decouple disconnection of the main channel from client destruction

Fixes rhbz#918169

Some channels make direct calls to reds/main_channel routines. If
these routines try to read/write to the socket, and they get socket
error, main_channel_client_on_disconnect is called, and triggers
red_client_destroy. In order to prevent accessing expired references
to RedClient, RedChannelClient, or other objects (inside the original call, after
red_client_destroy has been called) I made the call to
red_client_destroy asynchronous with respect to main_channel_client_on_disconnect.
I added MAIN_DISPATCHER_CLIENT_DISCONNECT to main_dispatcher.
main_channel_client_on_disconnect pushes this msg to the dispatcher,
instead of calling directly to reds_client_disconnect.

The patch uses RedClient ref-count in order to handle a case where
reds_client_disconnect is called directly (e.g., when a new client connects while
another one is connected), while there is already CLIENT_DISCONNECT msg
pending in the main_dispatcher.

Examples:
(1) snd_worker.c

    snd_disconnect_channel()
        channel->cleanup() //snd_playback_cleanup
            reds_enable_mm_timer()
                .
                .
                main_channel_push_multi_media_time()...socket_error
                    .
                    .
                    red_client_destory()
                        .
                        .
                        snd_disconnect_channel()
                            channel->cleanup()
                                celt051_encoder_destroy()
            celt051_encoder_destory() // double release

Note that this bug could have been solved by changing the order of
calls: e.g., channel->stream = NULL before calling cleanup, and
some other changes + reference counting. However, I found other
places in the code with similar problems, and I looked for a general
solution, at least till we redesign red_channel to handle reference
counting more consistently.

(2) inputs_channel.c

    inputs_connect()
        main_channel_client_push_notify()...socket_error
                .
                .
            red_client_destory()
                .
                .
        red_channel_client_create() // refers to client which is already destroyed

(3) reds.c

    reds_handle_main_link()
       main_channel_push_init() ...socket error
                .
                .
            red_client_destory()
                .
                .
       main_channel_client_start_net_test(mcc) // refers to mcc which is already destroyed

    This can explain the assert in rhbz#964136, comment #1 (but not the hang that occurred before).

1 files changed, 5 insertions, 4 deletions

diff --git a/server/main_channel.c b/server/main_channel.c
index 233e633d..fe032a65 100644
--- a/server/main_channel.c
+++ b/server/main_channel.c
@@ -46,6 +46,7 @@
 #include "red_common.h"
 #include "reds.h"
 #include "migration_protocol.h"
+#include "main_dispatcher.h"
 
 #define ZERO_BUF_SIZE 4096
 
@@ -175,13 +176,13 @@ int main_channel_is_connected(MainChannel *main_chan)
     return red_channel_is_connected(&main_chan->base);
 }
 
-// when disconnection occurs, let reds shutdown all channels. This will trigger the
-// real disconnection of main channel
+/*
+ * When the main channel is disconnected, disconnect the entire client.
+ */
 static void main_channel_client_on_disconnect(RedChannelClient *rcc)
 {
     spice_printerr("rcc=%p", rcc);
-    reds_client_disconnect(rcc->client);
-//    red_channel_client_disconnect(rcc);
+    main_dispatcher_client_disconnect(rcc->client);
 }
 
 RedClient *main_channel_get_client_by_link_id(MainChannel *main_chan, uint32_t connection_id)