esp32/modsocket: Correctly handle reading from a peer-closed socket.

If a socket is cleanly shut down by the peer then reads on this socket
should continue to return zero bytes.  The lwIP socket API does not have
this behaviour (it only returns zero once, then blocks on subsequent calls)
so this patch adds explicit checks and logic for peer closed sockets.

ports/esp32/modsocket.c

@@ -63,6 +63,7 @@ typedef struct _socket_obj_t {
     uint8_t domain;
     uint8_t type;
     uint8_t proto;
+    bool peer_closed;
     unsigned int retries;
     #if MICROPY_PY_USOCKET_EVENTS
     mp_obj_t events_callback;
@@ -233,6 +234,7 @@ STATIC mp_obj_t socket_accept(const mp_obj_t arg0) {
     sock->domain = self->domain;
     sock->type = self->type;
     sock->proto = self->proto;
+    sock->peer_closed = false;
     _socket_settimeout(sock, UINT64_MAX);
 
     // make the return value
@@ -354,23 +356,57 @@ STATIC mp_obj_t socket_setblocking(const mp_obj_t arg0, const mp_obj_t arg1) {
 }
 STATIC MP_DEFINE_CONST_FUN_OBJ_2(socket_setblocking_obj, socket_setblocking);
 
+// XXX this can end up waiting a very long time if the content is dribbled in one character
+// at a time, as the timeout resets each time a recvfrom succeeds ... this is probably not
+// good behaviour.
+STATIC mp_uint_t _socket_read_data(mp_obj_t self_in, void *buf, size_t size,
+    struct sockaddr *from, socklen_t *from_len, int *errcode) {
+    socket_obj_t *sock = MP_OBJ_TO_PTR(self_in);
+
+    // If the peer closed the connection then the lwIP socket API will only return "0" once
+    // from lwip_recvfrom_r and then block on subsequent calls.  To emulate POSIX behaviour,
+    // which continues to return "0" for each call on a closed socket, we set a flag when
+    // the peer closed the socket.
+    if (sock->peer_closed) {
+        return 0;
+    }
+
+    // XXX Would be nicer to use RTC to handle timeouts
+    for (int i = 0; i <= sock->retries; ++i) {
+        MP_THREAD_GIL_EXIT();
+        int r = lwip_recvfrom_r(sock->fd, buf, size, 0, from, from_len);
+        MP_THREAD_GIL_ENTER();
+        if (r == 0) {
+            sock->peer_closed = true;
+        }
+        if (r >= 0) {
+            return r;
+        }
+        if (errno != EWOULDBLOCK) {
+            *errcode = errno;
+            return MP_STREAM_ERROR;
+        }
+        check_for_exceptions();
+    }
+
+    *errcode = sock->retries == 0 ? MP_EWOULDBLOCK : MP_ETIMEDOUT;
+    return MP_STREAM_ERROR;
+}
+
 mp_obj_t _socket_recvfrom(mp_obj_t self_in, mp_obj_t len_in,
         struct sockaddr *from, socklen_t *from_len) {
-    socket_obj_t *sock = MP_OBJ_TO_PTR(self_in);
     size_t len = mp_obj_get_int(len_in);
     vstr_t vstr;
     vstr_init_len(&vstr, len);
 
-    // XXX Would be nicer to use RTC to handle timeouts
-    for (int i=0; i<=sock->retries; i++) {
-        MP_THREAD_GIL_EXIT();
-        int r = lwip_recvfrom_r(sock->fd, vstr.buf, len, 0, from, from_len);
-        MP_THREAD_GIL_ENTER();
-        if (r >= 0) { vstr.len = r; return mp_obj_new_str_from_vstr(&mp_type_bytes, &vstr); }
-        if (errno != EWOULDBLOCK) exception_from_errno(errno);
-        check_for_exceptions();
+    int errcode;
+    mp_uint_t ret = _socket_read_data(self_in, vstr.buf, len, from, from_len, &errcode);
+    if (ret == MP_STREAM_ERROR) {
+        exception_from_errno(errcode);
     }
-    mp_raise_OSError(MP_ETIMEDOUT);
+
+    vstr.len = ret;
+    return mp_obj_new_str_from_vstr(&mp_type_bytes, &vstr);
 }
 
 STATIC mp_obj_t socket_recv(mp_obj_t self_in, mp_obj_t len_in) {
@@ -468,25 +504,8 @@ STATIC mp_obj_t socket_makefile(size_t n_args, const mp_obj_t *args) {
 }
 STATIC MP_DEFINE_CONST_FUN_OBJ_VAR_BETWEEN(socket_makefile_obj, 1, 3, socket_makefile);
 
-
-// XXX this can end up waiting a very long time if the content is dribbled in one character
-// at a time, as the timeout resets each time a recvfrom succeeds ... this is probably not
-// good behaviour.
-
 STATIC mp_uint_t socket_stream_read(mp_obj_t self_in, void *buf, mp_uint_t size, int *errcode) {
-    socket_obj_t *sock = self_in;
-
-    // XXX Would be nicer to use RTC to handle timeouts
-    for (int i=0; i<=sock->retries; i++) {
-        MP_THREAD_GIL_EXIT();
-        int r = lwip_recvfrom_r(sock->fd, buf, size, 0, NULL, NULL);
-        MP_THREAD_GIL_ENTER();
-        if (r >= 0) return r;
-        if (r < 0 && errno != EWOULDBLOCK) { *errcode = errno; return MP_STREAM_ERROR; }
-        check_for_exceptions();
-    }
-    *errcode = sock->retries == 0 ? MP_EWOULDBLOCK : MP_ETIMEDOUT;
-    return MP_STREAM_ERROR;
+    return _socket_read_data(self_in, buf, size, NULL, NULL, errcode);
 }
 
 STATIC mp_uint_t socket_stream_write(mp_obj_t self_in, const void *buf, mp_uint_t size, int *errcode) {
@@ -592,6 +611,7 @@ STATIC mp_obj_t get_socket(size_t n_args, const mp_obj_t *args) {
     sock->domain = AF_INET;
     sock->type = SOCK_STREAM;
     sock->proto = 0;
+    sock->peer_closed = false;
     if (n_args > 0) {
         sock->domain = mp_obj_get_int(args[0]);
         if (n_args > 1) {