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, 7 insertions, 0 deletions

diff --git a/server/main_dispatcher.h b/server/main_dispatcher.h
index 0c79ca8d..522c7f91 100644
--- a/server/main_dispatcher.h
+++ b/server/main_dispatcher.h
@@ -7,6 +7,13 @@
 void main_dispatcher_channel_event(int event, SpiceChannelEventInfo *info);
 void main_dispatcher_seamless_migrate_dst_complete(RedClient *client);
 void main_dispatcher_set_mm_time_latency(RedClient *client, uint32_t latency);
+/*
+ * Disconnecting the client is always executed asynchronously,
+ * in order to protect from expired references in the routines
+ * that triggered the client destruction.
+ */
+void main_dispatcher_client_disconnect(RedClient *client);
+
 void main_dispatcher_init(SpiceCoreInterface *core);
 
 #endif //MAIN_DISPATCHER_H