From 4d7afb8c36398afc1ff674ef50fedb70d8e39c2f Mon Sep 17 00:00:00 2001
From: Minh Vu <vuhoangminh97@gmail.com>
Date: Sun, 18 Oct 2026 17:41:26 +0200
Subject: [PATCH] Fix empty atomic intrusive queue prepend

---
 .../exec/detail/atomic_intrusive_queue.hpp    |  5 ++
 test/exec/CMakeLists.txt                      |  1 +
 test/exec/test_atomic_intrusive_queue.cpp     | 68 +++++++++++++++++++
 3 files changed, 74 insertions(+)
 create mode 100644 test/exec/test_atomic_intrusive_queue.cpp

diff --git a/include/exec/detail/atomic_intrusive_queue.hpp b/include/exec/detail/atomic_intrusive_queue.hpp
index b205a7aa97..3fd53d0403 100644
--- a/include/exec/detail/atomic_intrusive_queue.hpp
+++ b/include/exec/detail/atomic_intrusive_queue.hpp
@@ -65,6 +65,11 @@ namespace experimental::execution
 
     void prepend(STDEXEC::__intrusive_queue<_NextPtr> queue) noexcept
     {
+      if (queue.empty())
+      {
+        return;
+      }
+
       __node_pointer __new_head = queue.front();
       __node_pointer __tail     = queue.back();
       __node_pointer __old_head = __head_.load(STDEXEC::__std::memory_order_relaxed);
diff --git a/test/exec/CMakeLists.txt b/test/exec/CMakeLists.txt
index 1d967bd964..9489a47df5 100644
--- a/test/exec/CMakeLists.txt
+++ b/test/exec/CMakeLists.txt
@@ -17,6 +17,7 @@
 set(exec_test_sources
     ../test_main.cpp
     test_bwos_lifo_queue.cpp
+    test_atomic_intrusive_queue.cpp
     test_any_sender.cpp
     test_task.cpp
     test_timed_thread_scheduler.cpp
diff --git a/test/exec/test_atomic_intrusive_queue.cpp b/test/exec/test_atomic_intrusive_queue.cpp
new file mode 100644
index 0000000000..279ca90726
--- /dev/null
+++ b/test/exec/test_atomic_intrusive_queue.cpp
@@ -0,0 +1,68 @@
+/*
+ * Copyright (c) 2026 NVIDIA Corporation
+ *
+ * Licensed under the Apache License, Version 2.0 with LLVM Exceptions
+ * (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ *
+ *   https://llvm.org/LICENSE.txt
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include <catch2/catch_all.hpp>
+
+#include <exec/detail/atomic_intrusive_queue.hpp>
+
+#include <utility>
+
+namespace
+{
+  struct test_node
+  {
+    test_node* next_{nullptr};
+  };
+
+  using test_queue = STDEXEC::__intrusive_queue<&test_node::next_>;
+  using atomic_queue = exec::__atomic_intrusive_queue<&test_node::next_>;
+
+  TEST_CASE("atomic_intrusive_queue::prepend handles empty input", "[atomic_intrusive_queue]")
+  {
+    atomic_queue queue;
+    queue.prepend(test_queue{});
+    CHECK(queue.empty());
+
+    test_node existing{};
+    REQUIRE(queue.push_front(&existing));
+    queue.prepend(test_queue{});
+    CHECK_FALSE(queue.empty());
+
+    auto contents = queue.pop_all();
+    REQUIRE(contents.pop_front() == &existing);
+    CHECK(contents.empty());
+  }
+
+  TEST_CASE("atomic_intrusive_queue::prepend preserves queue order", "[atomic_intrusive_queue]")
+  {
+    atomic_queue queue;
+    test_node    existing{};
+    test_node    first{};
+    test_node    second{};
+    REQUIRE(queue.push_front(&existing));
+
+    test_queue incoming;
+    incoming.push_back(&first);
+    incoming.push_back(&second);
+    queue.prepend(std::move(incoming));
+
+    auto contents = queue.pop_all();
+    CHECK(contents.pop_front() == &first);
+    CHECK(contents.pop_front() == &second);
+    CHECK(contents.pop_front() == &existing);
+    CHECK(contents.empty());
+  }
+}  // namespace