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Fix a bad race condition that causes intermittent socket receive failures #171

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Fix a bad race condition that causes intermittent socket receive failures #171

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13 changes: 11 additions & 2 deletions adafruit_wiznet5k/adafruit_wiznet5k_socketpool.py
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Original file line number Diff line number Diff line change
Expand Up @@ -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()
Expand All @@ -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,
):
Expand Down