serialize signatures

This took a surprising number of goose chases to work out... I had to
find out
- what is the gmser prefix for a signature (sg_)
- what is the gmb wrapper for a signature (none)
- what errors gmser can report when a signature is invalid
- what an example of a valid signature is
- what that example signature serializes to

src/hz.erl

@@ -1806,29 +1806,20 @@ coerce({O, N, integer},  S, to_fate) when is_list(S) ->
         error:badarg -> single_error({invalid, O, N, S})
     end;
 coerce({O, N, address},  S, to_fate) ->
-    try
+    coerce_chain_object(O, N, address, account_pubkey, S);
-        case gmser_api_encoder:decode(unicode:characters_to_binary(S)) of
-            {account_pubkey, Key} -> {ok, {address, Key}};
-            _                     -> single_error({invalid, O, N, S})
-        end
-    catch
-        error:_ -> single_error({invalid, O, N, S})
-    end;
 coerce({_, _, address}, {address, Bin}, from_fate) ->
     Address = gmser_api_encoder:encode(account_pubkey, Bin),
     {ok, unicode:characters_to_list(Address)};
 coerce({O, N, contract}, S, to_fate) ->
-    try
+    coerce_chain_object(O, N, contract, contract_pubkey, S);
-        case gmser_api_encoder:decode(unicode:characters_to_binary(S)) of
-            {contract_pubkey, Key} -> {ok, {contract, Key}};
-            _                      -> single_error({invalid, O, N, S})
-        end
-    catch
-        error:_ -> single_error({invalid, O, N, S})
-    end;
 coerce({_, _, contract}, {contract, Bin}, from_fate) ->
     Address = gmser_api_encoder:encode(contract_pubkey, Bin),
     {ok, unicode:characters_to_list(Address)};
+coerce({O, N, signature}, S, to_fate) ->
+    coerce_chain_object(O, N, signature, signature, S);
+coerce({_, _, signature}, Bin, from_fate) ->
+    Address = gmser_api_encoder:encode(signature, Bin),
+    {ok, unicode:characters_to_list(Address)};
 coerce({_, _, boolean}, true, _) ->
     {ok, true};
 coerce({_, _, boolean}, false, _) ->
@@ -1897,6 +1888,27 @@ coerce({O, N, _}, Data, from_fate) ->
     {ok, Data};
 coerce({O, N, _}, Data, _) -> single_error({invalid, O, N, Data}).
 
+coerce_chain_object(O, N, T, Tag, S) ->
+    case decode_chain_object(Tag, S) of
+        {ok, Data}      -> {ok, coerce_chain_object2(T, Data)};
+        {error, Reason} -> single_error({Reason, O, N, S})
+    end.
+
+coerce_chain_object2(address, Data)   -> {address, Data};
+coerce_chain_object2(contract, Data)  -> {contract, Data};
+coerce_chain_object2(signature, Data) -> Data.
+
+decode_chain_object(Tag, S) ->
+    try
+        case gmser_api_encoder:decode(unicode:characters_to_binary(S)) of
+            {Tag, Data} -> {ok, Data};
+            {_, _} -> {error, wrong_prefix}
+        end
+    catch
+        error:missing_prefix -> {error, missing_prefix};
+        error:incorrect_size -> {error, incorrect_size}
+    end.
+
 coerce_list(Type, Elements, Direction) ->
     % 0 index since it represents a sophia list
     coerce_list(Type, Elements, Direction, 0, [], []).
@@ -2427,6 +2439,8 @@ try_coerce(Type, Sophia, Fate) ->
         _ ->
             erlang:error({from_fate_failed, Sophia, SophiaActual})
     end,
+    % Finally, check that the FATE result is something that gmb understands.
+    gmb_fate_encoding:serialize(Fate),
     ok.
 
 coerce_int_test() ->
@@ -2449,6 +2463,15 @@ coerce_contract_test() ->
                             167,208,53,78,40,235,2,163,132,36,47,183,228,151,9,
                             210,39,214>>}).
 
+coerce_signature_test() ->
+    {ok, Type} = annotate_type(signature, #{}),
+    try_coerce(Type,
+               "sg_XDyF8LJC4tpMyAySvpaG1f5V9F2XxAbRx9iuVjvvdNMwVracLhzAuXhRM5kXAFtpwW1DCHuz5jGehUayCah4jub32Ti2n",
+               <<231,4,97,129,16,173,37,42,194,249,28,94,134,163,208,84,22,135,
+                 169,85,212,142,14,12,233,252,97,50,193,158,229,51,123,206,222,
+                 249,2,3,85,173,106,150,243,253,89,128,248,52,195,140,95,114,
+                 233,110,119,143,206,137,124,36,63,154,85,7>>).
+
 coerce_bool_test() ->
     {ok, Type} = annotate_type(boolean, #{}),
     try_coerce(Type, true, true),