Wrap EOF with helpful message for empty JSON request body

binding/json.go

@@ -7,6 +7,7 @@ package binding
 import (
 	"bytes"
 	"errors"
+	"fmt"
 	"io"
 	"net/http"
 
@@ -50,7 +51,11 @@ func decodeJSON(r io.Reader, obj any) error {
 		decoder.DisallowUnknownFields()
 	}
 	if err := decoder.Decode(obj); err != nil {
+		if err == io.EOF {
+			return fmt.Errorf("empty request body: %w", err)
+		}
 		return err
 	}
+
 	return validate(obj)
 }

binding/json_external_test.go

@@ -0,0 +1,38 @@
+package binding_test
+
+import (
+	"bytes"
+	"io"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/gin-gonic/gin"
+	"github.com/stretchr/testify/assert"
+)
+
+func TestJSONBindingEmptyBodyReturnsHelpfulError(t *testing.T) {
+	type Req struct {
+		Name string `json:"name" binding:"required"`
+	}
+
+	w := httptest.NewRecorder()
+	c, _ := gin.CreateTestContext(w)
+
+	req, err := http.NewRequest(http.MethodPost, "/", bytes.NewBuffer(nil))
+	assert.NoError(t, err)
+	req.Header.Set("Content-Type", "application/json")
+
+	c.Request = req
+
+	var r Req
+	err = c.ShouldBindJSON(&r)
+
+	assert.Error(t, err)
+
+	// Current behavior returns plain "EOF", which is not helpful.
+	assert.NotEqual(t, "EOF", err.Error(), "error message should not be plain EOF")
+	assert.Contains(t, err.Error(), "empty request body")
+	assert.ErrorIs(t, err, io.EOF)
+
+}