[PATCH v2 3/7] migration: Introduce migrate_has_error()

[Peter Xu <peterx@redhat.com>]

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()).

Signed-off-by: Peter Xu <peterx@redhat.com>
---
 migration/migration.h |  1 +
 migration/migration.c | 15 +++++++++++++++
 2 files changed, 16 insertions(+)

diff --git a/migration/migration.h b/migration/migration.h
index 507f2f111e..7d916e13e1 100644
--- a/migration/migration.h
+++ b/migration/migration.h
@@ -483,6 +483,7 @@ 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_error(MigrationState *s, Error *error);
 
 void migrate_fd_connect(MigrationState *s, Error *error_in);
diff --git a/migration/migration.c b/migration/migration.c
index e5d207699b..c54c195603 100644
--- a/migration/migration.c
+++ b/migration/migration.c
@@ -1222,6 +1222,21 @@ void migrate_set_error(MigrationState *s, Error *error)
     }
 }
 
+/*
+ * Whether the migration state has error set?
+ *
+ * Note this function explicitly didn't use error_mutex, because it only
+ * reads the error pointer for a boolean status.
+ *
+ * As long as the Error* is set, it shouldn't be freed before migration
+ * cleanup, so any thread can use this helper to safely detect whether
+ * there's anything wrong happened already.
+ */
+bool migrate_has_error(MigrationState *s)
+{
+    return qatomic_read(&s->error);
+}
+
 static void migrate_error_free(MigrationState *s)
 {
     QEMU_LOCK_GUARD(&s->error_mutex);
-- 
2.41.0