More gracefully handle inputs that exceed the limit

server/http.go

@@ -24,6 +24,7 @@ var (
 	ErrMissingBody        = errors.New("missing body")
 	ErrInvalidRequest     = errors.New("invalid request")
 	ErrInvalidRequestBody = errors.New("invalid request body")
+	ErrBodyTooLarge       = errors.New("body too large")
 )
 
 type contextKey int
@@ -215,15 +216,27 @@ func createRequestHandler(logger *logrus.Logger, svc Service, validators []Valid
 func getRequestFromHTTPRequest(r *http.Request) (tySugRequest, error) {
 	var req tySugRequest
 
-	b, err := ioutil.ReadAll(io.LimitReader(r.Body, maxBodySize))
+	var maxSizePlusOne int64 = maxBodySize + 1
+	b, err := ioutil.ReadAll(io.LimitReader(r.Body, maxSizePlusOne))
 	if err != nil {
 		if err == io.EOF {
 			return req, ErrMissingBody
 		}
 		return req, ErrInvalidRequest
 	}
 
-	err = json.Unmarshal(b, &req)
+	if int64(len(b)) == maxSizePlusOne {
+		return req, ErrBodyTooLarge
+	}
+
+	var limit int
+	if len(b) > maxBodySize {
+		limit = maxBodySize
+	} else {
+		limit = len(b)
+	}
+
+	err = json.Unmarshal(b[:limit], &req)
 	if err != nil {
 		return req, ErrInvalidRequestBody
 	}

server/http_test.go

@@ -0,0 +1,44 @@
+package server
+
+import (
+	"bytes"
+	"encoding/json"
+	"io"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+)
+
+func TestGetRequestFromHTTPRequest(t *testing.T) {
+
+	t.Run("empty payload", func(t *testing.T) {
+		request, err := createStubbedTySugRequest(strings.NewReader("{}"))
+		if err != nil {
+			t.Errorf("Expected error was thrown '%s'\n%+v", err, request)
+		}
+	})
+
+	t.Run("payload too large", func(t *testing.T) {
+		i := strings.Repeat("a", maxBodySize+1)
+		payload, _ := json.Marshal(tySugRequest{Input: i})
+		req := httptest.NewRequest(http.MethodPost, "/", bytes.NewReader(payload))
+
+		request, err := getRequestFromHTTPRequest(req)
+		if err != ErrBodyTooLarge {
+			t.Errorf("Expected an error to be thrown %+v", request)
+		}
+	})
+
+	t.Run("invalid request body", func(t *testing.T) {
+		request, err := createStubbedTySugRequest(strings.NewReader("{Input: nil}"))
+		if err != ErrInvalidRequestBody {
+			t.Errorf("Expected an error to be thrown %+v", request)
+		}
+	})
+}
+
+func createStubbedTySugRequest(r io.Reader) (tySugRequest, error) {
+	req := httptest.NewRequest(http.MethodPost, "/", r)
+	return getRequestFromHTTPRequest(req)
+}