lwIP sockets: a failed listen or connect leaves the socket lock held, a later close hangs forever

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Assessment

Difficulty
2/5
Estimated time
1-3 hours
Newbie friendliness
84/100
Issue type
Bug
Clarity
Clearly specified
Activity status
Active
Tech stack
c, erlang
Domain
embedded-iot

Research direction

Start in src/libAtomVM/otp_socket.c and read nif_socket_listen and the lwIP branch of nif_socket_connect. Check how #2345 handles the socket lock on the tcp_close error path, then reproduce the failed listen or connect on an SMP RP2 build. Done when each error path releases the read lock and socket:close returns after the failure.

Written by the indexing model from the issue text.

Description

Affected: release-0.7 at ca1583068, lwIP socket backend (RP2 port), SMP builds
(the default on RP2). Not changed by #2345.

What happens

nif_socket_listen and nif_socket_connect take the socket read lock, then
return on an lwIP error without releasing it:

  • nif_socket_listen: if (new_pcb == NULL) { return make_lwip_err_tuple(err, ctx); }
  • nif_socket_connect (lwIP branch): if (err != ERR_OK) { ... return {error, closed} / RAISE_ERROR(...) }

The read lock is never released. Every later operation that takes the write lock
on that socket blocks forever and pins a scheduler thread: socket:close/1,
socket:recv/1,2,3, socket:accept/1,2, socket:send/2 (through the select
NIFs) and the monitor callback socket_down.

How to reproduce (Pico W)

Connect, with the network down or no route to the address (tcp_connect fails
with ERR_RTE):

{ok, S} = socket:open(inet, stream, tcp),
{error, closed} = socket:connect(S, #{family => inet, addr => {192, 168, 0, 1}, port => 80}),
ok = socket:close(S).   % never returns

Listen on a socket that is already connected (tcp_listen fails with ERR_CLSD):

{error, _} = socket:listen(ConnectedSocket),
ok = socket:close(ConnectedSocket).   % never returns

Where

src/libAtomVM/otp_socket.c, nif_socket_listen and nif_socket_connect,
#elif OTP_SOCKET_LWIP branches.

Suggested fix

SMP_RWLOCK_UNLOCK(rsrc_obj->socket_lock) before returning on both error
paths, as #2345 did for the failing tcp_close in nif_socket_close.

Dominant language
Erlang
Stars
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Forks
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Avg merge
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Merged PRs (30d)
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