From 3e518861a0fdd0b743028344de904855808a216a Mon Sep 17 00:00:00 2001
From: Nikolai Kochetov <nik-kochetov@yandex-team.ru>
Date: Thu, 7 Oct 2021 12:23:22 +0300
Subject: [PATCH] Add coroutines example.

---
 src/Core/examples/CMakeLists.txt |   3 +
 src/Core/examples/coro.cpp       | 164 +++++++++++++++++++++++++++++++
 2 files changed, 167 insertions(+)
 create mode 100644 src/Core/examples/coro.cpp

diff --git a/src/Core/examples/CMakeLists.txt b/src/Core/examples/CMakeLists.txt
index 6b07dfbbfa6..c8846eb1743 100644
--- a/src/Core/examples/CMakeLists.txt
+++ b/src/Core/examples/CMakeLists.txt
@@ -13,3 +13,6 @@ target_link_libraries (mysql_protocol PRIVATE dbms)
 if(USE_SSL)
     target_include_directories (mysql_protocol SYSTEM PRIVATE ${OPENSSL_INCLUDE_DIR})
 endif()
+
+add_executable (coro coro.cpp)
+target_link_libraries (coro PRIVATE clickhouse_common_io)
diff --git a/src/Core/examples/coro.cpp b/src/Core/examples/coro.cpp
new file mode 100644
index 00000000000..20525a1da3b
--- /dev/null
+++ b/src/Core/examples/coro.cpp
@@ -0,0 +1,164 @@
+#include <iostream>
+
+#include <cassert>
+#include <iostream>
+#include <experimental/coroutine>
+#include <string>
+#include <optional>
+
+#include <Common/Exception.h>
+
+template <typename T>
+using coroutine_handle = std::experimental::coroutine_handle<T>;
+
+using default_coroutine_handle = std::experimental::coroutine_handle<>;
+
+using suspend_never = std::experimental::suspend_never;
+using suspend_always = std::experimental::suspend_always;
+
+template <typename T>
+struct suspend_never_val
+{
+    constexpr bool await_ready() const noexcept { return true; }
+    constexpr void await_suspend(default_coroutine_handle) const noexcept {}
+    constexpr T await_resume() const noexcept
+    {
+        std::cout << "  ret " << val << std::endl;
+        return val;
+    }
+
+    T val;
+};
+
+template <typename T>
+struct resumable {
+  struct promise_type {
+    using coro_handle = coroutine_handle<promise_type>;
+    auto get_return_object() { return coro_handle::from_promise(*this); }
+    auto initial_suspend() { return suspend_never(); }
+    auto final_suspend() noexcept { return suspend_never_val<T>{*r->value}; }
+    //void return_void() {}
+    void return_value(T value_) { r->value = value_; }
+    void unhandled_exception() { r->exception = std::current_exception(); }
+
+    explicit promise_type(std::string tag_) : tag(tag_) {}
+    ~promise_type() { std::cout << "~promise_type " << tag << std::endl; }
+    std::string tag;
+    coro_handle next;
+    resumable * r = nullptr;
+  };
+
+  using coro_handle = coroutine_handle<promise_type>;
+
+  bool await_ready() const noexcept { return false; }
+  void await_suspend(coro_handle g) noexcept
+  {
+      std::cout << "  await_suspend " << my.promise().tag << std::endl;
+      std::cout << "  g tag " << g.promise().tag << std::endl;
+      g.promise().next = my;
+  }
+  T await_resume() noexcept
+  {
+      std::cout << "  await_res " << my.promise().tag << std::endl;
+    return *value;
+  }
+
+  resumable(coro_handle handle) : my(handle), tag(handle.promise().tag)
+  {
+      assert(handle);
+      my.promise().r = this;
+      std::cout << "    resumable " << tag << std::endl;
+  }
+  resumable(resumable &) = delete;
+  resumable(resumable &&rhs) : my(rhs.my), tag(rhs.tag)
+  {
+      rhs.my = {};
+      std::cout << "    resumable&& " << tag << std::endl;
+  }
+  static bool resume_impl(resumable *r)
+  {
+      if (r->value)
+        return false;
+
+      auto & next = r->my.promise().next;
+
+      if (next)
+      {
+          if (resume_impl(next.promise().r))
+            return true;
+          next = {};
+      }
+
+      if (!r->value)
+      {
+        r->my.resume();
+        if (r->exception)
+            std::rethrow_exception(r->exception);
+      }
+      return !r->value;
+  }
+
+  bool resume() {
+      return resume_impl(this);
+  }
+
+  T res()
+  {
+      return *value;
+  }
+
+  ~resumable() {
+      std::cout << "    ~resumable " << tag << std::endl;
+  }
+
+private:
+  coro_handle my;
+  std::string tag;
+  std::optional<T> value;
+  std::exception_ptr exception;
+};
+
+resumable<int> boo(std::string tag)
+{
+    std::cout << "x" << std::endl;
+    co_await suspend_always();
+    std::cout << StackTrace().toString();
+    std::cout << "y" << std::endl;
+    co_return 1;
+}
+
+resumable<int> bar(std::string tag)
+{
+    std::cout << "a" << std::endl;
+    int res1 = co_await boo("boo1");
+    std::cout << "b " << res1 << std::endl;
+    int res2 = co_await boo("boo2");
+    if (res2 == 1)
+        throw DB::Exception(1, "hello");
+    std::cout << "c " << res2 << std::endl;
+    co_return res1 + res2;  // 1 + 1 = 2
+}
+
+resumable<int> foo(std::string tag) {
+  std::cout << "Hello" << std::endl;
+  auto res1 = co_await bar("bar1");
+  std::cout << "Coro " << res1 << std::endl;
+  auto res2 = co_await bar("bar2");
+  std::cout << "World " << res2 << std::endl;
+  co_return res1 * res2; // 2 * 2 = 4
+}
+
+int main()
+try
+{
+  auto t = foo("foo");
+  std::cout << ".. started" << std::endl;
+  while (t.resume())
+    std::cout << ".. yielded" << std::endl;
+  std::cout << ".. done: " << t.res() << std::endl;
+}
+catch (DB::Exception & e)
+{
+    std::cout << "Got exception " << e.what() << std::endl;
+    std::cout << e.getStackTraceString() << std::endl;
+}