
+ * The Eclipse Public License is available at
+ *    http://www.eclipse.org/legal/epl-v20.html
+ * and the Eclipse Distribution License is available at
+ *    http://www.eclipse.org/org/documents/edl-v10.html.
+ *
+ * Contributors:
+ *     Sierra Wireless - initial API and implementation
+ *******************************************************************************/
+package org.eclipse.leshan.core.util;
+
+import org.eclipse.leshan.core.request.ContentFormat;
+import org.junit.jupiter.params.converter.ArgumentConversionException;
+import org.junit.jupiter.params.converter.SimpleArgumentConverter;
+
+public class ContentFormatArgumentConverter extends SimpleArgumentConverter {
+    @Override
+    protected Object convert(Object source, Class<?> targetType) throws ArgumentConversionException {
+        if (targetType.equals(ContentFormat.class)) {
+            return ContentFormat.fromName((String) source);
+        }
+        return null;
+    }
+}