[PATCH v2] migration: Don't allow migration if vm is in POSTMIGRATE state

[Tuguoyi <tu.guoyi@h3c.com>]

The following steps will cause qemu assertion failure:
- pause vm by executing 'virsh suspend'
- create external snapshot of memory and disk using 'virsh snapshot-create-as'
- doing the above operation again will cause qemu crash

The backtrace looks like:
#0  0x00007fbf958c5c37 in raise () from /lib/x86_64-linux-gnu/libc.so.6
#1  0x00007fbf958c9028 in abort () from /lib/x86_64-linux-gnu/libc.so.6
#2  0x00007fbf958bebf6 in ?? () from /lib/x86_64-linux-gnu/libc.so.6
#3  0x00007fbf958beca2 in __assert_fail () from /lib/x86_64-linux-gnu/libc.so.6
#4  0x000055ca8decd39d in bdrv_inactivate_recurse (bs=0x55ca90c80400) at /build/qemu-5.0/block.c:5724
#5  0x000055ca8dece967 in bdrv_inactivate_all () at /build//qemu-5.0/block.c:5792
#6  0x000055ca8de5539d in qemu_savevm_state_complete_precopy_non_iterable (inactivate_disks=true, in_postcopy=false, f=0x55ca907044b0)
    at /build/qemu-5.0/migration/savevm.c:1401
#7  qemu_savevm_state_complete_precopy (f=0x55ca907044b0, iterable_only=iterable_only@entry=false, inactivate_disks=inactivate_disks@entry=true)
    at /build/qemu-5.0/migration/savevm.c:1453
#8  0x000055ca8de4f581 in migration_completion (s=0x55ca8f64d9f0) at /build/qemu-5.0/migration/migration.c:2941
#9  migration_iteration_run (s=0x55ca8f64d9f0) at /build/qemu-5.0/migration/migration.c:3295
#10 migration_thread (opaque=opaque@entry=0x55ca8f64d9f0) at /build/qemu-5.0/migration/migration.c:3459
#11 0x000055ca8dfc6716 in qemu_thread_start (args=<optimized out>) at /build/qemu-5.0/util/qemu-thread-posix.c:519
#12 0x00007fbf95c5f184 in start_thread () from /lib/x86_64-linux-gnu/libpthread.so.0
#13 0x00007fbf9598cbed in clone () from /lib/x86_64-linux-gnu/libc.so.6

When the first migration completes, bs->open_flags will set BDRV_O_INACTIVE
flag by bdrv_inactivate_all(), and during the second migration the
bdrv_inactivate_recurse assert that the bs->open_flags is already
BDRV_O_INACTIVE enabled which cause crash.

As Vladimir suggested, this patch just make migration job error-out with a
message in  migrate_fd_connect() if the vm is in RUN_STATE_POSTMIGRATE state.

Signed-off-by: Tuguoyi <tu.guoyi@h3c.com>
---
 migration/migration.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/migration/migration.c b/migration/migration.c
index 87a9b59..4091678 100644
--- a/migration/migration.c
+++ b/migration/migration.c
@@ -3622,6 +3622,13 @@ void migrate_fd_connect(MigrationState *s, Error *error_in)
         return;
     }
 
+    if (runstate_check(RUN_STATE_POSTMIGRATE)) {
+        error_report("Can't migrate the vm that is in POSTMIGRATE state");
+        migrate_set_state(&s->state, s->state, MIGRATION_STATUS_FAILED);
+        migrate_fd_cleanup(s);
+        return;
+    }
+
     if (resume) {
         /* This is a resumed migration */
         rate_limit = s->parameters.max_postcopy_bandwidth /
-- 
2.7.4

[Patch v1]: https://lists.gnu.org/archive/html/qemu-devel/2020-11/msg05950.html