handle lambda errors correctly in mock server (#104)

motivation: better mock server

changes:
* add handler for :requestID/error
* return .accepted to lambda
* return .internalServerError to client + error json
* small refactoring of response code in mock server to make it DRYer

Author: tomerd

Sources/AWSLambdaRuntimeCore/Lambda+LocalServer.swift

@@ -149,6 +149,7 @@ private enum LocalLambda {
                 case .waitingForLambdaRequest, .waitingForLambdaResponse:
                     Self.invocations.append(invocation)
                 }
+
             // /next endpoint is called by the lambda polling for work
             case (.GET, let url) where url.hasSuffix(Consts.getNextInvocationURLSuffix):
                 // check if our server is in the correct state
@@ -168,7 +169,7 @@ private enum LocalLambda {
                         switch result {
                         case .failure(let error):
                             self.logger.error("invocation error: \(error)")
-                            self.writeResponse(context: context, response: .init(status: .internalServerError))
+                            self.writeResponse(context: context, status: .internalServerError)
                         case .success(let invocation):
                             Self.invocationState = .waitingForLambdaResponse(invocation)
                             self.writeResponse(context: context, response: invocation.makeResponse())
@@ -180,33 +181,61 @@ private enum LocalLambda {
                     Self.invocationState = .waitingForLambdaResponse(invocation)
                     self.writeResponse(context: context, response: invocation.makeResponse())
                 }
+
             // :requestID/response endpoint is called by the lambda posting the response
             case (.POST, let url) where url.hasSuffix(Consts.postResponseURLSuffix):
                 let parts = request.head.uri.split(separator: "/")
                 guard let requestID = parts.count > 2 ? String(parts[parts.count - 2]) : nil else {
                     // the request is malformed, since we were expecting a requestId in the path
-                    return self.writeResponse(context: context, response: .init(status: .badRequest))
+                    return self.writeResponse(context: context, status: .badRequest)
                 }
                 guard case .waitingForLambdaResponse(let invocation) = Self.invocationState else {
                     // a response was send, but we did not expect to receive one
                     self.logger.error("invalid invocation state \(Self.invocationState)")
-                    return self.writeResponse(context: context, response: .init(status: .unprocessableEntity))
+                    return self.writeResponse(context: context, status: .unprocessableEntity)
                 }
                 guard requestID == invocation.requestID else {
                     // the request's requestId is not matching the one we are expecting
                     self.logger.error("invalid invocation state request ID \(requestID) does not match expected \(invocation.requestID)")
-                    return self.writeResponse(context: context, response: .init(status: .badRequest))
+                    return self.writeResponse(context: context, status: .badRequest)
                 }
 
                 invocation.responsePromise.succeed(.init(status: .ok, body: request.body))
-                self.writeResponse(context: context, response: .init(status: .accepted))
+                self.writeResponse(context: context, status: .accepted)
+                Self.invocationState = .waitingForLambdaRequest
+
+            // :requestID/error endpoint is called by the lambda posting an error response
+            case (.POST, let url) where url.hasSuffix(Consts.postErrorURLSuffix):
+                let parts = request.head.uri.split(separator: "/")
+                guard let requestID = parts.count > 2 ? String(parts[parts.count - 2]) : nil else {
+                    // the request is malformed, since we were expecting a requestId in the path
+                    return self.writeResponse(context: context, status: .badRequest)
+                }
+                guard case .waitingForLambdaResponse(let invocation) = Self.invocationState else {
+                    // a response was send, but we did not expect to receive one
+                    self.logger.error("invalid invocation state \(Self.invocationState)")
+                    return self.writeResponse(context: context, status: .unprocessableEntity)
+                }
+                guard requestID == invocation.requestID else {
+                    // the request's requestId is not matching the one we are expecting
+                    self.logger.error("invalid invocation state request ID \(requestID) does not match expected \(invocation.requestID)")
+                    return self.writeResponse(context: context, status: .badRequest)
+                }
+
+                invocation.responsePromise.succeed(.init(status: .internalServerError, body: request.body))
+                self.writeResponse(context: context, status: .accepted)
                 Self.invocationState = .waitingForLambdaRequest
+
             // unknown call
             default:
-                self.writeResponse(context: context, response: .init(status: .notFound))
+                self.writeResponse(context: context, status: .notFound)
             }
         }
 
+        func writeResponse(context: ChannelHandlerContext, status: HTTPResponseStatus) {
+            self.writeResponse(context: context, response: .init(status: status))
+        }
+
         func writeResponse(context: ChannelHandlerContext, response: Response) {
             var headers = HTTPHeaders(response.headers ?? [])
             headers.add(name: "content-length", value: "\(response.body?.readableBytes ?? 0)")