grpc-native: Add callback future

Signed-off-by: Johannes Zottele <[email protected]>

Author: Jozott00

grpc/grpc-core/src/nativeMain/kotlin/kotlinx/rpc/grpc/internal/CallbackFuture.kt

@@ -0,0 +1,29 @@
+/*
+ * Copyright 2023-2025 JetBrains s.r.o and contributors. Use of this source code is governed by the Apache 2.0 license.
+ */
+
+package kotlinx.rpc.grpc.internal
+
+import kotlinx.atomicfu.atomic
+
+internal class CallbackFuture<T : Any> {
+    private val value = atomic<T?>(null)
+    private val callback = atomic<((T) -> Unit)?>(null)
+
+    fun complete(result: T) {
+        if (value.compareAndSet(null, result)) {
+            callback.getAndSet(null)?.invoke(result)
+        } else {
+            error("Already completed")
+        }
+    }
+
+    fun onComplete(cb: (T) -> Unit) {
+        val r = value.value
+        if (r != null) cb(r)
+        else if (!callback.compareAndSet(null, cb)) {
+            // Already someone registered → run immediately
+            value.value?.let(cb)
+        }
+    }
+}