Fix #5176: Return actual null from JSON_EXTRACT for missing/null paths (#5196)

* Fix #5176: Return actual null from JSON_EXTRACT for missing/null paths

The doJsonize() method in JsonExtractFunctionImpl was explicitly converting
null values to the string "null". When JSON_EXTRACT encounters a missing
path or an explicit JSON null value, it should return actual null instead
of the string "null".

Signed-off-by: opensearchpplteam <opensearchpplteam@gmail.com>

* Add integration and unit tests for JSON_EXTRACT null handling (#5176)

Address review feedback:
- Add integration test for null-return behavior (missing path and explicit null)
- Add integration test for multi-path extraction with missing path
- Add unit tests for multi-path extraction where some/all paths are missing

Signed-off-by: opensearchpplteam <opensearchpplteam@gmail.com>

* Fix spotless formatting in JsonExtractFunctionImplTest

Signed-off-by: opensearchpplteam <opensearchpplteam@gmail.com>

* Add YAML REST tests for JSON_EXTRACT null handling (#5176)

Signed-off-by: opensearchpplteam <opensearchpplteam@gmail.com>

---------

Signed-off-by: opensearchpplteam <opensearchpplteam@gmail.com>

Author: opensearchpplteam

core/src/main/java/org/opensearch/sql/expression/function/jsonUDF/JsonExtractFunctionImpl.java

@@ -94,7 +94,7 @@ private static boolean isScalarObject(Object obj) {
 
   private static String doJsonize(Object candidate) {
     if (candidate == null) {
-      return "null"; // Matches isScalarObject, but not toString-able.
+      return null;
     } else if (isScalarObject(candidate)) {
       return candidate.toString();
     } else {

core/src/test/java/org/opensearch/sql/expression/function/jsonUDF/JsonExtractFunctionImplTest.java

@@ -0,0 +1,78 @@
+/*
+ * Copyright OpenSearch Contributors
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+package org.opensearch.sql.expression.function.jsonUDF;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertNull;
+
+import org.junit.jupiter.api.Test;
+
+public class JsonExtractFunctionImplTest {
+
+  @Test
+  public void testMissingPathReturnsNull() {
+    Object result = JsonExtractFunctionImpl.eval("{}", "name");
+    assertNull(result, "Missing path should return actual null, not string \"null\"");
+  }
+
+  @Test
+  public void testExplicitNullValueReturnsNull() {
+    Object result = JsonExtractFunctionImpl.eval("{\"name\": null}", "name");
+    assertNull(result, "Explicit JSON null value should return actual null, not string \"null\"");
+  }
+
+  @Test
+  public void testNoArgsReturnsNull() {
+    assertNull(JsonExtractFunctionImpl.eval());
+  }
+
+  @Test
+  public void testSingleArgReturnsNull() {
+    assertNull(JsonExtractFunctionImpl.eval("{}"));
+  }
+
+  @Test
+  public void testExtractStringValue() {
+    Object result = JsonExtractFunctionImpl.eval("{\"name\": \"John\"}", "name");
+    assertEquals("John", result);
+  }
+
+  @Test
+  public void testExtractNumericValue() {
+    Object result = JsonExtractFunctionImpl.eval("{\"age\": 30}", "age");
+    assertEquals("30", result);
+  }
+
+  @Test
+  public void testExtractNestedObject() {
+    Object result = JsonExtractFunctionImpl.eval("{\"user\": {\"name\": \"John\"}}", "user");
+    assertEquals("{\"name\":\"John\"}", result);
+  }
+
+  @Test
+  public void testExtractArray() {
+    Object result = JsonExtractFunctionImpl.eval("{\"items\": [1, 2, 3]}", "items");
+    assertEquals("[1,2,3]", result);
+  }
+
+  @Test
+  public void testExtractBooleanValue() {
+    Object result = JsonExtractFunctionImpl.eval("{\"active\": true}", "active");
+    assertEquals("true", result);
+  }
+
+  @Test
+  public void testMultiPathWithMissingPath() {
+    Object result = JsonExtractFunctionImpl.eval("{\"name\": \"John\"}", "name", "age");
+    assertEquals("[\"John\",null]", result);
+  }
+
+  @Test
+  public void testMultiPathAllMissing() {
+    Object result = JsonExtractFunctionImpl.eval("{}", "name", "age");
+    assertEquals("[null,null]", result);
+  }
+}

integ-test/src/test/java/org/opensearch/sql/calcite/remote/CalcitePPLJsonBuiltinFunctionIT.java

@@ -194,6 +194,36 @@ public void testJsonExtractWithMultiplyResult() throws IOException {
     verifyDataRows(actual, rows("[[\"Bridge of Sighs\",\"Ponte della Paglia\"],8981.0]"));
   }
 
+  @Test
+  public void testJsonExtractReturnsNullForMissingPathAndNullValue() throws IOException {
+    JSONObject actual =
+        executeQuery(
+            String.format(
+                "source=%s | head 1 | eval a = json_extract('{}', 'name'),"
+                    + " b = json_extract('{\\\"name\\\": null}', 'name')"
+                    + " | fields a, b | head 1",
+                TEST_INDEX_PEOPLE2));
+
+    verifySchema(actual, schema("a", "string"), schema("b", "string"));
+
+    verifyDataRows(actual, rows(null, null));
+  }
+
+  @Test
+  public void testJsonExtractMultiPathWithMissingPath() throws IOException {
+    JSONObject actual =
+        executeQuery(
+            String.format(
+                "source=%s | head 1 | eval a = json_extract('{\\\"name\\\": \\\"John\\\"}',"
+                    + " 'name', 'age')"
+                    + " | fields a | head 1",
+                TEST_INDEX_PEOPLE2));
+
+    verifySchema(actual, schema("a", "string"));
+
+    verifyDataRows(actual, rows("[\"John\",null]"));
+  }
+
   @Test
   public void testJsonKeys() throws IOException {
     JSONObject actual =

integ-test/src/yamlRestTest/resources/rest-api-spec/test/issues/5176.yml

@@ -0,0 +1,85 @@
+setup:
+  - do:
+      query.settings:
+        body:
+          transient:
+            plugins.calcite.enabled: true
+
+  - do:
+      indices.create:
+        index: issue5176
+        body:
+          settings:
+            number_of_shards: 1
+            number_of_replicas: 0
+          mappings:
+            properties:
+              id:
+                type: integer
+
+  - do:
+      bulk:
+        refresh: true
+        body:
+          - '{"index": {"_index": "issue5176", "_id": "1"}}'
+          - '{"id": 1}'
+
+---
+teardown:
+  - do:
+      indices.delete:
+        index: issue5176
+        ignore_unavailable: true
+  - do:
+      query.settings:
+        body:
+          transient:
+            plugins.calcite.enabled: false
+
+---
+"Issue 5176: json_extract returns null for missing path":
+  - skip:
+      features:
+        - headers
+  - do:
+      headers:
+        Content-Type: 'application/json'
+      ppl:
+        body:
+          query: source=issue5176 | eval a = json_extract('{}', 'name') | fields a
+
+  - match: { total: 1 }
+  - match: { schema: [ { name: a, type: string } ] }
+  - match: { datarows: [ [ null ] ] }
+
+---
+"Issue 5176: json_extract returns null for explicit JSON null value":
+  - skip:
+      features:
+        - headers
+  - do:
+      headers:
+        Content-Type: 'application/json'
+      ppl:
+        body:
+          query: "source=issue5176 | eval a = json_extract('{\"name\": null}', 'name') | fields a"
+
+  - match: { total: 1 }
+  - match: { schema: [ { name: a, type: string } ] }
+  - match: { datarows: [ [ null ] ] }
+
+---
+"Issue 5176: json_extract multi-path with missing path returns array with null":
+  - skip:
+      features:
+        - headers
+  - do:
+      headers:
+        Content-Type: 'application/json'
+      ppl:
+        body:
+          query: "source=issue5176 | eval a = json_extract('{\"name\": \"John\"}', 'name', 'age') | fields a"
+
+  - match: { total: 1 }
+  - match: { schema: [ { name: a, type: string } ] }
+  - match: { datarows: [ [ "[\"John\",null]" ] ] }