[PATCH] migration/multifd: fix hangup with TLS-Multifd due to blocking handshake

[Chuan Zheng <zhengchuan@huawei.com>]

The qemu main loop could hang up forever when we enable TLS+Multifd.
The Src multifd_send_0 invokes tls handshake, it sends hello to sever
and wait response.
However, the Dst main qemu loop has been waiting recvmsg() for multifd_recv_1.
Both of Src and Dst main qemu loop are blocking and waiting for reponse which
results in hanging up forever.

Src: (multifd_send_0)                                              Dst: (multifd_recv_1)
multifd_channel_connect                                            migration_channel_process_incoming
  multifd_tls_channel_connect                                        migration_tls_channel_process_incoming
    multifd_tls_channel_connect                                        qio_channel_tls_handshake_task
       qio_channel_tls_handshake                                         gnutls_handshake
          qio_channel_tls_handshake_task                                       ...
            qcrypto_tls_session_handshake                                      ...
              gnutls_handshake                                                 ...
                   ...                                                         ...
                recvmsg (Blocking I/O waiting for response)                recvmsg (Blocking I/O waiting for response)

Fix this by offloadinig handshake work to a background thread.

Reported-by: Yan Jin <jinyan12@huawei.com>
Suggested-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Chuan Zheng <zhengchuan@huawei.com>
---
 migration/multifd.c | 23 +++++++++++++++++------
 1 file changed, 17 insertions(+), 6 deletions(-)

diff --git a/migration/multifd.c b/migration/multifd.c
index 68b171f..88486b9 100644
--- a/migration/multifd.c
+++ b/migration/multifd.c
@@ -739,6 +739,19 @@ static void multifd_tls_outgoing_handshake(QIOTask *task,
     multifd_channel_connect(p, ioc, err);
 }
 
+static void *multifd_tls_handshake_thread(void *opaque)
+{
+    MultiFDSendParams *p = opaque;
+    QIOChannelTLS *tioc = QIO_CHANNEL_TLS(p->c);
+
+    qio_channel_tls_handshake(tioc,
+                              multifd_tls_outgoing_handshake,
+                              p,
+                              NULL,
+                              NULL);
+    return NULL;
+}
+
 static void multifd_tls_channel_connect(MultiFDSendParams *p,
                                         QIOChannel *ioc,
                                         Error **errp)
@@ -754,12 +767,10 @@ static void multifd_tls_channel_connect(MultiFDSendParams *p,
 
     trace_multifd_tls_outgoing_handshake_start(ioc, tioc, hostname);
     qio_channel_set_name(QIO_CHANNEL(tioc), "multifd-tls-outgoing");
-    qio_channel_tls_handshake(tioc,
-                              multifd_tls_outgoing_handshake,
-                              p,
-                              NULL,
-                              NULL);
-
+    p->c = QIO_CHANNEL(tioc);
+    qemu_thread_create(&p->thread, "multifd-tls-handshake-worker",
+                       multifd_tls_handshake_thread, p,
+                       QEMU_THREAD_JOINABLE);
 }
 
 static bool multifd_channel_connect(MultiFDSendParams *p,
-- 
1.8.3.1