Use client earlyCapabilities to determine channel join count

We always now indicate we support skipping channel joins. If the client
indicates this too, expect no channel join requests from the client.

If we do get some, process them anyway.

(cherry picked from commit f4d7305)

common/ms-rdpbcgr.h

@@ -80,6 +80,7 @@
 #define RNS_UD_CS_WANT_32BPP_SESSION         0x0002
 #define RNS_UD_CS_SUPPORT_MONITOR_LAYOUT_PDU 0x0040
 #define RNS_UD_CS_SUPPORT_DYNVC_GFX_PROTOCOL 0x0100
+#define RNS_UD_CS_SUPPORT_SKIP_CHANNELJOIN   0x0800
 
 /* Client Core Data: connectionType  (2.2.1.3.2) */
 #define CONNECTION_TYPE_MODEM          0x01

libxrdp/xrdp_mcs.c

@@ -1129,11 +1129,20 @@ handle_channel_join_requests(struct xrdp_mcs *self,
                              struct stream *s, int *appid)
 {
     int rv = 0;
-    /*
-     * Expect a channel join request PDU for each of the static virtual
-     * channels, plus the user channel (self->chanid) and the I/O channel
-     * (MCS_GLOBAL_CHANNEL) */
-    unsigned int expected_join_count = self->channel_list->count + 2;
+    unsigned int expected_join_count = 0;
+    if ((self->sec_layer->rdp_layer->client_info.mcs_early_capability_flags &
+            RNS_UD_CS_SUPPORT_SKIP_CHANNELJOIN) == 0)
+    {
+        /*
+         * The client has indicated it does not support skipping channel
+         * join request/confirm PDUs.
+         *
+         * Expect a channel join request PDU for each of the static
+         * virtual channels, plus the user channel (self->chanid) and
+         * the I/O channel (MCS_GLOBAL_CHANNEL) */
+        expected_join_count = self->channel_list->count + 2;
+    }
+
     unsigned int actual_join_count = 0;
 
     while (*appid == MCS_CJRQ)