Update docs with return payload

signature-aggregator/README.md

@@ -29,10 +29,20 @@ The only exposed endpoint is `/aggregate-signatures`  expecting `application/jso
     "message": "",            // (string) hex-encoded unsigned message bytes to be signed
     "justification": "",      // (string) hex-encoded bytes to supply to the validators as justification
     "signing-subnet-id": "",  // (string) hex or cb58 encoded signing subnet ID. Defaults to source blockchain's subnet from data if omitted.
-    "quorum-percentage": 67  // (int) quorum percentage required to sign the message. Defaults to 67 if omitted
+    "quorum-percentage": 67   // (int) quorum percentage required to sign the message. Defaults to 67 if omitted
 }
 ```
 
+The successful `HTTP 200` response format is
+
+```json
+{
+    "signed-message": ""  // (string) hex-encoded signed message bytes signed by at least `quorum-percentage` of the validator set.
+}
+```
+
+Unsuccessful responses will include a `40x-50x` status codes with an explanatory `application/json` encoded message in the body of the response.
+
 ## Sample workflow
 If you want to manually test a locally running service pointed to the Fuji testnet you can do so with the following steps.
 

signature-aggregator/api/api.go

@@ -58,6 +58,7 @@ func HandleAggregateSignaturesByRawMsgRequest(
 func writeJSONError(
 	logger logging.Logger,
 	w http.ResponseWriter,
+	httpStatusCode int,
 	errorMsg string,
 ) {
 	resp, err := json.Marshal(struct{ error string }{error: errorMsg})
@@ -68,6 +69,7 @@ func writeJSONError(
 	}
 
 	w.Header().Set("Content-Type", "application/json")
+	w.WriteHeader(httpStatusCode)
 
 	w.Write(resp)
 	if err != nil {
@@ -89,7 +91,7 @@ func signatureAggregationAPIHandler(
 		if err != nil {
 			msg := "Could not decode request body"
 			logger.Warn(msg, zap.Error(err))
-			writeJSONError(logger, w, msg)
+			writeJSONError(logger, w, http.StatusBadRequest, msg)
 			return
 		}
 		var decodedMessage []byte
@@ -103,14 +105,14 @@ func signatureAggregationAPIHandler(
 				zap.String("msg", req.UnsignedMessage),
 				zap.Error(err),
 			)
-			writeJSONError(logger, w, msg)
+			writeJSONError(logger, w, http.StatusBadRequest, msg)
 			return
 		}
 		unsignedMessage, err := types.UnpackWarpMessage(decodedMessage)
 		if err != nil {
 			msg := "Error unpacking warp message"
 			logger.Warn(msg, zap.Error(err))
-			writeJSONError(logger, w, msg)
+			writeJSONError(logger, w, http.StatusBadRequest, msg)
 			return
 		}
 		quorumPercentage := req.QuorumPercentage
@@ -119,7 +121,7 @@ func signatureAggregationAPIHandler(
 		} else if req.QuorumPercentage > 100 {
 			msg := "Invalid quorum number"
 			logger.Warn(msg, zap.Uint64("quorum-num", req.QuorumPercentage))
-			writeJSONError(logger, w, msg)
+			writeJSONError(logger, w, http.StatusBadRequest, msg)
 			return
 		}
 		var signingSubnetID ids.ID
@@ -134,7 +136,7 @@ func signatureAggregationAPIHandler(
 					zap.Error(err),
 					zap.String("input", req.SigningSubnetID),
 				)
-				writeJSONError(logger, w, msg)
+				writeJSONError(logger, w, http.StatusBadRequest, msg)
 				return
 			}
 		}
@@ -147,7 +149,7 @@ func signatureAggregationAPIHandler(
 		if err != nil {
 			msg := "Failed to aggregate signatures"
 			logger.Warn(msg, zap.Error(err))
-			writeJSONError(logger, w, msg)
+			writeJSONError(logger, w, http.StatusInternalServerError, msg)
 			return
 		}
 		resp, err := json.Marshal(
@@ -161,7 +163,7 @@ func signatureAggregationAPIHandler(
 		if err != nil {
 			msg := "Failed to marshal response"
 			logger.Error(msg, zap.Error(err))
-			writeJSONError(logger, w, msg)
+			writeJSONError(logger, w, http.StatusInternalServerError, msg)
 			return
 		}
 		w.Header().Set("Content-Type", "application/json")