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

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

(Subscribing Dan Berrange as char person).

Is there any chance you could bisect to see the exact change?

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

commit 271094474b65de1ad7aaf729938de3d9b9d0d36f (refs/bisect/bad)
Author: Dima Stepanov <email address hidden>
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 <email address hidden>
    Reviewed-by: Marc-André Lureau <email address hidden>
    Message-Id: <email address hidden>
    Reviewed-by: Michael S. Tsirkin <email address hidden>
    Signed-off-by: Michael S. Tsirkin <email address hidden>



https://lists.gnu.org/archive/html/qemu-devel/2020-09/msg04809.html

Released with QEMU v5.2.0.