MEGA S4 + Inconsistent part size

Hi,

I've noticed that my backups haven't been working for a while now and I just can't get it to work anymore. I use MEGA S4 as an S3 object storage provider with this configuration

```yaml
apiVersion: barmancloud.cnpg.io/v1
kind: ObjectStore
metadata:
name: my-objectstore-name
namespace: mynamespace
spec:
configuration:
data:
compression: snappy
jobs: 1
destinationPath: s3://my-bucket-name/
endpointURL: https://s3.eu-luxembourg.megas4.com
s3Credentials:
accessKeyId:
key: ACCESS_KEY_ID
name: mega-s3
secretAccessKey:
key: ACCESS_SECRET_KEY
name: mega-s3
wal:
compression: snappy
maxParallel: 1
logLevel: info
retentionPolicyIntervalSeconds: 1800
retentionPolicy: 14d
```

WAL uploads are still working fine. But on-demand backups and scheduled backups are failing with this error message

```
failure uploading data (An error occurred (InvalidPart) when calling the UploadPart operation: Inconsistent part size. S4 requires parts to have the same size (except a smaller size can be allowed for the last part) and consecutive sequential part numbers starting by 1)
```

The full json output

```json
{"level":"info","ts":"2026-06-24T16:48:07.752576576Z","logger":"barman-cloud-backup","msg":"2026-06-24 16:48:07,752 [104] ERROR: Backup failed uploading data (An error occurred (InvalidPart) when calling the UploadPart operation: Inconsistent part size. S4 requires parts to have the same size (except a smaller size can be allowed for the last part) and consecutive sequential part numbers starting by 1)","pipe":"stderr","logging_pod":"my-db-1"}
{"level":"info","ts":"2026-06-24T16:48:08.893659577Z","logger":"barman-cloud-backup","msg":"Exception ignored in: ","pipe":"stderr","logging_pod":"my-db-1"}
{"level":"info","ts":"2026-06-24T16:48:08.893914989Z","logger":"barman-cloud-backup","msg":"Traceback (most recent call last):","pipe":"stderr","logging_pod":"my-db-1"}
{"level":"info","ts":"2026-06-24T16:48:08.894077144Z","logger":"barman-cloud-backup","msg":" File \"/usr/lib/python3.13/tarfile.py\", line 415, in __del__","pipe":"stderr","logging_pod":"my-db-1"}
{"level":"info","ts":"2026-06-24T16:48:08.897247336Z","logger":"barman-cloud-backup","msg":" self.close()","pipe":"stderr","logging_pod":"my-db-1"}
{"level":"info","ts":"2026-06-24T16:48:08.897379955Z","logger":"barman-cloud-backup","msg":" File \"/usr/lib/python3.13/tarfile.py\", line 466, in close","pipe":"stderr","logging_pod":"my-db-1"}
{"level":"info","ts":"2026-06-24T16:48:08.897399926Z","logger":"barman-cloud-backup","msg":" self.fileobj.write(self.buf)","pipe":"stderr","logging_pod":"my-db-1"}
{"level":"info","ts":"2026-06-24T16:48:08.897432384Z","logger":"barman-cloud-backup","msg":" File \"/venv/lib/python3.13/site-packages/barman/cloud.py\", line 247, in write","pipe":"stderr","logging_pod":"my-db-1"}
{"level":"info","ts":"2026-06-24T16:48:08.897481828Z","logger":"barman-cloud-backup","msg":" if self.buffer and self.buffer.tell() > self.chunk_size:","pipe":"stderr","logging_pod":"my-db-1"}
{"level":"info","ts":"2026-06-24T16:48:08.897539826Z","logger":"barman-cloud-backup","msg":" File \"/usr/lib/python3.13/tempfile.py\", line 690, in func_wrapper","pipe":"stderr","logging_pod":"my-db-1"}
{"level":"info","ts":"2026-06-24T16:48:08.897543751Z","logger":"barman-cloud-backup","msg":" return func(*args, **kwargs)","pipe":"stderr","logging_pod":"my-db-1"}
{"level":"info","ts":"2026-06-24T16:48:08.897551773Z","logger":"barman-cloud-backup","msg":"ValueError: I/O operation on closed file","pipe":"stderr","logging_pod":"my-db-1"}
```

I'm pretty sure it has worked in the past. I've been searching a lot, but I haven't been able to find anything that might help me. Does anyone have any ideas? I think this might be something specific to MEGA. Does anyone know of a workaround for this, or has anyone seen something like this before?

Thanks in advance for any advice!

The version of the plugin being used is: **v0.13.0**

Best regards,
Paul

Start by inspecting the barman/cloud.py write path named in the traceback and the backup upload flow used by plugin v0.13.0. Reproduce the failure with the supplied ObjectStore configuration and MEGA S4, then compare the multipart behavior with the provider's required part sizing. Done means on-demand and scheduled backups upload successfully without breaking the WAL upload path.

Written by the indexing model from the issue text.