We introduced the auto-read-only option to fix the problem that block
jobs that reopen a backing file read-write don't work any more when all
nodes are created individually with -blockdev. The reason is that
bs->file of these backing files doesn't inherit the read-only option
from the format layer node any more if it's created separately.

The way auto-read-only was designed to fix this is that it just always
opens the file node read-write if it can, so reopening the format layer
node is enough to make the backing file writable when necessary.

This works in principle, but not when libvirt uses sVirt: Then QEMU
doesn't even have the permissions to open the image file read-write
until libvirt performs an operation where write access is needed.

This series changes auto-read-only so that it works dynamically and
automatically reopens the file read-only or read-write depending on the
permissions that users attached to the node requested.

See also: https://bugzilla.redhat.com/show_bug.cgi?id=1685989

v2:
- Added test for and fixed snapshot=on,read-only=on regression [Berto]
- Added test for and fixed commit regression [Peter]

Kevin Wolf (10):
  tests/virtio-blk-test: Disable auto-read-only
  qemu-iotests: commit to backing file with auto-read-only
  block: Avoid useless local_err
  block: Make permission changes in reopen less wrong
  file-posix: Fix bdrv_open_flags() for snapshot=on
  file-posix: Factor out raw_reconfigure_getfd()
  file-posix: Store BDRVRawState.reopen_state during reopen
  file-posix: Lock new fd in raw_reopen_prepare()
  file-posix: Prepare permission code for fd switching
  file-posix: Make auto-read-only dynamic

 block.c                       |  46 ++++---
 block/file-posix.c            | 248 ++++++++++++++++++++++++----------
 tests/virtio-blk-test.c       |   2 +-
 tests/qemu-iotests/051        |   7 +
 tests/qemu-iotests/051.out    |   9 ++
 tests/qemu-iotests/051.pc.out |   9 ++
 tests/qemu-iotests/232        |  31 +++++
 tests/qemu-iotests/232.out    |  32 ++++-
 8 files changed, 284 insertions(+), 100 deletions(-)

-- 
2.20.1

On Mon, Mar 11, 2019 at 17:50:07 +0100, Kevin Wolf wrote:
> We introduced the auto-read-only option to fix the problem that block
> jobs that reopen a backing file read-write don't work any more when all
> nodes are created individually with -blockdev. The reason is that
> bs->file of these backing files doesn't inherit the read-only option
> from the format layer node any more if it's created separately.
> 
> The way auto-read-only was designed to fix this is that it just always
> opens the file node read-write if it can, so reopening the format layer
> node is enough to make the backing file writable when necessary.
> 
> This works in principle, but not when libvirt uses sVirt: Then QEMU
> doesn't even have the permissions to open the image file read-write
> until libvirt performs an operation where write access is needed.
> 
> This series changes auto-read-only so that it works dynamically and
> automatically reopens the file read-only or read-write depending on the
> permissions that users attached to the node requested.
> 
> See also: https://bugzilla.redhat.com/show_bug.cgi?id=1685989
> 
> v2:
> - Added test for and fixed snapshot=on,read-only=on regression [Berto]
> - Added test for and fixed commit regression [Peter]

I've quickly tested it with non-active commit and it works in this
scenario as expected.

I'll give it a bit more thorough test in the morning.