Fix a bad race condition that causes intermittent socket receive

failures.

The issue is caused by the code checking socket status *after* checking
if bytes are available. Bytes may have become available between the
last byte available check and the socket status check. This causes us to
incorrectly ignore the newly available bytes and return 0 to the caller,
which in any blocking/timeout scenario tells the caller no more bytes
will ever be available.

This was introduced in commit 89b9f10
See also
adafruit#151
as a fix for
adafruit#135

Author: aggieNick02

adafruit_wiznet5k/adafruit_wiznet5k_socketpool.py

@@ -601,7 +601,14 @@ def recv_into(  # pylint: disable=unused-argument
                 self._buffer = self._buffer[bytes_to_read:]
                 # explicitly recheck num_to_read to avoid extra checks
                 continue
-
+            # We need to read the socket status here before seeing if any bytes are available.
+            # Why? Because otherwise we have a bad race condition in the if/elif/... logic below
+            # that can cause recv_into to fail when the other side closes the connection after
+            # sending bytes. The problem is that initially we can have num_avail=0 while the
+            # socket state is not in CLOSED or CLOSED_WAIT. Then after the if but before the elif
+            # that checks the socket state, bytes arrive and the other end closes the connection.
+            # So now bytes are available but we see the CLOSED/CLOSED_WAIT state and ignore them.
+            status_before_getting_available = self._status
             num_avail = self._available()
             if num_avail > 0:
                 last_read_time = ticks_ms()
@@ -620,7 +627,9 @@ def recv_into(  # pylint: disable=unused-argument
             elif num_read > 0:
                 # We got a message, but there are no more bytes to read, so we can stop.
                 break
-            elif self._status in (
+            # See note where we set status_before_getting_available for why we can't just check
+            # _status here
+            elif status_before_getting_available in (
                 wiznet5k.adafruit_wiznet5k.SNSR_SOCK_CLOSED,
                 wiznet5k.adafruit_wiznet5k.SNSR_SOCK_CLOSE_WAIT,
             ):