TCPIP: fix stale accept futex notifications

A TCP child socket can become visible and accept-ready to `FreeRTOS_accept()`
before `on_tcp_connect()` increments the accept futex. If a waiter accepts the
child while the counter is zero, the old futex consume helper does not decrement
it to -1, because -1 represents an invalid futex. The delayed `on_tcp_connect`
callback then increments the counter to one, though no connection is pending.
This leaves pending multi-waiters with a misleading futex. They will repeatedly
treat the socket as ready, call `accept()`, and never block, defeating the purpose
of multi-waiter waiting.

Use a signed counter and always decrement after a successful accept. An early
accept records a temporary negative debt that the callback later repays. Reserve
INT32_MIN for an invalid futex so that normal negative debt cannot conflict with
the sentinel value.

Author: nding0405

include/NetAPI.h

@@ -51,7 +51,7 @@ enum SocketEventType : uint8_t
 static constexpr size_t NumFutexTypes = SocketEventType::SocketAcceptEvent + 1;
 /// Sentinel value stored in a socket event futex after the socket has been
 /// torn down.
-static constexpr uint32_t SocketNotAvailable = -1;
+static constexpr int32_t SocketNotAvailable = INT32_MIN;
 
 /**
  * Enumeration defining the connection type.

lib/tcpip/network_wrapper.cc

@@ -405,7 +405,7 @@ int SealedSocket::signal_event_futex(SocketEventType type)
 	{
 		return -EINVAL;
 	}
-	uint32_t current = futex.load();
+	int32_t current = futex.load();
 	while (current != SocketNotAvailable)
 	{
 		/*
@@ -427,8 +427,8 @@ int SealedSocket::signal_event_futex(SocketEventType type)
 int SealedSocket::consume_event_futex(SocketEventType type)
 {
 	auto    &futex   = eventFutexState[type];
-	uint32_t current = futex.load();
-	while (current != SocketNotAvailable && current != 0)
+	int32_t current = futex.load();
+	while (current != SocketNotAvailable)
 	{
 		if (futex.compare_exchange_strong(current, current - 1))
 		{

lib/tcpip/tcpip-internal.h

@@ -49,7 +49,7 @@ struct SealedSocket
 	 * feature. Different events increment different futexes in the array and
 	 * wake the corresponding waiting threads.
 	 */
-	std::atomic<uint32_t> eventFutexState[NumFutexTypes];
+	std::atomic<int32_t> eventFutexState[NumFutexTypes];
 	/**
 	 * Increments the futex and notifies all waiters if the futex is still
 	 * valid.