migration: Add error_desc for file channel errors

Currently, there is no information about error if outgoing migration was failed
because of file channel errors.
Example (QMP session):
-> { "execute": "migrate", "arguments": { "uri": "exec:head -c 1" }}
<- { "return": {} }
...
-> { "execute": "query-migrate" }
<- { "return": { "status": "failed" }} // There is not error's description

And even in the QEMU's output there is nothing.

This patch
1) Adds errp for the most of QEMUFileOps
2) Adds qemu_file_get_error_obj/qemu_file_set_error_obj
3) And finally using of qemu_file_get_error_obj in migration.c

And now, the status for the mentioned fail will be:
-> { "execute": "query-migrate" }
<- { "return": { "status": "failed",
                 "error-desc": "Unable to write to command: Broken pipe" }}

Signed-off-by: Yury Kotov <yury-kotov@yandex-team.ru>
Message-Id: <20190422103420.15686-1-yury-kotov@yandex-team.ru>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

1 files changed, 8 insertions, 2 deletions

diff --git a/migration/migration.c b/migration/migration.c
index 8a607fe..2834296 100644
--- a/migration/migration.c
+++ b/migration/migration.c
@@ -2963,6 +2963,7 @@ static MigThrError migration_detect_error(MigrationState *s)
 {
     int ret;
     int state = s->state;
+    Error *local_error = NULL;
 
     if (state == MIGRATION_STATUS_CANCELLING ||
         state == MIGRATION_STATUS_CANCELLED) {
@@ -2971,13 +2972,18 @@ static MigThrError migration_detect_error(MigrationState *s)
     }
 
     /* Try to detect any file errors */
-    ret = qemu_file_get_error(s->to_dst_file);
-
+    ret = qemu_file_get_error_obj(s->to_dst_file, &local_error);
     if (!ret) {
         /* Everything is fine */
+        assert(!local_error);
         return MIG_THR_ERR_NONE;
     }
 
+    if (local_error) {
+        migrate_set_error(s, local_error);
+        error_free(local_error);
+    }
+
     if (state == MIGRATION_STATUS_POSTCOPY_ACTIVE && ret == -EIO) {
         /*
          * For postcopy, we allow the network to be down for a