rgw/putobj: RadosWriter uses part head object for multipart parts

the cleanup logic in the RadosWrite destructor was using the wrong
`head_obj` to avoid races between cleanup and part re-uploads. it
pointed at the final location of the multipart upload, rather than the
head object of the current part

Fixes: https://tracker.ceph.com/issues/63642

Signed-off-by: Casey Bodley <cbodley@redhat.com>
(cherry picked from commit bbbf5c5)

Conflicts:
	rgw_putobj_processor.* uses sal::RGWObject instead of rgw_obj

src/rgw/rgw_putobj_processor.cc

@@ -74,6 +74,11 @@ static int process_completed(const AioResultList& completed, RawObjSet *written)
   return error.value_or(0);
 }
 
+void RadosWriter::set_head_obj(std::unique_ptr<rgw::sal::RGWObject> head)
+{
+  head_obj = std::move(head);
+}
+
 int RadosWriter::set_stripe_obj(const rgw_raw_obj& raw_obj)
 {
   stripe_obj = store->svc()->rados->obj(raw_obj);
@@ -389,6 +394,9 @@ int MultipartObjectProcessor::prepare_head()
   head_obj->raw_obj_to_obj(stripe_obj);
   head_obj->set_hash_source(target_obj->get_name());
 
+  // point part uploads at the part head instead of the final multipart head
+  writer.set_head_obj(head_obj->clone());
+
   r = writer.set_stripe_obj(stripe_obj);
   if (r < 0) {
     return r;

src/rgw/rgw_putobj_processor.h

@@ -104,6 +104,9 @@ class RadosWriter : public DataProcessor {
 
   ~RadosWriter();
 
+  // change the head object
+  void set_head_obj(std::unique_ptr<rgw::sal::RGWObject> head);
+
   // change the current stripe object
   int set_stripe_obj(const rgw_raw_obj& obj);