pseudotcp: Factor out state changes and validate them

By validating state changes, we can verify that the implementation
conforms to the RFC 793 state machine at runtime. This will become more
important when FIN–ACK support is added, adding more states.

diff --git a/agent/pseudotcp.c b/agent/pseudotcp.c
index 00650ef..05fee3a 100644 (file)
--- a/agent/pseudotcp.c
+++ b/agent/pseudotcp.c
@@ -511,6 +511,7 @@ static void parse_options (PseudoTcpSocket *self, const guint8 *data,
     guint32 len);
 static void resize_send_buffer (PseudoTcpSocket *self, guint32 new_size);
 static void resize_receive_buffer (PseudoTcpSocket *self, guint32 new_size);
+static void set_state (PseudoTcpSocket *self, PseudoTcpState new_state);
 
 
 // The following logging is for detailed (packet-level) pseudotcp analysis only.
@@ -783,8 +784,7 @@ pseudo_tcp_socket_connect(PseudoTcpSocket *self)
     return FALSE;
   }
 
-  priv->state = TCP_SYN_SENT;
-  DEBUG (PSEUDO_TCP_DEBUG_NORMAL, "State: TCP_SYN_SENT");
+  set_state (self, TCP_SYN_SENT);
 
   queue_connect_message (self);
   attempt_send(self, sfNone);
@@ -1246,11 +1246,10 @@ process(PseudoTcpSocket *self, Segment *seg)
       parse_options (self, (guint8 *) &seg->data[1], seg->len - 1);
 
       if (priv->state == TCP_LISTEN) {
-        priv->state = TCP_SYN_RECEIVED;
+        set_state (self, TCP_SYN_RECEIVED);
         queue_connect_message (self);
       } else if (priv->state == TCP_SYN_SENT) {
-        priv->state = TCP_ESTABLISHED;
-        DEBUG (PSEUDO_TCP_DEBUG_NORMAL, "State: TCP_ESTABLISHED");
+        set_state (self, TCP_ESTABLISHED);
         adjustMTU(self);
         if (priv->callbacks.PseudoTcpOpened)
           priv->callbacks.PseudoTcpOpened(self, priv->callbacks.user_data);
@@ -1384,8 +1383,7 @@ process(PseudoTcpSocket *self, Segment *seg)
 
   // !?! A bit hacky
   if ((priv->state == TCP_SYN_RECEIVED) && !bConnect) {
-    priv->state = TCP_ESTABLISHED;
-    DEBUG (PSEUDO_TCP_DEBUG_NORMAL, "State: TCP_ESTABLISHED");
+    set_state (self, TCP_ESTABLISHED);
     adjustMTU(self);
     if (priv->callbacks.PseudoTcpOpened)
       priv->callbacks.PseudoTcpOpened(self, priv->callbacks.user_data);
@@ -1713,8 +1711,7 @@ closedown(PseudoTcpSocket *self, guint32 err)
 {
   PseudoTcpSocketPrivate *priv = self->priv;
 
-  DEBUG (PSEUDO_TCP_DEBUG_NORMAL, "State: TCP_CLOSED");
-  priv->state = TCP_CLOSED;
+  set_state (self, TCP_CLOSED);
   if (priv->callbacks.PseudoTcpClosed)
     priv->callbacks.PseudoTcpClosed(self, err, priv->callbacks.user_data);
 }
@@ -1908,3 +1905,56 @@ pseudo_tcp_socket_get_available_send_space (PseudoTcpSocket *self)
 
   return ret;
 }
+
+/* State names are capitalised and formatted as in RFC 793. */
+static const gchar *
+pseudo_tcp_state_get_name (PseudoTcpState state)
+{
+  switch (state) {
+  case TCP_LISTEN: return "LISTEN";
+  case TCP_SYN_SENT: return "SYN-SENT";
+  case TCP_SYN_RECEIVED: return "SYN-RECEIVED";
+  case TCP_ESTABLISHED: return "ESTABLISHED";
+  case TCP_CLOSED: return "CLOSED";
+  default: return "UNKNOWN";
+  }
+}
+
+static void
+set_state (PseudoTcpSocket *self, PseudoTcpState new_state)
+{
+  PseudoTcpSocketPrivate *priv = self->priv;
+  PseudoTcpState old_state = priv->state;
+
+  if (new_state == old_state)
+    return;
+
+  DEBUG (PSEUDO_TCP_DEBUG_NORMAL, "State %s → %s.",
+      pseudo_tcp_state_get_name (old_state),
+      pseudo_tcp_state_get_name (new_state));
+
+  /* Check whether it’s a valid state transition. */
+#define TRANSITION(OLD, NEW) \
+    (old_state == TCP_##OLD && \
+     new_state == TCP_##NEW)
+
+  /* Valid transitions. See: RFC 793, p23. */
+  g_assert (TRANSITION (CLOSED, SYN_SENT) ||
+            TRANSITION (SYN_SENT, CLOSED) ||
+            TRANSITION (CLOSED, LISTEN) ||
+            TRANSITION (LISTEN, CLOSED) ||
+            TRANSITION (LISTEN, SYN_SENT) ||
+            TRANSITION (LISTEN, SYN_RECEIVED) ||
+            TRANSITION (SYN_SENT, SYN_RECEIVED) ||
+            TRANSITION (SYN_RECEIVED, ESTABLISHED) ||
+            TRANSITION (SYN_SENT, ESTABLISHED) ||
+            /* These differ from RFC 793: */
+            TRANSITION (LISTEN, CLOSED) ||
+            TRANSITION (SYN_SENT, CLOSED) ||
+            TRANSITION (SYN_RECEIVED, CLOSED) ||
+            TRANSITION (ESTABLISHED, CLOSED));
+
+#undef TRANSITION
+
+  priv->state = new_state;
+}