The test:
```
def test_directory_metadata_indexer_single_root_dir(self):
        metadata_indexer = DirectoryMetadataIndexer(config=DIRECTORY_METADATA_CONFIG)
        fill_obj_storage(metadata_indexer.objstorage)
        fill_storage(metadata_indexer.storage)
```

Error thrown in `collections.py`:
```
class ImmutableDict(...):
    ...
    def __getitem__(self, key):
        for (k, v) in self.data:
            if k == key:
                return v
        raise KeyError(key)
```

Call stack that led to this:

```
__getitem__ (/home/vmz/swh-environment/swh-model/swh/model/collections.py:40)
__attrs_post_init__ (/home/vmz/swh-environment/swh-model/swh/model/model.py:873)----------------- 4
StorageData (/home/vmz/swh-environment/swh-storage/swh/storage/tests/storage_data.py:338)-------- 3
<module> (/home/vmz/swh-environment/swh-storage/swh/storage/tests/storage_data.py:40)------------ 2
<module> (/home/vmz/swh-environment/swh-storage/swh/storage/pytest_plugin.py:15) ---------------- 1
```

1:
```
from swh.storage.tests.storage_data import StorageData
```

2:
```
class StorageData:
```

3:
```
hg_revision = Revision(
        id=hash_to_bytes("951c9503541e7beaf002d7aebf2abd1629084c68"),
        message=b"hello",
        author=Person(
            name=b"Nicolas Dandrimont",
            email=b"nicolas@example.com",
            fullname=b"Nicolas Dandrimont <nicolas@example.com> ",
        ),
        date=TimestampWithTimezone(
            timestamp=Timestamp(seconds=1234567890, microseconds=0),
            offset_bytes=b"+0200",
        ),
        committer=Person(
            name=b"St\xc3fano Zacchiroli",
            email=b"stefano@example.com",
            fullname=b"St\xc3fano Zacchiroli <stefano@example.com>",
        ),
        committer_date=TimestampWithTimezone(
            timestamp=Timestamp(seconds=1123456789, microseconds=0),
            offset_bytes=b"+0200",
        ),
        parents=(),
        type=RevisionType.MERCURIAL,
        directory=directory.id,
        metadata={
            "checksums": {
                "sha1": "tarball-sha1",
                "sha256": "tarball-sha256",
            },
            "signed-off-by": "some-dude",
            "node": "a316dfb434af2b451c1f393496b7eaeda343f543",
        },
        extra_headers=(),
        synthetic=True,
    )
```

4:
```
class Revision(HashableObjectWithManifest, BaseModel):
    def __attrs_post_init__(self):
        super().__attrs_post_init__()
        # ensure metadata is a deep copy of whatever was given, and if needed
        # extract extra_headers from there
        if self.metadata:
            metadata = self.metadata
**          if not self.extra_headers and "extra_headers" in metadata:
                (extra_headers, metadata) = metadata.copy_pop("extra_headers")
                object.__setattr__(
                    self,
                    "extra_headers",
                    tuplify_extra_headers(extra_headers),
                )
                attr.validate(self)
            object.__setattr__(self, "metadata", metadata)

```