migration/qemu-file: don't make incoming fds blocking again

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

@@ -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;
 

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;

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()) {