[Bug 1893691] Re: Chardev logfile is not written (regression between 5.0 and 5.1)

[Daniel Berrange <1893691@bugs.launchpad.net>]

This regression is specific to the "socket" chardev and was caused by
this commit:

commit 271094474b65de1ad7aaf729938de3d9b9d0d36f (refs/bisect/bad)
Author: Dima Stepanov <dimastep@yandex-team.ru>
Date:   Thu May 28 12:11:18 2020 +0300

    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>

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https://bugs.launchpad.net/bugs/1893691

Title:
  Chardev logfile is not written (regression between 5.0 and 5.1)

Status in QEMU:
  New

Bug description:
  After update from version 5.0 to 5.1, logfile stopped being populated
  with console output. The file is being created, but remains empty.

  Relevant command line options:

  -mon chardev=char0
  -serial chardev:char0
  -chardev socket,host=127.0.0.10,port=2323,server,nowait,telnet,mux=on,id=char0,logfile=/home/jurkiew/.machiner/runs/2020-08-31T21:46:55-0/internal/log

  
  Full command line:

  qemu-system-x86_64
  -nodefaults
  -no-user-config
  -snapshot
  -enable-kvm
  -cpu
  host
  -nographic
  -echr
  17
  -mon
  chardev=char0
  -serial
  chardev:char0
  -rtc
  clock=vm
  -object
  rng-random,filename=/dev/urandom,id=rng0
  -device
  virtio-rng-pci,rng=rng0,max-bytes=512,period=1000
  -name
  2020-08-31T21:46:55-0,debug-threads=on
  -smp
  sockets=1,cores=9,threads=2
  -m
  251G
  -overcommit
  cpu-pm=on
  -pidfile
  /home/jurkiew/.machiner/runs/2020-08-31T21:46:55-0/internal/pid
  -net
  nic,model=virtio
  -net
  user,hostfwd=tcp:127.0.0.10:2222-:22,hostfwd=tcp:127.0.0.10:8000-:8000,hostfwd=tcp:127.0.0.10:9000-:9000
  -fsdev
  local,id=machiner_internal_dir,security_model=none,path=/home/jurkiew/.machiner/runs/2020-08-31T21:46:55-0/internal
  -device
  virtio-9p-pci,fsdev=machiner_internal_dir,mount_tag=machiner_internal_dir
  -fsdev
  local,id=machiner_lower_dir,security_model=none,readonly,path=.
  -device
  virtio-9p-pci,fsdev=machiner_lower_dir,mount_tag=machiner_lower_dir
  -fsdev
  local,id=machiner_upper_dir,security_model=mapped-xattr,fmode=0777,dmode=0777,path=/home/jurkiew/.machiner/runs/2020-08-31T21:46:55-0
  -device
  virtio-9p-pci,fsdev=machiner_upper_dir,mount_tag=machiner_upper_dir
  -device
  virtio-scsi
  -drive
  if=none,file=/home/jurkiew/.machiner/images/famtar/image.qcow2,discard=on,id=famtar
  -device
  scsi-hd,drive=famtar
  -chardev
  socket,host=127.0.0.10,port=2323,server,nowait,telnet,mux=on,id=char0,logfile=/home/jurkiew/.machiner/runs/2020-08-31T21:46:55-0/internal/log

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