char-socket: return -1 in case of disconnect during tcp_chr_write

char-socket: return -1 in case of disconnect during tcp_chr_write

During testing of the vhost-user-blk reconnect functionality the qemu
SIGSEGV was triggered:
 start qemu as:
 x86_64-softmmu/qemu-system-x86_64 -m 1024M -M q35 \
   -object memory-backend-file,id=ram-node0,size=1024M,mem-path=/dev/shm/qemu,share=on \
   -numa node,cpus=0,memdev=ram-node0 \
   -chardev socket,id=chardev0,path=./vhost.sock,noserver,reconnect=1 \
   -device vhost-user-blk-pci,chardev=chardev0,num-queues=4 --enable-kvm
 start vhost-user-blk daemon:
 ./vhost-user-blk -s ./vhost.sock -b test-img.raw

If vhost-user-blk will be killed during the vhost initialization
process, for instance after getting VHOST_SET_VRING_CALL command, then
QEMU will fail with the following backtrace:

Thread 1 "qemu-system-x86" received signal SIGSEGV, Segmentation fault.
0x00005555559272bb in vhost_user_read (dev=0x7fffef2d53e0, msg=0x7fffffffd5b0)
    at ./hw/virtio/vhost-user.c:260
260         CharBackend *chr = u->user->chr;

 #0  0x00005555559272bb in vhost_user_read (dev=0x7fffef2d53e0, msg=0x7fffffffd5b0)
    at ./hw/virtio/vhost-user.c:260
 #1  0x000055555592acb8 in vhost_user_get_config (dev=0x7fffef2d53e0, config=0x7fffef2d5394 "", config_len=60)
    at ./hw/virtio/vhost-user.c:1645
 #2  0x0000555555925525 in vhost_dev_get_config (hdev=0x7fffef2d53e0, config=0x7fffef2d5394 "", config_len=60)
    at ./hw/virtio/vhost.c:1490
 #3  0x00005555558cc46b in vhost_user_blk_device_realize (dev=0x7fffef2d51a0, errp=0x7fffffffd8f0)
    at ./hw/block/vhost-user-blk.c:429
 #4  0x0000555555920090 in virtio_device_realize (dev=0x7fffef2d51a0, errp=0x7fffffffd948)
    at ./hw/virtio/virtio.c:3615
 #5  0x0000555555a9779c in device_set_realized (obj=0x7fffef2d51a0, value=true, errp=0x7fffffffdb88)
    at ./hw/core/qdev.c:891
 ...

The problem is that vhost_user_write doesn't get an error after
disconnect and try to call vhost_user_read(). The tcp_chr_write()
routine should return -1 in case of disconnect. Indicate the EIO error
if this routine is called in the disconnected state.

Signed-off-by: Dima Stepanov <dimastep@yandex-team.ru>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <aeb7806bfc945faadf09f64dcfa30f59de3ac053.1590396396.git.dimastep@yandex-team.ru>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>