migration: Update error description whenever migration fails

There are places in migration.c where the migration is marked failed with
MIGRATION_STATUS_FAILED, but the failure reason is never updated. Hence
libvirt doesn't know why the migration failed when it queries for it.

Reviewed-by: Daniel P. Berrangé <berrange@redhat.com
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Tejus GK <tejus.gk@nutanix.com>
Message-ID: <20230621130940.178659-2-tejus.gk@nutanix.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>

migration/migration.c

@@ -1689,7 +1689,7 @@ void qmp_migrate(const char *uri, bool has_blk, bool blk,
         if (!resume_requested) {
             yank_unregister_instance(MIGRATION_YANK_INSTANCE);
         }
-        error_setg(errp, QERR_INVALID_PARAMETER_VALUE, "uri",
+        error_setg(&local_err, QERR_INVALID_PARAMETER_VALUE, "uri",
                    "a valid migration protocol");
         migrate_set_state(&s->state, MIGRATION_STATUS_SETUP,
                           MIGRATION_STATUS_FAILED);
@@ -2082,7 +2082,7 @@ migration_wait_main_channel(MigrationState *ms)
  * Switch from normal iteration to postcopy
  * Returns non-0 on error
  */
-static int postcopy_start(MigrationState *ms)
+static int postcopy_start(MigrationState *ms, Error **errp)
 {
     int ret;
     QIOChannelBuffer *bioc;
@@ -2192,7 +2192,7 @@ static int postcopy_start(MigrationState *ms)
      */
     ret = qemu_file_get_error(ms->to_dst_file);
     if (ret) {
-        error_report("postcopy_start: Migration stream errored (pre package)");
+        error_setg(errp, "postcopy_start: Migration stream errored (pre package)");
         goto fail_closefb;
     }
 
@@ -2229,7 +2229,7 @@ static int postcopy_start(MigrationState *ms)
 
     ret = qemu_file_get_error(ms->to_dst_file);
     if (ret) {
-        error_report("postcopy_start: Migration stream errored");
+        error_setg(errp, "postcopy_start: Migration stream errored");
         migrate_set_state(&ms->state, MIGRATION_STATUS_POSTCOPY_ACTIVE,
                               MIGRATION_STATUS_FAILED);
     }
@@ -2750,6 +2750,7 @@ typedef enum {
 static MigIterateState migration_iteration_run(MigrationState *s)
 {
     uint64_t must_precopy, can_postcopy;
+    Error *local_err = NULL;
     bool in_postcopy = s->state == MIGRATION_STATUS_POSTCOPY_ACTIVE;
     bool can_switchover = migration_can_switchover(s);
 
@@ -2773,8 +2774,9 @@ static MigIterateState migration_iteration_run(MigrationState *s)
     /* Still a significant amount to transfer */
     if (!in_postcopy && must_precopy <= s->threshold_size && can_switchover &&
         qatomic_read(&s->start_postcopy)) {
-        if (postcopy_start(s)) {
-            error_report("%s: postcopy failed to start", __func__);
+        if (postcopy_start(s, &local_err)) {
+            migrate_set_error(s, local_err);
+            error_report_err(local_err);
         }
         return MIG_ITERATE_SKIP;
     }
@@ -3265,8 +3267,10 @@ void migrate_fd_connect(MigrationState *s, Error *error_in)
      */
     if (migrate_postcopy_ram() || migrate_return_path()) {
         if (open_return_path_on_source(s, !resume)) {
-            error_report("Unable to open return-path for postcopy");
+            error_setg(&local_err, "Unable to open return-path for postcopy");
             migrate_set_state(&s->state, s->state, MIGRATION_STATUS_FAILED);
+            migrate_set_error(s, local_err);
+            error_report_err(local_err);
             migrate_fd_cleanup(s);
             return;
         }
@@ -3290,6 +3294,7 @@ void migrate_fd_connect(MigrationState *s, Error *error_in)
     }
 
     if (multifd_save_setup(&local_err) != 0) {
+        migrate_set_error(s, local_err);
         error_report_err(local_err);
         migrate_set_state(&s->state, MIGRATION_STATUS_SETUP,
                           MIGRATION_STATUS_FAILED);