def populate_hash_if_needed(msg: ForwardMsg) -> str:
"""Computes and assigns the unique hash for a ForwardMsg.
If the ForwardMsg already has a hash, this is a no-op.
Parameters
----------
msg : ForwardMsg
Returns
-------
string
The message's hash, returned here for convenience. (The hash
will also be assigned to the ForwardMsg; callers do not need
to do this.)
"""
if msg.hash == "":
# Move the message's metadata aside. It's not part of the
# hash calculation.
metadata = msg.metadata
msg.ClearField("metadata")
# MD5 is good enough for what we need, which is uniqueness.
hasher = hashlib.md5()
hasher.update(msg.SerializeToString())
msg.hash = hasher.hexdigest()
# Restore metadata.
msg.metadata.CopyFrom(metadata)
return msg.hash
