block: move drain outside of bdrv_try_change_aio_context()

block: move drain outside of bdrv_try_change_aio_context()

This is part of resolving the deadlock mentioned in commit "block:
move draining out of bdrv_change_aio_context() and mark GRAPH_RDLOCK".

Convert the function to a _locked() version that has to be called with
the graph lock held and add a convenience wrapper that has to be
called with the graph unlocked, which drains and takes the lock
itself. Since bdrv_try_change_aio_context() is global state code, the
wrapper is too.

Callers are adapted to use the appropriate variant, depending on
whether the caller already holds the lock. In the
test_set_aio_context() unit test, prior drains can be removed, because
draining already happens inside the new wrapper.

Note that bdrv_attach_child_common_abort(), bdrv_attach_child_common()
and bdrv_root_unref_child() hold the graph lock and are not actually
allowed to drain either. This will be addressed in the following
commits.

Functions like qmp_blockdev_mirror() query the nodes to act on before
draining and locking. In theory, draining could invalidate those nodes.
This kind of issue is not addressed by these commits.

Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Message-ID: <20250530151125.955508-10-f.ebner@proxmox.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>