Don't check the author_signature when the author is the parent author

The signature from the Magic Envelope is enough for that scenario,
because it's all from the same person.

docs/federation/relayable.md

@@ -24,14 +24,17 @@ All relayables have the following properties:
 
 ## Relaying
 
-The author of the relayable sends the entity to the parent author. The author must include the `author_signature`.
+If the author is not the same as the parent author, the author of the relayable sends the entity to the parent author
+and the author must include the `author_signature`.
 
 The parent author then must envelop it in a new [Magic Envelope][magicsig] and send the entity to all the recipients
 of the parent entity. If the author and the parent author are on the same server, the author must sign the
 `author_signature` and the parent author needs to sign the Magic Envelope.
 
 If someone other then the parent author receives a relayable without a valid Magic Envelope signed from
-the parent author, it must be ignored. If the `author_signature` is missing or invalid, it also must be ignored.
+the parent author, it must be ignored. If the author is not the same as the parent author and the `author_signature`
+is missing or invalid, it also must be ignored. If the author is the same as the parent author, the `author_signature`
+can be missing, because a valid signature in the Magic Envelope from the author is enough in that case.
 
 ## Signatures
 

lib/diaspora_federation/entities/relayable.rb

@@ -70,13 +70,13 @@ module DiasporaFederation
         super(data)
       end
 
-      # Verifies the +author_signature+.
+      # Verifies the +author_signature+ if needed.
       # @see DiasporaFederation::Entities::Signable#verify_signature
       #
       # @raise [SignatureVerificationFailed] if the signature is not valid
       # @raise [PublicKeyNotFound] if no public key is found
       def verify_signature
-        super(author, :author_signature)
+        super(author, :author_signature) unless author == parent.author
       end
 
       def sender_valid?(sender)

spec/lib/diaspora_federation/entities/relayable_spec.rb

@@ -69,6 +69,15 @@ module DiasporaFederation
         }.to raise_error Entities::Relayable::SignatureVerificationFailed
       end
 
+      it "doesn't raise when no author signature was passed, but the author is also the parent author" do
+        hash[:author_signature] = nil
+        hash[:parent] = Fabricate(:related_entity, author: author, local: false)
+
+        expect {
+          Entities::SomeRelayable.new(hash, signature_order).verify_signature
+        }.not_to raise_error
+      end
+
       it "raises when bad author signature was passed" do
         hash[:author_signature] = sign_with_key(author_pkey, "bad signed string")