block-coroutine-wrapper.py: introduce annotations that take the graph rdlock

Add co_wrapper_bdrv_rdlock and co_wrapper_mixed_bdrv_rdlock option to
the block-coroutine-wrapper.py script.

This "_bdrv_rdlock" option takes and releases the graph rdlock when a
coroutine function is created.

This means that when used together with "_mixed", the function marked
with co_wrapper_mixed_bdrv_rdlock will support both coroutine and
non-coroutine case, and in the latter case it will create a coroutine
that takes and releases the rdlock. When called from a coroutine, the
caller must already hold the graph lock.

Example:
void co_wrapper_mixed_bdrv_rdlock bdrv_f1();

Becomes

static void bdrv_co_enter_f1()
{
    bdrv_graph_co_rdlock();
    bdrv_co_function();
    bdrv_graph_co_rdunlock();
}

void bdrv_f1()
{
    if (qemu_in_coroutine) {
        assume_graph_lock();
        bdrv_co_function();
    } else {
        qemu_co_enter(bdrv_co_enter_f1);
        ...
    }
}

When used alone, the function will not work in coroutine context, and
when called in non-coroutine context it will create a new coroutine that
takes care of taking and releasing the rdlock automatically.

Example:
void co_wrapper_bdrv_rdlock bdrv_f1();

Becomes

static void bdrv_co_enter_f1()
{
    bdrv_graph_co_rdlock();
    bdrv_co_function();
    bdrv_graph_co_rdunlock();
}

void bdrv_f1()
{
    assert(!qemu_in_coroutine());
    qemu_co_enter(bdrv_co_enter_f1);
    ...
}

About their usage:
- co_wrapper does not take the rdlock, so it can be used also outside
  the block layer.
- co_wrapper_mixed will be used by many blk_* functions, since the
  coroutine function needs to call blk_wait_while_drained() and
  the rdlock *must* be taken afterwards, otherwise it's a deadlock.
  In the future this annotation will go away, and blk_* will use
  co_wrapper directly.
- co_wrapper_bdrv_rdlock will be used by BlockDriver callbacks, ideally
  by all of them in the future.
- co_wrapper_mixed_bdrv_rdlock will be used by the remaining functions
  that are still called by coroutine and non-coroutine context. In the
  future this annotation will go away, as we will split such mixed
  functions.

Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20221207131838.239125-17-kwolf@redhat.com>
Reviewed-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

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