migration: Don't assert() if MIGRATE_DATA comes before attaching the agent

During seamless migration, after switching host, if a client was connected
during the migration, it will have data to send back to the new
qemu/spice-server instance. This is handled through MIGRATE_DATA messages.
SPICE char devices use such MIGRATE_DATA messages to restore their state.

However, the MIGRATE_DATA message can arrive any time after the new qemu
instance has started, this can happen before or after the SPICE char
devices have been created. In order to handle this, if the migrate data
arrives early, it's stored in reds->agent_state.mig_data, and
attach_to_red_agent() will restore the agent state as appropriate.

Unfortunately this does not work as expected, for main
channel (agent messages).
If attach_to_red_agent() is called before the MIGRATE_DATA
message reaches the server, all goes well,
but if MIGRATE_DATA reaches the server before
attach_to_red_agent() gets called, then some assert() gets
triggered in spice_char_device_state_restore():

((null):32507): Spice-ERROR **: char_device.c:937:spice_char_device_state_restore: assertion `dev->num_clients == 1 && dev->wait_for_migrate_data' failed
Thread 3 (Thread 0x7f406b543700 (LWP 32543)):
Thread 2 (Thread 0x7f40697ff700 (LWP 32586)):
Thread 1 (Thread 0x7f4079b45a40 (LWP 32507)):

When restoring state, a client must already be added to the
spice-char-device.
What happens is that a client is not being added to the char-device
when when MIGRATE_DATA arrives first, which leaves both
dev->num_clients and dev->wait_for_migrate_data value at 0.

This commit changes the logic in spice_server_char_device_add_interface(),
such that if there is migrate data pending in reds->agent_state.mig_data
but no client was added to the spice-char-device yet,
then first the client is added to the device by calling
spice_char_device_client_add(), and only then the state is restored.

=== How to Reproduce
To reproduce, add delays to the migration connection between
qmeu-kvm on the source host (SRC) and on the destination (DST).

Specifically I added a man in the middle DLY host between
migration ports from SRC to DST.

+-----+    +-----+     +-----+
| SRC |--> | DLY | --> | DST |
+-----+    +-----+     +-----+

DLY listens on port P1 (e.g. 4444) and DST listens on port
PINCOMING (e.g. 4444, from qemu-kvm '-incoming' command line option)

Precondition: make sure port P1 on DLY is accessible in iptables.
Option 1: use ssh tcp port forwarding
On DLY host run ssh:
  ssh DLY:P1:DST:PINCOMING DST
Then use the following migration command (on qemu-kvm monitor):
  client_migrate_info spice DST PSPICE
  migrate -d tcp:DLY:P1

Option 2: Use a simple proxy program that forwards
packets from SRC to DST while adding some delays.
The program runs on DLY, listens to port D1, upon
accept connects to DST:PINCOMING and forward all
packets from DLY:D1 to DST:PINCOMING.
Then use the same migrate command as in option 1:
  client_migrate_info spice DST PSPICE
  migrate -d tcp:DLY:P1

=== How to Reproduce Ends

This fixes https://bugzilla.redhat.com/show_bug.cgi?id=1035184

Based-on-a-patch-by: Christophe Fergeau <cfergeau@redhat.com>

1 files changed, 28 insertions, 11 deletions

diff --git a/server/reds.c b/server/reds.c
index 6864d364..ef7ff620 100644
--- a/server/reds.c
+++ b/server/reds.c
@@ -1274,6 +1274,7 @@ int reds_handle_migrate_data(MainChannelClient *mcc, SpiceMigrateDataMain *mig_d
 {
     VDIPortState *agent_state = &reds->agent_state;
 
+    spice_debug("main-channel: got migrate data");
     /*
      * Now that the client has switched to the target server, if main_channel
      * controls the mm-time, we update the client's mm-time.
@@ -1295,15 +1296,18 @@ int reds_handle_migrate_data(MainChannelClient *mcc, SpiceMigrateDataMain *mig_d
                     main_channel_push_agent_disconnected(reds->main_channel);
                     main_channel_push_agent_connected(reds->main_channel);
                 } else {
+                    spice_debug("restoring state from mig_data");
                     return reds_agent_state_restore(mig_data);
                 }
             }
         } else {
             /* restore agent starte when the agent gets attached */
+            spice_debug("saving mig_data");
             spice_assert(agent_state->plug_generation == 0);
             agent_state->mig_data = spice_memdup(mig_data, size);
         }
     } else {
+        spice_debug("agent was not attached on the source host");
         if (vdagent) {
             /* spice_char_device_client_remove disables waiting for migration data */
             spice_char_device_client_remove(agent_state->base,
@@ -2853,17 +2857,15 @@ static SpiceCharDeviceState *attach_to_red_agent(SpiceCharDeviceInstance *sin)
     state->read_filter.discard_all = FALSE;
     reds->agent_state.plug_generation++;
 
-    if (reds->agent_state.mig_data) {
-        spice_assert(reds->agent_state.plug_generation == 1);
-        reds_agent_state_restore(reds->agent_state.mig_data);
-        free(reds->agent_state.mig_data);
-        reds->agent_state.mig_data = NULL;
-    } else if (!red_channel_waits_for_migrate_data(&reds->main_channel->base)) {
-        /* we will assoicate the client with the char device, upon reds_on_main_agent_start,
-         * in response to MSGC_AGENT_START */
-        main_channel_push_agent_connected(reds->main_channel);
-    } else {
-       spice_debug("waiting for migration data");
+    if (reds->agent_state.mig_data ||
+        red_channel_waits_for_migrate_data(&reds->main_channel->base)) {
+        /* Migration in progress (code is running on the destination host):
+         * 1.  Add the client to spice char device, if it was not already added.
+         * 2.a If this (qemu-kvm state load side of migration) happens first
+         *     then wait for spice migration data to arrive. Otherwise
+         * 2.b If this happens second ==> we already have spice migrate data
+         *     then restore state
+         */
         if (!spice_char_device_client_exists(reds->agent_state.base, reds_get_client())) {
             int client_added;
 
@@ -2879,9 +2881,24 @@ static SpiceCharDeviceState *attach_to_red_agent(SpiceCharDeviceInstance *sin)
                 spice_warning("failed to add client to agent");
                 reds_disconnect();
             }
+        }
 
+        if (reds->agent_state.mig_data) {
+            spice_debug("restoring state from stored migration data");
+            spice_assert(reds->agent_state.plug_generation == 1);
+            reds_agent_state_restore(reds->agent_state.mig_data);
+            free(reds->agent_state.mig_data);
+            reds->agent_state.mig_data = NULL;
+        }
+        else {
+            spice_debug("waiting for migration data");
         }
+    } else {
+        /* we will associate the client with the char device, upon reds_on_main_agent_start,
+         * in response to MSGC_AGENT_START */
+        main_channel_push_agent_connected(reds->main_channel);
     }
+
     return state->base;
 }