block: move drain outside of bdrv_attach_child_common(_abort)()

block: move drain outside of bdrv_attach_child_common(_abort)()

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

The function bdrv_attach_child_common_abort() is used only as the
abort callback in bdrv_attach_child_common_drv transactions, so the
tran_finalize() calls of such transactions need to be in drained
sections too.

All code paths are covered:
The bdrv_attach_child_common_drv transactions are only used in
bdrv_attach_child_common(), so it is enough to check callers of
bdrv_attach_child_common() following the transactions.

bdrv_attach_child_common() is called by:
1. bdrv_attach_child_noperm(), which does not finalize the
   transaction yet.
2. bdrv_root_attach_child(), where a drained section is introduced.

bdrv_attach_child_noperm() is called by:
1. bdrv_attach_child(), where a drained section is introduced.
2. bdrv_set_file_or_backing_noperm(), which does not finalize the
   transaction yet.
3. bdrv_append(), where a drained section is introduced.

bdrv_set_file_or_backing_noperm() is called by:
1. bdrv_set_backing_hd_drained(), where a drained section is
   introduced.
2. bdrv_reopen_parse_file_or_backing(), which does not finalize the
   transaction yet. Draining the old child bs currently happens under
   the graph lock there. This is replaced with an assertion, because
   the drain will be moved further up to the caller.

bdrv_reopen_parse_file_or_backing() is called by:
1. bdrv_reopen_prepare(), which does not finalize the transaction yet.

bdrv_reopen_prepare() is called by:
1. bdrv_reopen_multiple(), which does finalize the transaction. It is
   called after bdrv_reopen_queue(), which starts a drained section.
   The drained section ends, when bdrv_reopen_queue_free() is called
   at the end of bdrv_reopen_multiple().

This resolves all code paths.

The functions bdrv_set_backing_hd_drained(), bdrv_attach_child() and
bdrv_root_attach_child() run under the graph lock, so they are not
actually allowed to drain. This will be addressed in the following
commits.

Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>
Message-ID: <20250530151125.955508-11-f.ebner@proxmox.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>