Ondřej Surý authored Feb 08, 2022 and Michał Kępień committed Mar 16, 2022

When sock->closehandle_cb is set, we need to run nmhandle_detach_cb()
asynchronously to ensure correct order of multiple packets processing in
the isc__nm_process_sock_buffer().  When not run asynchronously, it
would cause:

  a) out-of-order processing of the return codes from processbuffer();

  b) stack growth because the next TCP DNS message read callback will
     be called from within the current TCP DNS message read callback.

The sock->closehandle_cb is set to isc__nm_resume_processing() for TCP
sockets which calls isc__nm_process_sock_buffer().  If the read callback
(called from isc__nm_process_sock_buffer()->processbuffer()) doesn't
attach to the nmhandle (f.e. because it wants to drop the processing or
we send the response directly via uv_try_write()), the
isc__nm_resume_processing() (via .closehandle_cb) would call
isc__nm_process_sock_buffer() recursively.

The below shortened code path shows how the stack can grow:

 1: ns__client_request(handle, ...);
 2: isc_nm_tcpdns_sequential(handle);
 3: ns_query_start(client, handle);
 4:   query_lookup(qctx);
 5:     query_send(qctcx->client);
 6:       isc__nmhandle_detach(&client->reqhandle);
 7:         nmhandle_detach_cb(&handle);
 8:           sock->closehandle_cb(sock); // isc__nm_resume_processing
 9:             isc__nm_process_sock_buffer(sock);
10:               processbuffer(sock); // isc__nm_tcpdns_processbuffer
11:                 isc_nmhandle_attach(req->handle, &handle);
12:                 isc__nm_readcb(sock, req, ISC_R_SUCCESS);
13:                   isc__nm_async_readcb(NULL, ...);
14:                     uvreq->cb.recv(...); // ns__client_request

Instead, if 'sock->closehandle_cb' is set, we need to run detach the
handle asynchroniously in 'isc__nmhandle_detach', so that on line 8 in
the code flow above does not start this recursion. This ensures the
correct order when processing multiple packets in the function
'isc__nm_process_sock_buffer()' and prevents the stack growth.

When not run asynchronously, the out-of-order processing leaves the
first TCP socket open until all requests on the stream have been
processed.

If the pipelining is disabled on the TCP via `keep-response-order`
configuration option, named would keep the first socket in lingering
CLOSE_WAIT state when the client sends an incomplete packet and then
closes the connection from the client side.