diff --git a/swh/model/model.py b/swh/model/model.py
index 7f14e88..65ffdfd 100644
--- a/swh/model/model.py
+++ b/swh/model/model.py
@@ -1,1074 +1,1074 @@
 # Copyright (C) 2018-2020 The Software Heritage developers
 # See the AUTHORS file at the top-level directory of this distribution
 # License: GNU General Public License version 3, or any later version
 # See top-level LICENSE file for more information
 
 from abc import ABCMeta, abstractmethod
 import datetime
 from enum import Enum
 from hashlib import sha256
 from typing import Any, Dict, Iterable, Optional, Tuple, TypeVar, Union
 
 import attr
 from attrs_strict import type_validator
 import dateutil.parser
 import iso8601
 from typing_extensions import Final
 
 from .collections import ImmutableDict
 from .hashutil import DEFAULT_ALGORITHMS, MultiHash, hash_to_bytes
 from .identifiers import (
     SWHID,
     directory_identifier,
     normalize_timestamp,
     parse_swhid,
     release_identifier,
     revision_identifier,
     snapshot_identifier,
 )
 
 
 class MissingData(Exception):
     """Raised by `Content.with_data` when it has no way of fetching the
     data (but not when fetching the data fails)."""
 
     pass
 
 
 KeyType = Union[Dict[str, str], Dict[str, bytes], bytes]
 """The type returned by BaseModel.unique_key()."""
 
 
 SHA1_SIZE = 20
 
 # TODO: Limit this to 20 bytes
 Sha1Git = bytes
 Sha1 = bytes
 
 
 KT = TypeVar("KT")
 VT = TypeVar("VT")
 
 
 def freeze_optional_dict(
     d: Union[None, Dict[KT, VT], ImmutableDict[KT, VT]]  # type: ignore
 ) -> Optional[ImmutableDict[KT, VT]]:
     if isinstance(d, dict):
         return ImmutableDict(d)
     else:
         return d
 
 
 def dictify(value):
     "Helper function used by BaseModel.to_dict()"
     if isinstance(value, BaseModel):
         return value.to_dict()
     elif isinstance(value, SWHID):
         return str(value)
     elif isinstance(value, Enum):
         return value.value
     elif isinstance(value, (dict, ImmutableDict)):
         return {k: dictify(v) for k, v in value.items()}
     elif isinstance(value, tuple):
         return tuple(dictify(v) for v in value)
     else:
         return value
 
 
 ModelType = TypeVar("ModelType", bound="BaseModel")
 
 
 class BaseModel:
     """Base class for SWH model classes.
 
     Provides serialization/deserialization to/from Python dictionaries,
     that are suitable for JSON/msgpack-like formats."""
 
     __slots__ = ()
 
     def to_dict(self):
         """Wrapper of `attr.asdict` that can be overridden by subclasses
         that have special handling of some of the fields."""
         return dictify(attr.asdict(self, recurse=False))
 
     @classmethod
     def from_dict(cls, d):
         """Takes a dictionary representing a tree of SWH objects, and
         recursively builds the corresponding objects."""
         return cls(**d)
 
     def anonymize(self: ModelType) -> Optional[ModelType]:
         """Returns an anonymized version of the object, if needed.
 
         If the object model does not need/support anonymization, returns None.
         """
         return None
 
     def unique_key(self) -> KeyType:
         """Returns a unique key for this object, that can be used for
         deduplication."""
         raise NotImplementedError(f"unique_key for {self}")
 
 
 class HashableObject(metaclass=ABCMeta):
     """Mixin to automatically compute object identifier hash when
     the associated model is instantiated."""
 
     __slots__ = ()
 
     @abstractmethod
     def compute_hash(self) -> bytes:
         """Derived model classes must implement this to compute
         the object hash.
 
         This method is called by the object initialization if the `id`
         attribute is set to an empty value.
         """
         pass
 
     def __attrs_post_init__(self):
         if not self.id:
             obj_id = self.compute_hash()
             object.__setattr__(self, "id", obj_id)
 
     def unique_key(self) -> KeyType:
         return self.id  # type: ignore
 
 
 @attr.s(frozen=True, slots=True)
 class Person(BaseModel):
     """Represents the author/committer of a revision or release."""
 
     object_type: Final = "person"
 
     fullname = attr.ib(type=bytes, validator=type_validator())
     name = attr.ib(type=Optional[bytes], validator=type_validator())
     email = attr.ib(type=Optional[bytes], validator=type_validator())
 
     @classmethod
     def from_fullname(cls, fullname: bytes):
         """Returns a Person object, by guessing the name and email from the
         fullname, in the `name <email>` format.
 
         The fullname is left unchanged."""
         if fullname is None:
             raise TypeError("fullname is None.")
 
         name: Optional[bytes]
         email: Optional[bytes]
 
         try:
             open_bracket = fullname.index(b"<")
         except ValueError:
             name = fullname
             email = None
         else:
             raw_name = fullname[:open_bracket]
             raw_email = fullname[open_bracket + 1 :]
 
             if not raw_name:
                 name = None
             else:
                 name = raw_name.strip()
 
             try:
                 close_bracket = raw_email.rindex(b">")
             except ValueError:
                 email = raw_email
             else:
                 email = raw_email[:close_bracket]
 
         return Person(name=name or None, email=email or None, fullname=fullname,)
 
     def anonymize(self) -> "Person":
         """Returns an anonymized version of the Person object.
 
         Anonymization is simply a Person which fullname is the hashed, with unset name
         or email.
         """
         return Person(fullname=sha256(self.fullname).digest(), name=None, email=None,)
 
 
 @attr.s(frozen=True, slots=True)
 class Timestamp(BaseModel):
     """Represents a naive timestamp from a VCS."""
 
     object_type: Final = "timestamp"
 
     seconds = attr.ib(type=int, validator=type_validator())
     microseconds = attr.ib(type=int, validator=type_validator())
 
     @seconds.validator
     def check_seconds(self, attribute, value):
         """Check that seconds fit in a 64-bits signed integer."""
         if not (-(2 ** 63) <= value < 2 ** 63):
             raise ValueError("Seconds must be a signed 64-bits integer.")
 
     @microseconds.validator
     def check_microseconds(self, attribute, value):
         """Checks that microseconds are positive and < 1000000."""
         if not (0 <= value < 10 ** 6):
             raise ValueError("Microseconds must be in [0, 1000000[.")
 
 
 @attr.s(frozen=True, slots=True)
 class TimestampWithTimezone(BaseModel):
     """Represents a TZ-aware timestamp from a VCS."""
 
     object_type: Final = "timestamp_with_timezone"
 
     timestamp = attr.ib(type=Timestamp, validator=type_validator())
     offset = attr.ib(type=int, validator=type_validator())
     negative_utc = attr.ib(type=bool, validator=type_validator())
 
     @offset.validator
     def check_offset(self, attribute, value):
         """Checks the offset is a 16-bits signed integer (in theory, it
         should always be between -14 and +14 hours)."""
         if not (-(2 ** 15) <= value < 2 ** 15):
             # max 14 hours offset in theory, but you never know what
             # you'll find in the wild...
             raise ValueError("offset too large: %d minutes" % value)
 
     @negative_utc.validator
     def check_negative_utc(self, attribute, value):
         if self.offset and value:
             raise ValueError("negative_utc can only be True is offset=0")
 
     @classmethod
     def from_dict(cls, obj: Union[Dict, datetime.datetime, int]):
         """Builds a TimestampWithTimezone from any of the formats
         accepted by :func:`swh.model.normalize_timestamp`."""
         # TODO: this accept way more types than just dicts; find a better
         # name
         d = normalize_timestamp(obj)
         return cls(
             timestamp=Timestamp.from_dict(d["timestamp"]),
             offset=d["offset"],
             negative_utc=d["negative_utc"],
         )
 
     @classmethod
     def from_datetime(cls, dt: datetime.datetime):
         return cls.from_dict(dt)
 
     @classmethod
     def from_iso8601(cls, s):
         """Builds a TimestampWithTimezone from an ISO8601-formatted string.
         """
         dt = iso8601.parse_date(s)
         tstz = cls.from_datetime(dt)
         if dt.tzname() == "-00:00":
             tstz = attr.evolve(tstz, negative_utc=True)
         return tstz
 
 
 @attr.s(frozen=True, slots=True)
 class Origin(BaseModel):
     """Represents a software source: a VCS and an URL."""
 
     object_type: Final = "origin"
 
     url = attr.ib(type=str, validator=type_validator())
 
     def unique_key(self) -> KeyType:
         return {"url": self.url}
 
 
 @attr.s(frozen=True, slots=True)
 class OriginVisit(BaseModel):
     """Represents an origin visit with a given type at a given point in time, by a
     SWH loader."""
 
     object_type: Final = "origin_visit"
 
     origin = attr.ib(type=str, validator=type_validator())
     date = attr.ib(type=datetime.datetime, validator=type_validator())
     type = attr.ib(type=str, validator=type_validator())
     """Should not be set before calling 'origin_visit_add()'."""
     visit = attr.ib(type=Optional[int], validator=type_validator(), default=None)
 
     @date.validator
     def check_date(self, attribute, value):
         """Checks the date has a timezone."""
         if value is not None and value.tzinfo is None:
             raise ValueError("date must be a timezone-aware datetime.")
 
     def to_dict(self):
         """Serializes the date as a string and omits the visit id if it is
         `None`."""
         ov = super().to_dict()
         if ov["visit"] is None:
             del ov["visit"]
         return ov
 
     def unique_key(self) -> KeyType:
         return {"origin": self.origin, "date": str(self.date)}
 
 
 @attr.s(frozen=True, slots=True)
 class OriginVisitStatus(BaseModel):
     """Represents a visit update of an origin at a given point in time.
 
     """
 
     object_type: Final = "origin_visit_status"
 
     origin = attr.ib(type=str, validator=type_validator())
     visit = attr.ib(type=int, validator=type_validator())
 
     date = attr.ib(type=datetime.datetime, validator=type_validator())
     status = attr.ib(
         type=str,
         validator=attr.validators.in_(["created", "ongoing", "full", "partial"]),
     )
     snapshot = attr.ib(type=Optional[Sha1Git], validator=type_validator())
     metadata = attr.ib(
         type=Optional[ImmutableDict[str, object]],
         validator=type_validator(),
         converter=freeze_optional_dict,
         default=None,
     )
 
     @date.validator
     def check_date(self, attribute, value):
         """Checks the date has a timezone."""
         if value is not None and value.tzinfo is None:
             raise ValueError("date must be a timezone-aware datetime.")
 
     def unique_key(self) -> KeyType:
         return {"origin": self.origin, "visit": str(self.visit), "date": str(self.date)}
 
 
 class TargetType(Enum):
     """The type of content pointed to by a snapshot branch. Usually a
     revision or an alias."""
 
     CONTENT = "content"
     DIRECTORY = "directory"
     REVISION = "revision"
     RELEASE = "release"
     SNAPSHOT = "snapshot"
     ALIAS = "alias"
 
 
 class ObjectType(Enum):
     """The type of content pointed to by a release. Usually a revision"""
 
     CONTENT = "content"
     DIRECTORY = "directory"
     REVISION = "revision"
     RELEASE = "release"
     SNAPSHOT = "snapshot"
 
 
 @attr.s(frozen=True, slots=True)
 class SnapshotBranch(BaseModel):
     """Represents one of the branches of a snapshot."""
 
     object_type: Final = "snapshot_branch"
 
     target = attr.ib(type=bytes, validator=type_validator())
     target_type = attr.ib(type=TargetType, validator=type_validator())
 
     @target.validator
     def check_target(self, attribute, value):
         """Checks the target type is not an alias, checks the target is a
         valid sha1_git."""
         if self.target_type != TargetType.ALIAS and self.target is not None:
             if len(value) != 20:
                 raise ValueError("Wrong length for bytes identifier: %d" % len(value))
 
     @classmethod
     def from_dict(cls, d):
         return cls(target=d["target"], target_type=TargetType(d["target_type"]))
 
 
 @attr.s(frozen=True, slots=True)
 class Snapshot(HashableObject, BaseModel):
     """Represents the full state of an origin at a given point in time."""
 
     object_type: Final = "snapshot"
 
     branches = attr.ib(
         type=ImmutableDict[bytes, Optional[SnapshotBranch]],
         validator=type_validator(),
         converter=freeze_optional_dict,
     )
     id = attr.ib(type=Sha1Git, validator=type_validator(), default=b"")
 
     def compute_hash(self) -> bytes:
         return hash_to_bytes(snapshot_identifier(self.to_dict()))
 
     @classmethod
     def from_dict(cls, d):
         d = d.copy()
         return cls(
             branches=ImmutableDict(
                 (name, SnapshotBranch.from_dict(branch) if branch else None)
                 for (name, branch) in d.pop("branches").items()
             ),
             **d,
         )
 
 
 @attr.s(frozen=True, slots=True)
 class Release(HashableObject, BaseModel):
     object_type: Final = "release"
 
     name = attr.ib(type=bytes, validator=type_validator())
     message = attr.ib(type=Optional[bytes], validator=type_validator())
     target = attr.ib(type=Optional[Sha1Git], validator=type_validator())
     target_type = attr.ib(type=ObjectType, validator=type_validator())
     synthetic = attr.ib(type=bool, validator=type_validator())
     author = attr.ib(type=Optional[Person], validator=type_validator(), default=None)
     date = attr.ib(
         type=Optional[TimestampWithTimezone], validator=type_validator(), default=None
     )
     metadata = attr.ib(
         type=Optional[ImmutableDict[str, object]],
         validator=type_validator(),
         converter=freeze_optional_dict,
         default=None,
     )
     id = attr.ib(type=Sha1Git, validator=type_validator(), default=b"")
 
     def compute_hash(self) -> bytes:
         return hash_to_bytes(release_identifier(self.to_dict()))
 
     @author.validator
     def check_author(self, attribute, value):
         """If the author is `None`, checks the date is `None` too."""
         if self.author is None and self.date is not None:
             raise ValueError("release date must be None if author is None.")
 
     def to_dict(self):
         rel = super().to_dict()
         if rel["metadata"] is None:
             del rel["metadata"]
         return rel
 
     @classmethod
     def from_dict(cls, d):
         d = d.copy()
         if d.get("author"):
             d["author"] = Person.from_dict(d["author"])
         if d.get("date"):
             d["date"] = TimestampWithTimezone.from_dict(d["date"])
         return cls(target_type=ObjectType(d.pop("target_type")), **d)
 
     def anonymize(self) -> "Release":
         """Returns an anonymized version of the Release object.
 
         Anonymization consists in replacing the author with an anonymized Person object.
         """
         author = self.author and self.author.anonymize()
         return attr.evolve(self, author=author)
 
 
 class RevisionType(Enum):
     GIT = "git"
     TAR = "tar"
     DSC = "dsc"
     SUBVERSION = "svn"
     MERCURIAL = "hg"
 
 
 def tuplify_extra_headers(value: Iterable):
     return tuple((k, v) for k, v in value)
 
 
 @attr.s(frozen=True, slots=True)
 class Revision(HashableObject, BaseModel):
     object_type: Final = "revision"
 
     message = attr.ib(type=Optional[bytes], validator=type_validator())
     author = attr.ib(type=Person, validator=type_validator())
     committer = attr.ib(type=Person, validator=type_validator())
     date = attr.ib(type=Optional[TimestampWithTimezone], validator=type_validator())
     committer_date = attr.ib(
         type=Optional[TimestampWithTimezone], validator=type_validator()
     )
     type = attr.ib(type=RevisionType, validator=type_validator())
     directory = attr.ib(type=Sha1Git, validator=type_validator())
     synthetic = attr.ib(type=bool, validator=type_validator())
     metadata = attr.ib(
         type=Optional[ImmutableDict[str, object]],
         validator=type_validator(),
         converter=freeze_optional_dict,
         default=None,
     )
     parents = attr.ib(type=Tuple[Sha1Git, ...], validator=type_validator(), default=())
     id = attr.ib(type=Sha1Git, validator=type_validator(), default=b"")
     extra_headers = attr.ib(
         type=Tuple[Tuple[bytes, bytes], ...],
         validator=type_validator(),
         converter=tuplify_extra_headers,
         default=(),
     )
 
     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)
 
     def compute_hash(self) -> bytes:
         return hash_to_bytes(revision_identifier(self.to_dict()))
 
     @classmethod
     def from_dict(cls, d):
         d = d.copy()
         date = d.pop("date")
         if date:
             date = TimestampWithTimezone.from_dict(date)
 
         committer_date = d.pop("committer_date")
         if committer_date:
             committer_date = TimestampWithTimezone.from_dict(committer_date)
 
         return cls(
             author=Person.from_dict(d.pop("author")),
             committer=Person.from_dict(d.pop("committer")),
             date=date,
             committer_date=committer_date,
             type=RevisionType(d.pop("type")),
             parents=tuple(d.pop("parents")),  # for BW compat
             **d,
         )
 
     def anonymize(self) -> "Revision":
         """Returns an anonymized version of the Revision object.
 
         Anonymization consists in replacing the author and committer with an anonymized
         Person object.
         """
         return attr.evolve(
             self, author=self.author.anonymize(), committer=self.committer.anonymize()
         )
 
 
 @attr.s(frozen=True, slots=True)
 class DirectoryEntry(BaseModel):
     object_type: Final = "directory_entry"
 
     name = attr.ib(type=bytes, validator=type_validator())
     type = attr.ib(type=str, validator=attr.validators.in_(["file", "dir", "rev"]))
     target = attr.ib(type=Sha1Git, validator=type_validator())
     perms = attr.ib(type=int, validator=type_validator())
     """Usually one of the values of `swh.model.from_disk.DentryPerms`."""
 
 
-@attr.s(frozen=True)
+@attr.s(frozen=True, slots=True)
 class Directory(HashableObject, BaseModel):
     object_type: Final = "directory"
 
     entries = attr.ib(type=Tuple[DirectoryEntry, ...], validator=type_validator())
     id = attr.ib(type=Sha1Git, validator=type_validator(), default=b"")
 
     def compute_hash(self) -> bytes:
         return hash_to_bytes(directory_identifier(self.to_dict()))
 
     @classmethod
     def from_dict(cls, d):
         d = d.copy()
         return cls(
             entries=tuple(
                 DirectoryEntry.from_dict(entry) for entry in d.pop("entries")
             ),
             **d,
         )
 
 
 @attr.s(frozen=True, slots=True)
 class BaseContent(BaseModel):
     status = attr.ib(
         type=str, validator=attr.validators.in_(["visible", "hidden", "absent"])
     )
 
     @staticmethod
     def _hash_data(data: bytes):
         """Hash some data, returning most of the fields of a content object"""
         d = MultiHash.from_data(data).digest()
         d["data"] = data
         d["length"] = len(data)
 
         return d
 
     @classmethod
     def from_dict(cls, d, use_subclass=True):
         if use_subclass:
             # Chooses a subclass to instantiate instead.
             if d["status"] == "absent":
                 return SkippedContent.from_dict(d)
             else:
                 return Content.from_dict(d)
         else:
             return super().from_dict(d)
 
     def get_hash(self, hash_name):
         if hash_name not in DEFAULT_ALGORITHMS:
             raise ValueError("{} is not a valid hash name.".format(hash_name))
         return getattr(self, hash_name)
 
     def hashes(self) -> Dict[str, bytes]:
         """Returns a dictionary {hash_name: hash_value}"""
         return {algo: getattr(self, algo) for algo in DEFAULT_ALGORITHMS}
 
 
 @attr.s(frozen=True, slots=True)
 class Content(BaseContent):
     object_type: Final = "content"
 
     sha1 = attr.ib(type=bytes, validator=type_validator())
     sha1_git = attr.ib(type=Sha1Git, validator=type_validator())
     sha256 = attr.ib(type=bytes, validator=type_validator())
     blake2s256 = attr.ib(type=bytes, validator=type_validator())
 
     length = attr.ib(type=int, validator=type_validator())
 
     status = attr.ib(
         type=str,
         validator=attr.validators.in_(["visible", "hidden"]),
         default="visible",
     )
 
     data = attr.ib(type=Optional[bytes], validator=type_validator(), default=None)
 
     ctime = attr.ib(
         type=Optional[datetime.datetime],
         validator=type_validator(),
         default=None,
         eq=False,
     )
 
     @length.validator
     def check_length(self, attribute, value):
         """Checks the length is positive."""
         if value < 0:
             raise ValueError("Length must be positive.")
 
     @ctime.validator
     def check_ctime(self, attribute, value):
         """Checks the ctime has a timezone."""
         if value is not None and value.tzinfo is None:
             raise ValueError("ctime must be a timezone-aware datetime.")
 
     def to_dict(self):
         content = super().to_dict()
         if content["data"] is None:
             del content["data"]
         if content["ctime"] is None:
             del content["ctime"]
         return content
 
     @classmethod
     def from_data(cls, data, status="visible", ctime=None) -> "Content":
         """Generate a Content from a given `data` byte string.
 
         This populates the Content with the hashes and length for the data
         passed as argument, as well as the data itself.
         """
         d = cls._hash_data(data)
         d["status"] = status
         d["ctime"] = ctime
         return cls(**d)
 
     @classmethod
     def from_dict(cls, d):
         if isinstance(d.get("ctime"), str):
             d = d.copy()
             d["ctime"] = dateutil.parser.parse(d["ctime"])
         return super().from_dict(d, use_subclass=False)
 
     def with_data(self) -> "Content":
         """Loads the `data` attribute; meaning that it is guaranteed not to
         be None after this call.
 
         This call is almost a no-op, but subclasses may overload this method
         to lazy-load data (eg. from disk or objstorage)."""
         if self.data is None:
             raise MissingData("Content data is None.")
         return self
 
     def unique_key(self) -> KeyType:
         return self.sha1  # TODO: use a dict of hashes
 
 
 @attr.s(frozen=True, slots=True)
 class SkippedContent(BaseContent):
     object_type: Final = "skipped_content"
 
     sha1 = attr.ib(type=Optional[bytes], validator=type_validator())
     sha1_git = attr.ib(type=Optional[Sha1Git], validator=type_validator())
     sha256 = attr.ib(type=Optional[bytes], validator=type_validator())
     blake2s256 = attr.ib(type=Optional[bytes], validator=type_validator())
 
     length = attr.ib(type=Optional[int], validator=type_validator())
 
     status = attr.ib(type=str, validator=attr.validators.in_(["absent"]))
     reason = attr.ib(type=Optional[str], validator=type_validator(), default=None)
 
     origin = attr.ib(type=Optional[str], validator=type_validator(), default=None)
 
     ctime = attr.ib(
         type=Optional[datetime.datetime],
         validator=type_validator(),
         default=None,
         eq=False,
     )
 
     @reason.validator
     def check_reason(self, attribute, value):
         """Checks the reason is full if status != absent."""
         assert self.reason == value
         if value is None:
             raise ValueError("Must provide a reason if content is absent.")
 
     @length.validator
     def check_length(self, attribute, value):
         """Checks the length is positive or -1."""
         if value < -1:
             raise ValueError("Length must be positive or -1.")
 
     @ctime.validator
     def check_ctime(self, attribute, value):
         """Checks the ctime has a timezone."""
         if value is not None and value.tzinfo is None:
             raise ValueError("ctime must be a timezone-aware datetime.")
 
     def to_dict(self):
         content = super().to_dict()
         if content["origin"] is None:
             del content["origin"]
         if content["ctime"] is None:
             del content["ctime"]
         return content
 
     @classmethod
     def from_data(
         cls, data: bytes, reason: str, ctime: Optional[datetime.datetime] = None
     ) -> "SkippedContent":
         """Generate a SkippedContent from a given `data` byte string.
 
         This populates the SkippedContent with the hashes and length for the
         data passed as argument.
 
         You can use `attr.evolve` on such a generated content to nullify some
         of its attributes, e.g. for tests.
         """
         d = cls._hash_data(data)
         del d["data"]
         d["status"] = "absent"
         d["reason"] = reason
         d["ctime"] = ctime
         return cls(**d)
 
     @classmethod
     def from_dict(cls, d):
         d2 = d.copy()
         if d2.pop("data", None) is not None:
             raise ValueError('SkippedContent has no "data" attribute %r' % d)
         return super().from_dict(d2, use_subclass=False)
 
     def unique_key(self) -> KeyType:
         return self.hashes()
 
 
 class MetadataAuthorityType(Enum):
     DEPOSIT_CLIENT = "deposit_client"
     FORGE = "forge"
     REGISTRY = "registry"
 
 
 @attr.s(frozen=True, slots=True)
 class MetadataAuthority(BaseModel):
     """Represents an entity that provides metadata about an origin or
     software artifact."""
 
     object_type: Final = "metadata_authority"
 
     type = attr.ib(type=MetadataAuthorityType, validator=type_validator())
     url = attr.ib(type=str, validator=type_validator())
     metadata = attr.ib(
         type=Optional[ImmutableDict[str, Any]],
         default=None,
         validator=type_validator(),
         converter=freeze_optional_dict,
     )
 
     def to_dict(self):
         d = super().to_dict()
         if d["metadata"] is None:
             del d["metadata"]
         return d
 
     @classmethod
     def from_dict(cls, d):
         d["type"] = MetadataAuthorityType(d["type"])
         return super().from_dict(d)
 
     def unique_key(self) -> KeyType:
         return {"type": self.type.value, "url": self.url}
 
 
 @attr.s(frozen=True, slots=True)
 class MetadataFetcher(BaseModel):
     """Represents a software component used to fetch metadata from a metadata
     authority, and ingest them into the Software Heritage archive."""
 
     object_type: Final = "metadata_fetcher"
 
     name = attr.ib(type=str, validator=type_validator())
     version = attr.ib(type=str, validator=type_validator())
     metadata = attr.ib(
         type=Optional[ImmutableDict[str, Any]],
         default=None,
         validator=type_validator(),
         converter=freeze_optional_dict,
     )
 
     def to_dict(self):
         d = super().to_dict()
         if d["metadata"] is None:
             del d["metadata"]
         return d
 
     def unique_key(self) -> KeyType:
         return {"name": self.name, "version": self.version}
 
 
 class MetadataTargetType(Enum):
     """The type of object extrinsic metadata refer to."""
 
     CONTENT = "content"
     DIRECTORY = "directory"
     REVISION = "revision"
     RELEASE = "release"
     SNAPSHOT = "snapshot"
     ORIGIN = "origin"
 
 
 @attr.s(frozen=True, slots=True)
 class RawExtrinsicMetadata(BaseModel):
     object_type: Final = "raw_extrinsic_metadata"
 
     # target object
     type = attr.ib(type=MetadataTargetType, validator=type_validator())
     target = attr.ib(type=Union[str, SWHID], validator=type_validator())
     """URL if type=MetadataTargetType.ORIGIN, else core SWHID"""
 
     # source
     discovery_date = attr.ib(type=datetime.datetime, validator=type_validator())
     authority = attr.ib(type=MetadataAuthority, validator=type_validator())
     fetcher = attr.ib(type=MetadataFetcher, validator=type_validator())
 
     # the metadata itself
     format = attr.ib(type=str, validator=type_validator())
     metadata = attr.ib(type=bytes, validator=type_validator())
 
     # context
     origin = attr.ib(type=Optional[str], default=None, validator=type_validator())
     visit = attr.ib(type=Optional[int], default=None, validator=type_validator())
     snapshot = attr.ib(type=Optional[SWHID], default=None, validator=type_validator())
     release = attr.ib(type=Optional[SWHID], default=None, validator=type_validator())
     revision = attr.ib(type=Optional[SWHID], default=None, validator=type_validator())
     path = attr.ib(type=Optional[bytes], default=None, validator=type_validator())
     directory = attr.ib(type=Optional[SWHID], default=None, validator=type_validator())
 
     @target.validator
     def check_target(self, attribute, value):
         if self.type == MetadataTargetType.ORIGIN:
             if isinstance(value, SWHID) or value.startswith("swh:"):
                 raise ValueError(
                     "Got SWHID as target for origin metadata (expected an URL)."
                 )
         else:
             self._check_swhid(self.type.value, value)
 
     @discovery_date.validator
     def check_discovery_date(self, attribute, value):
         """Checks the discovery_date has a timezone."""
         if value is not None and value.tzinfo is None:
             raise ValueError("discovery_date must be a timezone-aware datetime.")
 
     @origin.validator
     def check_origin(self, attribute, value):
         if value is None:
             return
 
         if self.type not in (
             MetadataTargetType.SNAPSHOT,
             MetadataTargetType.RELEASE,
             MetadataTargetType.REVISION,
             MetadataTargetType.DIRECTORY,
             MetadataTargetType.CONTENT,
         ):
             raise ValueError(
                 f"Unexpected 'origin' context for {self.type.value} object: {value}"
             )
 
         if value.startswith("swh:"):
             # Technically this is valid; but:
             # 1. SWHIDs are URIs, not URLs
             # 2. if a SWHID gets here, it's very likely to be a mistake
             #    (and we can remove this check if it turns out there is a
             #    legitimate use for it).
             raise ValueError(f"SWHID used as context origin URL: {value}")
 
     @visit.validator
     def check_visit(self, attribute, value):
         if value is None:
             return
 
         if self.type not in (
             MetadataTargetType.SNAPSHOT,
             MetadataTargetType.RELEASE,
             MetadataTargetType.REVISION,
             MetadataTargetType.DIRECTORY,
             MetadataTargetType.CONTENT,
         ):
             raise ValueError(
                 f"Unexpected 'visit' context for {self.type.value} object: {value}"
             )
 
         if self.origin is None:
             raise ValueError("'origin' context must be set if 'visit' is.")
 
         if value <= 0:
             raise ValueError("Nonpositive visit id")
 
     @snapshot.validator
     def check_snapshot(self, attribute, value):
         if value is None:
             return
 
         if self.type not in (
             MetadataTargetType.RELEASE,
             MetadataTargetType.REVISION,
             MetadataTargetType.DIRECTORY,
             MetadataTargetType.CONTENT,
         ):
             raise ValueError(
                 f"Unexpected 'snapshot' context for {self.type.value} object: {value}"
             )
 
         self._check_swhid("snapshot", value)
 
     @release.validator
     def check_release(self, attribute, value):
         if value is None:
             return
 
         if self.type not in (
             MetadataTargetType.REVISION,
             MetadataTargetType.DIRECTORY,
             MetadataTargetType.CONTENT,
         ):
             raise ValueError(
                 f"Unexpected 'release' context for {self.type.value} object: {value}"
             )
 
         self._check_swhid("release", value)
 
     @revision.validator
     def check_revision(self, attribute, value):
         if value is None:
             return
 
         if self.type not in (MetadataTargetType.DIRECTORY, MetadataTargetType.CONTENT,):
             raise ValueError(
                 f"Unexpected 'revision' context for {self.type.value} object: {value}"
             )
 
         self._check_swhid("revision", value)
 
     @path.validator
     def check_path(self, attribute, value):
         if value is None:
             return
 
         if self.type not in (MetadataTargetType.DIRECTORY, MetadataTargetType.CONTENT,):
             raise ValueError(
                 f"Unexpected 'path' context for {self.type.value} object: {value}"
             )
 
     @directory.validator
     def check_directory(self, attribute, value):
         if value is None:
             return
 
         if self.type not in (MetadataTargetType.CONTENT,):
             raise ValueError(
                 f"Unexpected 'directory' context for {self.type.value} object: {value}"
             )
 
         self._check_swhid("directory", value)
 
     def _check_swhid(self, expected_object_type, swhid):
         if isinstance(swhid, str):
             raise ValueError(f"Expected SWHID, got a string: {swhid}")
 
         if swhid.object_type != expected_object_type:
             raise ValueError(
                 f"Expected SWHID type '{expected_object_type}', "
                 f"got '{swhid.object_type}' in {swhid}"
             )
 
         if swhid.metadata:
             raise ValueError(f"Expected core SWHID, but got: {swhid}")
 
     def to_dict(self):
         d = super().to_dict()
 
         context_keys = (
             "origin",
             "visit",
             "snapshot",
             "release",
             "revision",
             "directory",
             "path",
         )
         for context_key in context_keys:
             if d[context_key] is None:
                 del d[context_key]
         return d
 
     @classmethod
     def from_dict(cls, d):
         d = {
             **d,
             "type": MetadataTargetType(d["type"]),
             "authority": MetadataAuthority.from_dict(d["authority"]),
             "fetcher": MetadataFetcher.from_dict(d["fetcher"]),
         }
 
         if d["type"] != MetadataTargetType.ORIGIN:
             d["target"] = parse_swhid(d["target"])
 
         swhid_keys = ("snapshot", "release", "revision", "directory")
         for swhid_key in swhid_keys:
             if d.get(swhid_key):
                 d[swhid_key] = parse_swhid(d[swhid_key])
 
         return super().from_dict(d)
 
     def unique_key(self) -> KeyType:
         return {
             "type": self.type.value,
             "target": str(self.target),
             "authority_type": self.authority.type.value,
             "authority_url": self.authority.url,
             "discovery_date": str(self.discovery_date),
             "fetcher_name": self.fetcher.name,
             "fetcher_version": self.fetcher.version,
         }