[Maintenance-2930] Review use of JSON Flattener (opensearch-project#3674

)

### Description

Implement JsonFlattener helper class as written in opensearch-project#2926 to deprecate
the use of the unnecessary JsonFlattener third party module.

* Category (Enhancement, New feature, Bug fix, Test fix, Refactoring,
Maintenance, Documentation)

Maintenance

* Why these changes are required?

The JsonFlattener module was being utilized in only one place for one
specific purpose, so these functions can be implemented as part of the
OpenSearch codebase instead of importing an unnecessary third party
module.

* What is the old behavior before changes and new behavior after
changes?

Hopefully nothing.

### Issues Resolved
- opensearch-project#2930 

Is this a backport? If so, please add backport PR # and/or commits #

No

### Testing
Tests checked to make sure functions are not broken.

### Check List
- [x] New functionality includes testing
- [x] New functionality has been documented
- [x] Commits are signed per the DCO using --signoff

By submitting this pull request, I confirm that my contribution is made
under the terms of the Apache 2.0 license.
For more information on following Developer Certificate of Origin and
signing off your commits, please check
[here](https://github.com/opensearch-project/OpenSearch/blob/main/CONTRIBUTING.md#developer-certificate-of-origin).

---------

Signed-off-by: Prabhas Kurapati <[email protected]>

build.gradle

@@ -585,13 +585,6 @@ dependencies {
     implementation "io.jsonwebtoken:jjwt-api:${jjwt_version}"
     implementation "io.jsonwebtoken:jjwt-impl:${jjwt_version}"
     implementation "io.jsonwebtoken:jjwt-jackson:${jjwt_version}"
-    // JSON flattener
-    implementation ("com.github.wnameless.json:json-base:2.4.3") {
-        exclude group: "org.glassfish", module: "jakarta.json"
-        exclude group: "com.google.code.gson", module: "gson"
-        exclude group: "org.json", module: "json"
-    }
-    implementation 'com.github.wnameless.json:json-flattener:0.16.6'
     // JSON patch
     implementation 'com.flipkart.zjsonpatch:zjsonpatch:0.4.14'
     implementation 'org.apache.commons:commons-collections4:4.4'

src/main/java/org/opensearch/security/compliance/FieldReadCallback.java

@@ -33,11 +33,10 @@
 import org.opensearch.security.auditlog.AuditLog;
 import org.opensearch.security.dlic.rest.support.Utils;
 import org.opensearch.security.support.HeaderHelper;
+import org.opensearch.security.support.JsonFlattener;
 import org.opensearch.security.support.SourceFieldsContext;
 import org.opensearch.security.support.WildcardMatcher;
 
-import com.github.wnameless.json.flattener.JsonFlattener;
-
 //TODO  We need to deal with caching!!
 //Currently we disable caching (and realtime requests) when FLS or DLS is applied
 //Check if we can hook in into the caches

src/main/java/org/opensearch/security/support/JsonFlattener.java

@@ -0,0 +1,66 @@
+/*
+ * Copyright OpenSearch Contributors
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * The OpenSearch Contributors require contributions made to
+ * this file be licensed under the Apache-2.0 license or a
+ * compatible open source license.
+ *
+ */
+
+package org.opensearch.security.support;
+
+import java.io.IOException;
+import java.util.LinkedHashMap;
+import java.util.Map;
+
+import com.fasterxml.jackson.core.type.TypeReference;
+
+import org.opensearch.core.common.Strings;
+import org.opensearch.security.DefaultObjectMapper;
+
+public class JsonFlattener {
+
+    public static Map<String, Object> flattenAsMap(String jsonString) {
+        try {
+            final TypeReference<Map<String, Object>> typeReference = new TypeReference<>() {
+            };
+            final Map<String, Object> jsonMap = DefaultObjectMapper.objectMapper.readValue(jsonString, typeReference);
+            final Map<String, Object> flattenMap = new LinkedHashMap<>();
+            flattenEntries("", jsonMap.entrySet(), flattenMap);
+            return flattenMap;
+        } catch (final IOException ioe) {
+            throw new IllegalArgumentException("Unparseable json", ioe);
+        }
+    }
+
+    private static void flattenEntries(String prefix, final Iterable<Map.Entry<String, Object>> entries, final Map<String, Object> result) {
+        if (!Strings.isNullOrEmpty(prefix)) {
+            prefix += ".";
+        }
+
+        for (final Map.Entry<String, Object> e : entries) {
+            flattenElement(prefix.concat(e.getKey()), e.getValue(), result);
+        }
+    }
+
+    @SuppressWarnings("unchecked")
+    private static void flattenElement(String prefix, final Object source, final Map<String, Object> result) {
+        if (source instanceof Iterable) {
+            flattenCollection(prefix, (Iterable<Object>) source, result);
+        }
+        if (source instanceof Map) {
+            flattenEntries(prefix, ((Map<String, Object>) source).entrySet(), result);
+        }
+        result.put(prefix, source);
+    }
+
+    private static void flattenCollection(String prefix, final Iterable<Object> objects, final Map<String, Object> result) {
+        int counter = 0;
+        for (final Object o : objects) {
+            flattenElement(prefix + "[" + counter + "]", o, result);
+            counter++;
+        }
+    }
+
+}