migration: fix the multifd code when receiving less channels

In our current code, when multifd is used during migration, if there
is an error before the destination receives all new channels, the
source keeps running, however the destination does not exit but keeps
waiting until the source is killed deliberately.

Fix this by dumping the specific error and let users decide whether
to quit from the destination side when failing to receive packet via
some channel. And update the comment for multifd_recv_new_channel().

Cc: Dr. David Alan Gilbert <dgilbert@redhat.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Fei Li <fli@suse.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Message-Id: <20190113140849.38339-3-lifei1214@126.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

1 files changed, 14 insertions, 3 deletions

diff --git a/migration/ram.c b/migration/ram.c
index 1849979..47c7ab2 100644
--- a/migration/ram.c
+++ b/migration/ram.c
@@ -1322,8 +1322,13 @@ bool multifd_recv_all_channels_created(void)
     return thread_count == atomic_read(&multifd_recv_state->count);
 }
 
-/* Return true if multifd is ready for the migration, otherwise false */
-bool multifd_recv_new_channel(QIOChannel *ioc)
+/*
+ * Try to receive all multifd channels to get ready for the migration.
+ * - Return true and do not set @errp when correctly receving all channels;
+ * - Return false and do not set @errp when correctly receiving the current one;
+ * - Return false and set @errp when failing to receive the current channel.
+ */
+bool multifd_recv_new_channel(QIOChannel *ioc, Error **errp)
 {
     MultiFDRecvParams *p;
     Error *local_err = NULL;
@@ -1332,6 +1337,10 @@ bool multifd_recv_new_channel(QIOChannel *ioc)
     id = multifd_recv_initial_packet(ioc, &local_err);
     if (id < 0) {
         multifd_recv_terminate_threads(local_err);
+        error_propagate_prepend(errp, local_err,
+                                "failed to receive packet"
+                                " via multifd channel %d: ",
+                                atomic_read(&multifd_recv_state->count));
         return false;
     }
 
@@ -1340,6 +1349,7 @@ bool multifd_recv_new_channel(QIOChannel *ioc)
         error_setg(&local_err, "multifd: received id '%d' already setup'",
                    id);
         multifd_recv_terminate_threads(local_err);
+        error_propagate(errp, local_err);
         return false;
     }
     p->c = ioc;
@@ -1351,7 +1361,8 @@ bool multifd_recv_new_channel(QIOChannel *ioc)
     qemu_thread_create(&p->thread, p->name, multifd_recv_thread, p,
                        QEMU_THREAD_JOINABLE);
     atomic_inc(&multifd_recv_state->count);
-    return multifd_recv_state->count == migrate_multifd_channels();
+    return atomic_read(&multifd_recv_state->count) ==
+           migrate_multifd_channels();
 }
 
 /**