migration: Cleanup incoming migration setup state change

Destination QEMU can setup incoming ports for two purposes: either a fresh
new incoming migration, in which QEMU will switch to SETUP for channel
establishment, or a paused postcopy migration, in which QEMU will stay in
POSTCOPY_PAUSED until kicking off the RECOVER phase.

Now the state machine worked on dest node for the latter, only because
migrate_set_state() implicitly will become a noop if the current state
check failed.  It wasn't clear at all.

Clean it up by providing a helper migration_incoming_state_setup() doing
proper checks over current status.  Postcopy-paused will be explicitly
checked now, and then we can bail out for unknown states.

Reviewed-by: Fabiano Rosas <farosas@suse.de>
Signed-off-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Fabiano Rosas <farosas@suse.de>

migration/migration.c

@@ -618,6 +618,29 @@ bool migrate_uri_parse(const char *uri, MigrationChannel **channel,
     return true;
 }
 
+static bool
+migration_incoming_state_setup(MigrationIncomingState *mis, Error **errp)
+{
+    MigrationStatus current = mis->state;
+
+    if (current == MIGRATION_STATUS_POSTCOPY_PAUSED) {
+        /*
+         * Incoming postcopy migration will stay in PAUSED state even if
+         * reconnection happened.
+         */
+        return true;
+    }
+
+    if (current != MIGRATION_STATUS_NONE) {
+        error_setg(errp, "Illegal migration incoming state: %s",
+                   MigrationStatus_str(current));
+        return false;
+    }
+
+    migrate_set_state(&mis->state, current, MIGRATION_STATUS_SETUP);
+    return true;
+}
+
 static void qemu_start_incoming_migration(const char *uri, bool has_channels,
                                           MigrationChannelList *channels,
                                           Error **errp)
@@ -656,8 +679,9 @@ static void qemu_start_incoming_migration(const char *uri, bool has_channels,
         return;
     }
 
-    migrate_set_state(&mis->state, MIGRATION_STATUS_NONE,
-                      MIGRATION_STATUS_SETUP);
+    if (!migration_incoming_state_setup(mis, errp)) {
+        return;
+    }
 
     if (addr->transport == MIGRATION_ADDRESS_TYPE_SOCKET) {
         SocketAddress *saddr = &addr->u.socket;