CORDA-2782 allow un-whitelisted Comparable (#4940)

serialization/src/main/kotlin/net/corda/serialization/internal/amqp/SerializationHelper.kt

@@ -122,10 +122,9 @@ internal enum class CommonPropertyNames {
     IncludeInternalInfo,
 }
 
-
-
 fun ClassWhitelist.requireWhitelisted(type: Type) {
-    if (!this.isWhitelisted(type.asClass())) {
+    // See CORDA-2782 for explanation of the special exemption made for Comparable
+    if (!this.isWhitelisted(type.asClass()) && type.asClass() != java.lang.Comparable::class.java) {
         throw AMQPNotSerializableException(
                 type,
                 "Class \"$type\" is not on the whitelist or annotated with @CordaSerializable.")

serialization/src/test/kotlin/net/corda/serialization/internal/amqp/DeserializeSimpleTypesTests.kt

@@ -583,6 +583,35 @@ class DeserializeSimpleTypesTests {
         }
     }
 
+    // See CORDA-2782
+    @Test
+    fun comparableNotWhitelistedOk() {
+        @CordaSerializable
+        class Ok(val value: String) : java.lang.Comparable<Ok> {
+            override fun compareTo(o: Ok?): Int = value.compareTo(o?.value ?: "")
+        }
+
+        class NotOk(val value: String) : java.lang.Comparable<NotOk> {
+            override fun compareTo(o: NotOk?): Int = value.compareTo(o?.value ?: "")
+        }
+
+        @CordaSerializable
+        class OkComparable(val value: java.lang.Comparable<Ok>)
+
+        @CordaSerializable
+        class NotOkComparable(val value: java.lang.Comparable<NotOk>)
+
+        val factory = testDefaultFactoryWithWhitelist()
+
+        TestSerializationOutput(VERBOSE, factory).serialize(OkComparable(Ok("value")))
+        assertFailsWithMessage(
+                "Class \"class ${NotOk::class.java.name}\" " +
+                        "is not on the whitelist or annotated with @CordaSerializable.") {
+            TestSerializationOutput(VERBOSE, factory).serialize(NotOkComparable(NotOk("value")))
+        }
+
+    }
+
     private fun assertFailsWithMessage(expectedMessage: String, block: () -> Unit) {
         try {
             block()