windows 下写文件无法追加内容

[osabc]

coro_io::coro_file file{};
file.open("test.txt", std::ios::app);
co_await file.async_write("test");

[qicosmos]

windows 上work around 一下，创建coro_file 的时候设置为thread pool：

coro_io::basic_seq_coro_file<coro_io::execution_type::thread_pool> file{};
file.open("test.txt", std::ios::app);
co_await file.async_write("test");

[osabc]

解决方法，修改asio源码

diff --git "a/include/asio/detail/impl/win_iocp_file_service.ipp" "b/include/asio/detail/impl/win_iocp_file_service.ipp"
index a1a7503..a9cc87a 100644
--- "a/include/asio/detail/impl/win_iocp_file_service.ipp"
+++ "b/include/asio/detail/impl/win_iocp_file_service.ipp"
@@ -67,6 +67,8 @@ asio::error_code win_iocp_file_service::open(
     access = GENERIC_WRITE;
   else if ((open_flags & file_base::read_write) != 0)
     access = GENERIC_READ | GENERIC_WRITE;
+  else if ((open_flags & file_base::append) != 0)
+    access = GENERIC_WRITE;
 
   DWORD share = FILE_SHARE_READ | FILE_SHARE_WRITE;
 
@@ -82,6 +84,8 @@ asio::error_code win_iocp_file_service::open(
   {
     if ((open_flags & file_base::truncate) != 0)
       disposition = TRUNCATE_EXISTING;
+    else if ((open_flags & file_base::append) != 0)
+      disposition = OPEN_ALWAYS;
     else
       disposition = OPEN_EXISTING;
   }
@@ -94,6 +98,7 @@ asio::error_code win_iocp_file_service::open(
   if ((open_flags & file_base::sync_all_on_write) != 0)
     flags |= FILE_FLAG_WRITE_THROUGH;
 
+  impl.offset_ = 0;
   HANDLE handle = ::CreateFileA(path, access, share, 0, disposition, flags, 0);
   if (handle != INVALID_HANDLE_VALUE)
   {
@@ -109,10 +114,24 @@ asio::error_code win_iocp_file_service::open(
       }
     }
 
+    if (disposition == OPEN_ALWAYS || (open_flags & file_base::append) != 0) {
+      LARGE_INTEGER distance, new_offset;
+      distance.QuadPart = 0;
+      if (::SetFilePointerEx(handle, distance, &new_offset, FILE_END)) {
+        impl.offset_ = static_cast<uint64_t>(new_offset.QuadPart);
+      }
+      else {
+        DWORD last_error = ::GetLastError();
+        ::CloseHandle(handle);
+        ec.assign(last_error, asio::error::get_system_category());
+        ASIO_ERROR_LOCATION(ec);
+        return ec;
+      }
+    }
+
     handle_service_.assign(impl, handle, ec);
     if (ec)
       ::CloseHandle(handle);
-    impl.offset_ = 0;
     ASIO_ERROR_LOCATION(ec);
     return ec;
   }

[qicosmos]

或者你在asio 这个pr：chriskohlhoff/asio#1500
里回复一下正确的解决方法。

[qicosmos]

再发一个diff的截图吧。