Introduce a helper to detect whether MigrationState.error is set for
whatever reason.  It is intended to not taking the error_mutex here because
neither do we reference the pointer, nor do we modify the pointer.  State
why it's safe to do so.

This is preparation work for any thread (e.g. source return path thread) to
setup errors in an unified way to MigrationState, rather than relying on
its own way to set errors (mark_source_rp_bad()).

Reviewed-by: Fabiano Rosas <farosas@suse.de>
Signed-off-by: Peter Xu <peterx@redhat.com>
---
 migration/migration.h | 1 +
 migration/migration.c | 7 +++++++
 2 files changed, 8 insertions(+)

diff --git a/migration/migration.h b/migration/migration.h
index 76e35a5ecf..1ba38eecfa 100644
--- a/migration/migration.h
+++ b/migration/migration.h
@@ -466,6 +466,7 @@ bool  migration_has_all_channels(void);
 uint64_t migrate_max_downtime(void);
 
 void migrate_set_error(MigrationState *s, Error *error);
+bool migrate_has_error(MigrationState *s);
 
 void migrate_fd_connect(MigrationState *s, Error *error_in);
 
diff --git a/migration/migration.c b/migration/migration.c
index 0f3ca168ed..84c17dfbbb 100644
--- a/migration/migration.c
+++ b/migration/migration.c
@@ -1237,6 +1237,13 @@ void migrate_set_error(MigrationState *s, Error *error)
     }
 }
 
+bool migrate_has_error(MigrationState *s)
+{
+    /* The lock is not helpful here, but still follow the rule */
+    QEMU_LOCK_GUARD(&s->error_mutex);
+    return qatomic_read(&s->error);
+}
+
 static void migrate_error_free(MigrationState *s)
 {
     QEMU_LOCK_GUARD(&s->error_mutex);
-- 
2.41.0