[PATCH v2 1/1] migration: Unregister yank if migration setup fails

[Leonardo Bras]

Currently, if a qemu instance is started with "-incoming defer" and
an incorect parameter is passed to "migrate_incoming", it will print the
expected error and reply with "duplicate yank instance" for any upcoming
"migrate_incoming" command.

This renders current qemu process unusable, and requires a new qemu
process to be started before accepting a migration.

This is caused by a yank_register_instance() that happens in
qemu_start_incoming_migration() but is never reverted if any error
happens.

Solves this by unregistering the instance if anything goes wrong
in the function, allowing a new "migrate_incoming" command to be
accepted.

Fixes: b5eea99ec2f ("migration: Add yank feature", 2021-01-13)
Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=1974366
Signed-off-by: Leonardo Bras <leobras@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>

---
Changes since v1:
 - Add ERRP_GUARD() at the beginning of the function, so it deals with
   errp passed as NULL, and does correct error propagation.
---
 migration/migration.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/migration/migration.c b/migration/migration.c
index 4228635d18..af0c72609f 100644
--- a/migration/migration.c
+++ b/migration/migration.c
@@ -454,6 +454,7 @@ void migrate_add_address(SocketAddress *address)
 
 static void qemu_start_incoming_migration(const char *uri, Error **errp)
 {
+    ERRP_GUARD();
     const char *p = NULL;
 
     if (!yank_register_instance(MIGRATION_YANK_INSTANCE, errp)) {
@@ -474,9 +475,13 @@ static void qemu_start_incoming_migration(const char *uri, 
Error **errp)
     } else if (strstart(uri, "fd:", &p)) {
         fd_start_incoming_migration(p, errp);
     } else {
-        yank_unregister_instance(MIGRATION_YANK_INSTANCE);
         error_setg(errp, "unknown migration protocol: %s", uri);
     }
+
+    if (*errp) {
+        yank_unregister_instance(MIGRATION_YANK_INSTANCE);
+    }
+
 }
 
 static void process_incoming_migration_bh(void *opaque)
-- 
2.32.0