add coroutine file test

examples/linux/file_coroutines_test.cpp

@@ -0,0 +1,62 @@
+/*
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * 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 <unifex/config.hpp>
+
+// requires both coroutine and liburing support (for now)
+#if !UNIFEX_NO_COROUTINES and !UNIFEX_NO_LIBURING
+#  include <unifex/linux/io_uring_context.hpp>
+using io_context = unifex::linuxos::io_uring_context;
+
+#  include <unifex/sync_wait.hpp>
+#  include <unifex/task.hpp>
+
+using namespace unifex;
+
+int main() {
+  io_context ctx{};
+  auto sched = ctx.get_scheduler();
+  inplace_stop_source stopSource;
+  std::thread t{[&] {
+    ctx.run(stopSource.get_token());
+  }};
+  scope_guard stopOnExit = [&]() noexcept {
+    stopSource.request_stop();
+    t.join();
+  };
+  sync_wait([&]() -> task<void> {
+    auto this_file = open_file_read_only(sched, filesystem::path{__FILE__});
+    auto output_file = open_file_write_only(
+        sched, filesystem::path{"file_coroutine_test_copy.cpp"});
+    std::array<std::byte, 32> buffer{};
+    size_t offset = 0;
+    while (size_t read_bytes = co_await async_read_some_at(
+               this_file, offset, as_writable_bytes(span{buffer}))) {
+      co_await async_write_some_at(
+          output_file, offset, as_bytes(span{buffer.data(), read_bytes}));
+      offset += read_bytes;
+    }
+    std::printf("copied %zu bytes\n", offset);
+  }());
+  return 0;
+}
+#else
+#  include <cstdio>
+int main() {
+  printf("neither io_ring or coroutine support found\n");
+  return 0;
+}
+#endif