[PATCH v9 24/27] migration: Propagate last encountered error in vmstate_save_state_v() function

[Arun Menon]

We consider,
  - the saving of the device state (save or subsection save) and
  - running the cleanup after (post_save)
as an atomic operation. And that is why, post_save() is called regardless
of whether there is a preceeding error. This means that, it is possible
that we have 2 distict errors, one from the preceeding function and the
other from the post_save() function.

This commit changes the error handling behavior when two errors occur during
a save operation.

1) Preceding errors were propagated before, but they are now dismissed, if there
   is a new post_save() error.
    - A failure during the main save operation, means the system failed to
      reach a new desired state. A failure during the post-save cleanup,
      however, is a more critical issue as it can leave the system in an
      inconsistent or corrupted state. At present, all post_save() calls
      succeed. However, the introduction of error handling variants of these
      functions (post_save_errp) in the following commit, imply that we need
      to handle and propagate these errors. Therefore, we prioritize reporting
      the more severe error.

2) post_save() errors were dismissed before, but they are now propagated.
    - The original design implicitly assumed post-save hooks were infallible
      cleanup routines.
      This will not be the case after introducing the post/pre save/load errp
      variant functions. Dismissing these errors prevents users from
      identifying the specific operation that failed.
      The entire save-and-cleanup process is treated as a single
      logical operation, and all potential failures are communicated.

Signed-off-by: Arun Menon <arme...@redhat.com>
---
 migration/vmstate.c | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/migration/vmstate.c b/migration/vmstate.c
index 
fbc59caadbbcc75fe6de27b459aa9aa25e76aa0a..ef78a1e62ad92e9608de72d125da80ea496c8dd1
 100644
--- a/migration/vmstate.c
+++ b/migration/vmstate.c
@@ -554,6 +554,12 @@ static int vmstate_save_dispatch(QEMUFile *f,
                     error_setg(errp, "Save of field %s/%s failed",
                                 vmsd->name, field->name);
                     ps_ret = post_save_dispatch(vmsd, opaque, &local_err);
+                    if (ps_ret) {
+                        ret = ps_ret;
+                        error_free_or_abort(errp);
+                        error_setg(errp, "post-save for %s failed, ret: %d",
+                                   vmsd->name, ret);
+                    }
                     return ret;
                 }
 
@@ -603,10 +609,14 @@ int vmstate_save_state_v(QEMUFile *f, const 
VMStateDescription *vmsd,
     }
 
     ret = vmstate_subsection_save(f, vmsd, opaque, vmdesc, errp);
+
     ps_ret = post_save_dispatch(vmsd, opaque, &local_err);
-    if (!ret && ps_ret) {
+    if (ps_ret) {
+        if (ret) {
+            error_free_or_abort(errp);
+        }
         ret = ps_ret;
-        error_setg(errp, "post-save failed: %s", vmsd->name);
+        error_propagate(errp, local_err);
     }
 
     return ret;

-- 
2.50.1