- Updates DslList and ExtensionList to take a lazy supplier lambda () -> List<E>, retaining secondary constructors for binary backward compatibility with existing compiled proto code.

PiperOrigin-RevId: 966853174

java/kotlin/src/main/kotlin/com/google/protobuf/DslList.kt

@@ -30,6 +30,10 @@
 
 package com.google.protobuf.kotlin
 
+private val UNINITIALIZED_SUPPLIER: () -> Nothing = {
+  throw IllegalStateException("Should not be called")
+}
+
 /**
  * A simple wrapper around a [List] with an extra generic parameter that can be used to disambiguate
  * extension methods.
@@ -41,7 +45,42 @@ package com.google.protobuf.kotlin
 @Suppress("unused") // the unused type parameter
 class DslList<E, P : DslProxy>
 @OnlyForUseByGeneratedProtoCode
-constructor(private val delegate: List<E>) : List<E> by delegate {
+constructor(private val delegateSupplier: () -> List<E>) : List<E> {
+  @Suppress("UNCHECKED_CAST")
+  @OnlyForUseByGeneratedProtoCode
+  constructor(delegate: List<E>) : this(UNINITIALIZED_SUPPLIER as () -> List<E>) {
+    memoizedDelegate = delegate
+  }
+
+  private var memoizedDelegate: List<E>? = null
+
+  private val delegate: List<E>
+    get() {
+      var result = memoizedDelegate
+      if (result == null) {
+        result = delegateSupplier()
+        memoizedDelegate = result
+      }
+      return result
+    }
+
+  override val size: Int
+    get() = delegate.size
+
+  override fun isEmpty(): Boolean = delegate.isEmpty()
+
+  override fun contains(element: E): Boolean = delegate.contains(element)
+
+  override fun containsAll(elements: Collection<E>): Boolean = delegate.containsAll(elements)
+
+  override fun get(index: Int): E = delegate[index]
+
+  override fun indexOf(element: E): Int = delegate.indexOf(element)
+
+  override fun lastIndexOf(element: E): Int = delegate.lastIndexOf(element)
+
+  override fun subList(fromIndex: Int, toIndex: Int): List<E> = delegate.subList(fromIndex, toIndex)
+
   override fun iterator(): Iterator<E> = UnmodifiableIterator(delegate.iterator())
 
   override fun listIterator(): ListIterator<E> = UnmodifiableListIterator(delegate.listIterator())

java/kotlin/src/main/kotlin/com/google/protobuf/ExtensionList.kt

@@ -33,6 +33,10 @@ package com.google.protobuf.kotlin
 import com.google.protobuf.ExtensionLite
 import com.google.protobuf.MessageLite
 
+private val UNINITIALIZED_SUPPLIER: () -> Nothing = {
+  throw IllegalStateException("Should not be called")
+}
+
 /**
  * Implementation for ExtensionList and ExtensionListLite. Like [DslList], represents an
  * unmodifiable view of a repeated proto field -- in this case, an extension field -- but supports
@@ -40,8 +44,48 @@ import com.google.protobuf.MessageLite
  */
 class ExtensionList<E, M : MessageLite>
 @OnlyForUseByGeneratedProtoCode
-constructor(val extension: ExtensionLite<M, List<E>>, private val delegate: List<E>) :
-  List<E> by delegate {
+constructor(
+  val extension: ExtensionLite<M, List<E>>,
+  private val delegateSupplier: () -> List<E>,
+) : List<E> {
+  @Suppress("UNCHECKED_CAST")
+  @OnlyForUseByGeneratedProtoCode
+  constructor(
+    extension: ExtensionLite<M, List<E>>,
+    delegate: List<E>,
+  ) : this(extension, UNINITIALIZED_SUPPLIER as () -> List<E>) {
+    memoizedDelegate = delegate
+  }
+
+  private var memoizedDelegate: List<E>? = null
+
+  private val delegate: List<E>
+    get() {
+      var result = memoizedDelegate
+      if (result == null) {
+        result = delegateSupplier()
+        memoizedDelegate = result
+      }
+      return result
+    }
+
+  override val size: Int
+    get() = delegate.size
+
+  override fun isEmpty(): Boolean = delegate.isEmpty()
+
+  override fun contains(element: E): Boolean = delegate.contains(element)
+
+  override fun containsAll(elements: Collection<E>): Boolean = delegate.containsAll(elements)
+
+  override fun get(index: Int): E = delegate[index]
+
+  override fun indexOf(element: E): Int = delegate.indexOf(element)
+
+  override fun lastIndexOf(element: E): Int = delegate.lastIndexOf(element)
+
+  override fun subList(fromIndex: Int, toIndex: Int): List<E> = delegate.subList(fromIndex, toIndex)
+
   override fun iterator(): Iterator<E> = UnmodifiableIterator(delegate.iterator())
 
   override fun listIterator(): ListIterator<E> = UnmodifiableListIterator(delegate.listIterator())

java/kotlin/src/test/kotlin/com/google/protobuf/DslListTest.kt

@@ -85,13 +85,59 @@ class DslListTest {
   @Test
   fun equality() {
     EqualsTester()
-      .addEqualityGroup(DslList<Int, DummyProxy>(listOf(1, 2)), listOf(1, 2))
-      .addEqualityGroup(DslList<Int, DummyProxy>(listOf(2, 2)), listOf(2, 2))
+      .addEqualityGroup(
+        DslList<Int, DummyProxy>(listOf(1, 2)),
+        DslList<Int, DummyProxy> { listOf(1, 2) },
+        listOf(1, 2),
+      )
+      .addEqualityGroup(
+        DslList<Int, DummyProxy>(listOf(2, 2)),
+        DslList<Int, DummyProxy> { listOf(2, 2) },
+        listOf(2, 2),
+      )
       .addEqualityGroup(
         DslList<Int, DummyProxy>(emptyList()),
         DslList<String, DummyProxy>(emptyList()),
+        DslList<Int, DummyProxy> { emptyList() },
+        DslList<String, DummyProxy> { emptyList() },
         emptyList<Int>(),
       )
       .testEquals()
   }
+
+  @Test
+  fun supplierNotInvokedOnConstruction() {
+    var supplierCalled = false
+    val dslList = DslList<Int, DummyProxy> {
+      supplierCalled = true
+      listOf(1, 2, 3)
+    }
+    assertThat(supplierCalled).isFalse()
+    assertThat(dslList).containsExactly(1, 2, 3).inOrder()
+    assertThat(supplierCalled).isTrue()
+  }
+
+  @Test
+  fun supplierEvaluatedOnlyOnFirstReadOperation() {
+    var callCount = 0
+    val dslList = DslList<Int, DummyProxy> {
+      callCount++
+      listOf(1, 2, 1)
+    }
+    assertThat(callCount).isEqualTo(0)
+    assertThat(dslList.size).isEqualTo(3)
+    assertThat(callCount).isEqualTo(1)
+    assertThat(dslList[0]).isEqualTo(1)
+    assertThat(callCount).isEqualTo(1)
+    assertThat(dslList.isEmpty()).isFalse()
+    assertThat(dslList.contains(2)).isTrue()
+    assertThat(dslList.containsAll(listOf(1, 2))).isTrue()
+    assertThat(dslList.indexOf(1)).isEqualTo(0)
+    assertThat(dslList.lastIndexOf(1)).isEqualTo(2)
+    assertThat(dslList.subList(0, 2)).containsExactly(1, 2).inOrder()
+    assertThat(dslList.iterator().hasNext()).isTrue()
+    assertThat(dslList.listIterator().hasNext()).isTrue()
+    assertThat(dslList.listIterator(1).next()).isEqualTo(2)
+    assertThat(callCount).isEqualTo(1)
+  }
 }

java/kotlin/src/test/kotlin/com/google/protobuf/ExtensionListTest.kt

@@ -118,17 +118,71 @@ class ExtensionListTest {
     EqualsTester()
       .addEqualityGroup(
         ExtensionList<Int, ExampleExtensibleMessage>(TestProto.repeatedExtension, listOf(1, 2)),
+        ExtensionList<Int, ExampleExtensibleMessage>(TestProto.repeatedExtension) { listOf(1, 2) },
         ExtensionList<Int, ExampleExtensibleMessage>(TestProto.differentExtension, listOf(1, 2)),
+        ExtensionList<Int, ExampleExtensibleMessage>(TestProto.differentExtension) { listOf(1, 2) },
         listOf(1, 2),
       )
       .addEqualityGroup(
         ExtensionList<Int, ExampleExtensibleMessage>(TestProto.repeatedExtension, listOf(2, 2)),
+        ExtensionList<Int, ExampleExtensibleMessage>(TestProto.repeatedExtension) { listOf(2, 2) },
         listOf(2, 2),
       )
       .addEqualityGroup(
         ExtensionList<Int, ExampleExtensibleMessage>(TestProto.repeatedExtension, emptyList()),
+        ExtensionList<Int, ExampleExtensibleMessage>(TestProto.repeatedExtension) { emptyList() },
         emptyList<Int>(),
       )
       .testEquals()
   }
+
+  @Test
+  fun supplierNotInvokedOnConstruction() {
+    var supplierCalled = false
+    val extensionList =
+      ExtensionList<Int, ExampleExtensibleMessage>(TestProto.repeatedExtension) {
+        supplierCalled = true
+        listOf(1, 2, 3)
+      }
+    assertThat(supplierCalled).isFalse()
+    assertThat(extensionList).containsExactly(1, 2, 3).inOrder()
+    assertThat(supplierCalled).isTrue()
+  }
+
+  @Test
+  fun extensionPropertyAccessDoesNotInvokeSupplier() {
+    var supplierCalled = false
+    val extensionList =
+      ExtensionList<Int, ExampleExtensibleMessage>(TestProto.repeatedExtension) {
+        supplierCalled = true
+        listOf(1, 2, 3)
+      }
+    assertThat(extensionList.extension).isEqualTo(TestProto.repeatedExtension)
+    assertThat(supplierCalled).isFalse()
+  }
+
+  @Test
+  fun supplierEvaluatedOnlyOnFirstReadOperation() {
+    var callCount = 0
+    val extensionList =
+      ExtensionList<Int, ExampleExtensibleMessage>(TestProto.repeatedExtension) {
+        callCount++
+        listOf(1, 2, 1)
+      }
+    assertThat(callCount).isEqualTo(0)
+    assertThat(extensionList.size).isEqualTo(3)
+    assertThat(callCount).isEqualTo(1)
+    assertThat(extensionList[0]).isEqualTo(1)
+    assertThat(callCount).isEqualTo(1)
+    assertThat(extensionList.isEmpty()).isFalse()
+    assertThat(extensionList.contains(2)).isTrue()
+    assertThat(extensionList.containsAll(listOf(1, 2))).isTrue()
+    assertThat(extensionList.indexOf(1)).isEqualTo(0)
+    assertThat(extensionList.lastIndexOf(1)).isEqualTo(2)
+    assertThat(extensionList.subList(0, 2)).containsExactly(1, 2).inOrder()
+    assertThat(extensionList.iterator().hasNext()).isTrue()
+    assertThat(extensionList.listIterator().hasNext()).isTrue()
+    assertThat(extensionList.listIterator(1).next()).isEqualTo(2)
+    assertThat(callCount).isEqualTo(1)
+  }
 }