[Bug 1901892] [NEW] qemu-img create corrupts the qcow2 if the file already exists

[Pablo Schandin]

Public bug reported:

When creating a disk using qemu-img create command, if the destination
path of the qcow2 file already exists, it will show the error saying
that it cannot get a lock so it exits with exit status 1 but it will
corrupt the qcow2 file anyway.

Steps to reproduce:
1. Have a guest running with a root (vda) and a second device (vdc).
In my case is a clean Ubuntu 16.04 image with kernel 4.4.0-190-generic x86_64
vdc disk is called testadddisk-3.qcow2
2. vdc is an xfs over lvm.
pvcreacte /dev/vdc
vgcreate myVg /dev/vdc
lvcreate -l+100%FREE -n myLv myVg
mkfs.xfs /dev/mapper/myVg-myLv
mount /dev/mapper/myVg-myLv /mnt
3. Create disk IO on that device in the guest.
while true ; do dd if=/dev/zero of=/mnt/testfile bs=1024 count=1000 ; sleep 1; 
done
4. Execute the command to create a new device but use the same name of the 
device attached:
sudo qemu-img create -f qcow2 testadddisk-3.qcow2 20G
The output of the command is this:
Formatting 'testadddisk-3.qcow2', fmt=qcow2 size=21474836480 cluster_size=65536 
lazy_refcounts=off refcount_bits=16
qemu-img: testadddisk-3.qcow2: Failed to get "write" lock
Is another process using the image?

The write continues in the guest but when it is shutdown, when it is powered on 
again you get this:
error: Failed to start domain testadddisk
error: internal error: process exited while connecting to monitor: 
2020-10-27T22:00:51.628374Z qemu-system-x86_64: -drive 
file=/var/lib/vmImages/testadddisk-3.qcow2,format=qcow2,if=none,id=drive-virtio-disk2:
 Image is not in qcow2 format

I run the qemu-img create command with an strace and I believe that
first it tries to open the file in write mode, then does a truncate on
it and after that says it cannot get a lock. The output is in the file
attached. As well as the guest xml just in case.

The host: 
Ubuntu 18.04.5 LTS
4.15.0-112-generic x86_64
qemu packages installed:
ii  qemu-block-extra:amd64                 1:2.11+dfsg-1ubuntu7.32              
           amd64        extra block backend modules for qemu-system and 
qemu-utils
ii  qemu-kvm                               1:2.11+dfsg-1ubuntu7.31              
           amd64        QEMU Full virtualization on x86 hardware
ii  qemu-system-common                     1:2.11+dfsg-1ubuntu7.32              
           amd64        QEMU full system emulation binaries (common files)
ii  qemu-system-x86                        1:2.11+dfsg-1ubuntu7.31              
           amd64        QEMU full system emulation binaries (x86)
ii  qemu-utils                             1:2.11+dfsg-1ubuntu7.32              
           amd64        QEMU utilities

** Affects: qemu
     Importance: Undecided
         Status: New

** Attachment added: "testadddisk-strace-dumxml.txt"
   
https://bugs.launchpad.net/bugs/1901892/+attachment/5428520/+files/testadddisk-strace-dumxml.txt

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https://bugs.launchpad.net/bugs/1901892

Title:
  qemu-img create corrupts the qcow2 if the file already exists

Status in QEMU:
  New

Bug description:
  When creating a disk using qemu-img create command, if the destination
  path of the qcow2 file already exists, it will show the error saying
  that it cannot get a lock so it exits with exit status 1 but it will
  corrupt the qcow2 file anyway.

  Steps to reproduce:
  1. Have a guest running with a root (vda) and a second device (vdc).
  In my case is a clean Ubuntu 16.04 image with kernel 4.4.0-190-generic x86_64
  vdc disk is called testadddisk-3.qcow2
  2. vdc is an xfs over lvm.
  pvcreacte /dev/vdc
  vgcreate myVg /dev/vdc
  lvcreate -l+100%FREE -n myLv myVg
  mkfs.xfs /dev/mapper/myVg-myLv
  mount /dev/mapper/myVg-myLv /mnt
  3. Create disk IO on that device in the guest.
  while true ; do dd if=/dev/zero of=/mnt/testfile bs=1024 count=1000 ; sleep 
1; done
  4. Execute the command to create a new device but use the same name of the 
device attached:
  sudo qemu-img create -f qcow2 testadddisk-3.qcow2 20G
  The output of the command is this:
  Formatting 'testadddisk-3.qcow2', fmt=qcow2 size=21474836480 
cluster_size=65536 lazy_refcounts=off refcount_bits=16
  qemu-img: testadddisk-3.qcow2: Failed to get "write" lock
  Is another process using the image?

  The write continues in the guest but when it is shutdown, when it is powered 
on again you get this:
  error: Failed to start domain testadddisk
  error: internal error: process exited while connecting to monitor: 
2020-10-27T22:00:51.628374Z qemu-system-x86_64: -drive 
file=/var/lib/vmImages/testadddisk-3.qcow2,format=qcow2,if=none,id=drive-virtio-disk2:
 Image is not in qcow2 format

  I run the qemu-img create command with an strace and I believe that
  first it tries to open the file in write mode, then does a truncate on
  it and after that says it cannot get a lock. The output is in the file
  attached. As well as the guest xml just in case.

  The host: 
  Ubuntu 18.04.5 LTS
  4.15.0-112-generic x86_64
  qemu packages installed:
  ii  qemu-block-extra:amd64                 1:2.11+dfsg-1ubuntu7.32            
             amd64        extra block backend modules for qemu-system and 
qemu-utils
  ii  qemu-kvm                               1:2.11+dfsg-1ubuntu7.31            
             amd64        QEMU Full virtualization on x86 hardware
  ii  qemu-system-common                     1:2.11+dfsg-1ubuntu7.32            
             amd64        QEMU full system emulation binaries (common files)
  ii  qemu-system-x86                        1:2.11+dfsg-1ubuntu7.31            
             amd64        QEMU full system emulation binaries (x86)
  ii  qemu-utils                             1:2.11+dfsg-1ubuntu7.32            
             amd64        QEMU utilities

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