c++ – Boost::Asio async client – how to know the other side did TCP FIN?

I am using a boost::beast::tcp_stream which wraps around a boost::asio::basic_stream_socket for an asynchronous client.

No. Time           Source                Destination           Protocol Length Info
...
117 0.261388597    127.0.0.1             127.0.0.1             HTTP/JSON 125    HTTP/1.1 200 OK , JavaScript Object Notation (application/json)
119 0.261398240    127.0.0.1             127.0.0.1             TCP      66     43794 → 8090 [ACK] Seq=47 Ack=257 Win=65408 Len=0 TSval=4119348562 TSecr=4119348562
121 0.261485938    127.0.0.1             127.0.0.1             TCP      66     8090 → 43794 [FIN, ACK] Seq=257 Ack=47 Win=65536 Len=0 TSval=4119348562 TSecr=4119348562

122 0.273668580    127.0.0.1             127.0.0.1             HTTP     115    POST /api/clear HTTP/1.1 
123 0.273716187    127.0.0.1             127.0.0.1             TCP      54     8090 → 43794 [RST] Seq=258 Win=0 Len=0

The client sends a request to the server (not shown), and the server responds with HTTP 200 OK.

The client naturally returns an ACK.

The server is Python Flask, which has a habit of closing the connection even with HTTP 1.1, so it sends a FIN, ACK.

The client wishes to send the next HTTP request, and it definitely does a boost::asio::io_context::poll() to give it all the opportunity to receive this FIN from the server. The client then checks boost::beast::tcp_stream::socket().is_open(), and it falsely returns true, even though it has received the FIN from the server.

The only way I can see to check if the connection is closed is to attempt to write the next request, which the server complains about with an RST.

Surely there is a way to receive the FIN from the server?