migration: make .post_save() a void function

All other handlers now have _errp() variants. Should we go this way
for .post_save()? Actually it's rather strange, when the vmstate do
successful preparations in .pre_save(), then successfully save all
sections and subsections, end then fail when all the state is
successfully transferred to the target.

Happily, we have only three .post_save() realizations, all always
successful. Let's make this a rule.

Also note, that we call .post_save() in two places, and handle
its (theoretical) failure inconsistently. Fix that too.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
Reviewed-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Zhao Liu <zhao1.liu@intel.com> #rust
Link: https://lore.kernel.org/qemu-devel/20260304212303.667141-4-vsementsov@yandex-team.ru
Signed-off-by: Fabiano Rosas <farosas@suse.de>

2 files changed, 9 insertions, 9 deletions

diff --git a/rust/migration/src/migratable.rs b/rust/migration/src/migratable.rs
index 7748aac2f2..b9e5e1fc15 100644
--- a/rust/migration/src/migratable.rs
+++ b/rust/migration/src/migratable.rs
@@ -406,10 +406,8 @@ impl<T: ToMigrationStateShared> Migratable<T> {
         Ok(())
     }
 
-    fn post_save(&self) -> Result<(), InvalidError> {
-        let state = unsafe { Box::from_raw(self.migration_state.replace(ptr::null_mut())) };
-        drop(state);
-        Ok(())
+    fn post_save(&self) {
+        let _ = unsafe { Box::from_raw(self.migration_state.replace(ptr::null_mut())) };
     }
 
     fn pre_load(&self) -> Result<(), InvalidError> {
diff --git a/rust/migration/src/vmstate.rs b/rust/migration/src/vmstate.rs
index edc7c70265..f34a36f680 100644
--- a/rust/migration/src/vmstate.rs
+++ b/rust/migration/src/vmstate.rs
@@ -492,6 +492,11 @@ unsafe extern "C" fn vmstate_no_version_cb<
     into_neg_errno(result)
 }
 
+unsafe extern "C" fn vmstate_post_save_cb<T, F: for<'a> FnCall<(&'a T,), ()>>(opaque: *mut c_void) {
+    // SAFETY: the function is used in T's implementation of VMState.
+    F::call((unsafe { &*(opaque.cast::<T>()) },));
+}
+
 unsafe extern "C" fn vmstate_post_load_cb<
     T,
     F: for<'a> FnCall<(&'a T, u8), Result<(), impl Into<Errno>>>,
@@ -597,12 +602,9 @@ impl<T> VMStateDescriptionBuilder<T> {
     }
 
     #[must_use]
-    pub const fn post_save<F: for<'a> FnCall<(&'a T,), Result<(), impl Into<Errno>>>>(
-        mut self,
-        _f: &F,
-    ) -> Self {
+    pub const fn post_save<F: for<'a> FnCall<(&'a T,), ()>>(mut self, _f: &F) -> Self {
         self.0.post_save = if F::IS_SOME {
-            Some(vmstate_no_version_cb::<T, F>)
+            Some(vmstate_post_save_cb::<T, F>)
         } else {
             None
         };