From: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>

In migration we want to pass fd "as is", not changing its
blocking status.

The only current user of these fds is CPR state (through VMSTATE_FD),
which of-course doesn't want to modify fds on target when source is
still running and use these fds.

Suggested-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
---
 include/io/channel.h  |  1 +
 io/channel-socket.c   | 13 +++++++++----
 migration/qemu-file.c |  3 ++-
 3 files changed, 12 insertions(+), 5 deletions(-)

diff --git a/include/io/channel.h b/include/io/channel.h
index 234e5db70d..12266256a8 100644
--- a/include/io/channel.h
+++ b/include/io/channel.h
@@ -36,6 +36,7 @@ OBJECT_DECLARE_TYPE(QIOChannel, QIOChannelClass,
 
 #define QIO_CHANNEL_READ_FLAG_MSG_PEEK 0x1
 #define QIO_CHANNEL_READ_FLAG_RELAXED_EOF 0x2
+#define QIO_CHANNEL_READ_FLAG_FD_PRESERVE_BLOCKING 0x4
 
 typedef enum QIOChannelFeature QIOChannelFeature;
 
diff --git a/io/channel-socket.c b/io/channel-socket.c
index 3b7ca924ff..21f8f2e0c5 100644
--- a/io/channel-socket.c
+++ b/io/channel-socket.c
@@ -464,7 +464,8 @@ static void qio_channel_socket_finalize(Object *obj)
 
 #ifndef WIN32
 static void qio_channel_socket_copy_fds(struct msghdr *msg,
-                                        int **fds, size_t *nfds)
+                                        int **fds, size_t *nfds,
+                                        bool preserve_blocking)
 {
     struct cmsghdr *cmsg;
 
@@ -497,8 +498,10 @@ static void qio_channel_socket_copy_fds(struct msghdr *msg,
                 continue;
             }
 
-            /* O_NONBLOCK is preserved across SCM_RIGHTS so reset it */
-            qemu_socket_set_block(fd);
+            if (!preserve_blocking) {
+                /* O_NONBLOCK is preserved across SCM_RIGHTS so reset it */
+                qemu_socket_set_block(fd);
+            }
 
 #ifndef MSG_CMSG_CLOEXEC
             qemu_set_cloexec(fd);
@@ -556,7 +559,9 @@ static ssize_t qio_channel_socket_readv(QIOChannel *ioc,
     }
 
     if (fds && nfds) {
-        qio_channel_socket_copy_fds(&msg, fds, nfds);
+        qio_channel_socket_copy_fds(
+            &msg, fds, nfds,
+            flags & QIO_CHANNEL_READ_FLAG_FD_PRESERVE_BLOCKING);
     }
 
     return ret;
diff --git a/migration/qemu-file.c b/migration/qemu-file.c
index b6ac190034..d5c6e7ec61 100644
--- a/migration/qemu-file.c
+++ b/migration/qemu-file.c
@@ -340,7 +340,8 @@ static ssize_t coroutine_mixed_fn qemu_fill_buffer(QEMUFile *f)
 
     do {
         struct iovec iov = { f->buf + pending, IO_BUF_SIZE - pending };
-        len = qio_channel_readv_full(f->ioc, &iov, 1, pfds, pnfd, 0,
+        len = qio_channel_readv_full(f->ioc, &iov, 1, pfds, pnfd,
+                                     QIO_CHANNEL_READ_FLAG_FD_PRESERVE_BLOCKING,
                                      &local_error);
         if (len == QIO_CHANNEL_ERR_BLOCK) {
             if (qemu_in_coroutine()) {
-- 
2.50.1