blockdev: Return bs to the proper context on snapshot abort

external_snapshot_abort() calls to bdrv_set_backing_hd(), which
returns state->old_bs to the main AioContext, as it's intended to be
used then the BDS is going to be released. As that's not the case when
aborting an external snapshot, return it to the AioContext it was
before the call.

This issue can be triggered by issuing a transaction with two actions,
a proper blockdev-snapshot-sync and a bogus one, so the second will
trigger a transaction abort. This results in a crash with an stack
trace like this one:

 #0  0x00007fa1048b28df in __GI_raise (sig=sig@entry=6) at ../sysdeps/unix/sysv/linux/raise.c:50
 qemu#1  0x00007fa10489ccf5 in __GI_abort () at abort.c:79
 qemu#2  0x00007fa10489cbc9 in __assert_fail_base
     (fmt=0x7fa104a03300 "%s%s%s:%u: %s%sAssertion `%s' failed.\n%n", assertion=0x5572240b44d8 "bdrv_get_aio_context(old_bs) == bdrv_get_aio_context(new_bs)", file=0x557224014d30 "block.c", line=2240, function=<optimized out>) at assert.c:92
 qemu#3  0x00007fa1048aae96 in __GI___assert_fail
     (assertion=assertion@entry=0x5572240b44d8 "bdrv_get_aio_context(old_bs) == bdrv_get_aio_context(new_bs)", file=file@entry=0x557224014d30 "block.c", line=line@entry=2240, function=function@entry=0x5572240b5d60 <__PRETTY_FUNCTION__.31620> "bdrv_replace_child_noperm") at assert.c:101
 qemu#4  0x0000557223e631f8 in bdrv_replace_child_noperm (child=0x557225b9c980, new_bs=new_bs@entry=0x557225c42e40) at block.c:2240
 qemu#5  0x0000557223e68be7 in bdrv_replace_node (from=0x557226951a60, to=0x557225c42e40, errp=0x5572247d6138 <error_abort>) at block.c:4196
 qemu#6  0x0000557223d069c4 in external_snapshot_abort (common=0x557225d7e170) at blockdev.c:1731
 qemu#7  0x0000557223d069c4 in external_snapshot_abort (common=0x557225d7e170) at blockdev.c:1717
 qemu#8  0x0000557223d09013 in qmp_transaction (dev_list=<optimized out>, has_props=<optimized out>, props=0x557225cc7d70, errp=errp@entry=0x7ffe704c0c98) at blockdev.c:2360
 qemu#9  0x0000557223e32085 in qmp_marshal_transaction (args=<optimized out>, ret=<optimized out>, errp=0x7ffe704c0d08) at qapi/qapi-commands-transaction.c:44
 qemu#10 0x0000557223ee798c in do_qmp_dispatch (errp=0x7ffe704c0d00, allow_oob=<optimized out>, request=<optimized out>, cmds=0x5572247d3cc0 <qmp_commands>) at qapi/qmp-dispatch.c:132
 qemu#11 0x0000557223ee798c in qmp_dispatch (cmds=0x5572247d3cc0 <qmp_commands>, request=<optimized out>, allow_oob=<optimized out>) at qapi/qmp-dispatch.c:175
 qemu#12 0x0000557223e06141 in monitor_qmp_dispatch (mon=0x557225c69ff0, req=<optimized out>) at monitor/qmp.c:120
 qemu#13 0x0000557223e0678a in monitor_qmp_bh_dispatcher (data=<optimized out>) at monitor/qmp.c:209
 qemu#14 0x0000557223f2f366 in aio_bh_call (bh=0x557225b9dc60) at util/async.c:117
 qemu#15 0x0000557223f2f366 in aio_bh_poll (ctx=ctx@entry=0x557225b9c840) at util/async.c:117
 qemu#16 0x0000557223f32754 in aio_dispatch (ctx=0x557225b9c840) at util/aio-posix.c:459
 qemu#17 0x0000557223f2f242 in aio_ctx_dispatch (source=<optimized out>, callback=<optimized out>, user_data=<optimized out>) at util/async.c:260
 qemu#18 0x00007fa10913467d in g_main_dispatch (context=0x557225c28e80) at gmain.c:3176
 qemu#19 0x00007fa10913467d in g_main_context_dispatch (context=context@entry=0x557225c28e80) at gmain.c:3829
 qemu#20 0x0000557223f31808 in glib_pollfds_poll () at util/main-loop.c:219
 qemu#21 0x0000557223f31808 in os_host_main_loop_wait (timeout=<optimized out>) at util/main-loop.c:242
 qemu#22 0x0000557223f31808 in main_loop_wait (nonblocking=<optimized out>) at util/main-loop.c:518
 qemu#23 0x0000557223d13201 in main_loop () at vl.c:1828
 qemu#24 0x0000557223bbfb82 in main (argc=<optimized out>, argv=<optimized out>, envp=<optimized out>) at vl.c:4504

RHBZ: https://bugzilla.redhat.com/show_bug.cgi?id=1779036
Signed-off-by: Sergio Lopez <slp@redhat.com>

blockdev.c

@@ -1731,13 +1731,34 @@ static void external_snapshot_abort(BlkActionState *common)
     if (state->new_bs) {
         if (state->overlay_appended) {
             AioContext *aio_context;
+            AioContext *tmp_context;
+            int ret;
 
             aio_context = bdrv_get_aio_context(state->old_bs);
             aio_context_acquire(aio_context);
 
             bdrv_ref(state->old_bs);   /* we can't let bdrv_set_backind_hd()
                                           close state->old_bs; we need it */
             bdrv_set_backing_hd(state->new_bs, NULL, &error_abort);
+
+            /*
+             * The call to bdrv_set_backing_hd() above returns state->old_bs to
+             * the main AioContext. As we're still going to be using it, return
+             * it to the AioContext it was before.
+             */
+            tmp_context = bdrv_get_aio_context(state->old_bs);
+            if (aio_context != tmp_context) {
+                aio_context_release(aio_context);
+                aio_context_acquire(tmp_context);
+
+                ret = bdrv_try_set_aio_context(state->old_bs,
+                                               aio_context, NULL);
+                assert(ret == 0);
+
+                aio_context_release(tmp_context);
+                aio_context_acquire(aio_context);
+            }
+
             bdrv_replace_node(state->new_bs, state->old_bs, &error_abort);
             bdrv_unref(state->old_bs); /* bdrv_replace_node() ref'ed old_bs */