Kotlin: throw exception rather than returning null from getMappedRange()

In API review it was pointed out that Kotlin APIs should not use null
returns when a request is invalid.

Bug: 381392576
Test: BufferTest.*
Change-Id: Iaf9b15b1083d3c73ee6e26047339b4b0435fc357
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/218594
Commit-Queue: Alex Benton <[email protected]>
Reviewed-by: Alex Benton <[email protected]>

generator/templates/art/methods.cpp

@@ -42,7 +42,9 @@ namespace dawn::kotlin_api {
 
 jobject toByteBuffer(JNIEnv *env, const void* address, jlong size) {
     if (!address) {
-        return nullptr;
+      //* TODO(b/344805524): custom exception for Dawn.
+      env->ThrowNew(env->FindClass("java/lang/Error"), "Invalid byte buffer.");
+      return nullptr;
     }
     jclass byteBufferClass = env->FindClass("java/nio/ByteBuffer");
 

tools/android/webgpu/src/androidTest/java/android/dawn/BufferTest.kt

@@ -0,0 +1,44 @@
+package android.dawn
+
+import org.junit.Assert.assertThrows
+import org.junit.Test
+
+class BufferTest {
+    @Test
+    /**
+     * Test that calling getMappedRange() on a mapped buffer does not raise an exception.
+     */
+    fun bufferMapTest() {
+        dawnTestLauncher() { device ->
+            device.createBuffer(
+                BufferDescriptor(
+                    usage = BufferUsage.Vertex,
+                    size = 1024,
+                    mappedAtCreation = true
+                )
+            ).apply {
+                getMappedRange(size = size)
+            }
+        }
+    }
+
+    @Test
+    /**
+     * Test that calling getMappedRange() on a non-mapped buffer raises an exception.
+     */
+    fun bufferMapFailureTest() {
+        dawnTestLauncher() { device ->
+            assertThrows(Error::class.java) {
+                device.createBuffer(
+                    BufferDescriptor(
+                        usage = BufferUsage.Vertex,
+                        size = 1024,
+                        mappedAtCreation = false
+                    )
+                ).apply {
+                    getMappedRange(size = size)
+                }
+            }
+        }
+    }
+}