From 7ff7e0f043f036cba9511100e3b41bf0cff5e56a Mon Sep 17 00:00:00 2001
From: Gani Georgiev <gani.georgiev@gmail.com>
Date: Sun, 6 Sep 2026 19:51:06 +0300
Subject: [PATCH] handled io.ReadAll error with exact payload size

---
 apis/middlewares_body_limit.go      | 52 +++++++++++++++++++----------
 apis/middlewares_body_limit_test.go | 18 ++++++++++
 2 files changed, 52 insertions(+), 18 deletions(-)

diff --git a/apis/middlewares_body_limit.go b/apis/middlewares_body_limit.go
index 70b3fb5e..1c456156 100644
--- a/apis/middlewares_body_limit.go
+++ b/apis/middlewares_body_limit.go
@@ -84,57 +84,73 @@ func applyBodyLimit(e *core.RequestEvent, limitBytes int64) error {
 	}
 
 	// replace the request body
-	e.Request.Body = newLimitedReader(e.Request.Body, limitBytes)
+	e.Request.Body = newMaxBytesReader(e.Request.Body, limitBytes)
 
 	return nil
 }
 
-func newLimitedReader(body io.ReadCloser, limitBytes int64) *limitedReader {
-	return &limitedReader{
+func newMaxBytesReader(body io.ReadCloser, limitBytes int64) *maxBytesReader {
+	return &maxBytesReader{
 		ReadCloser: body,
 		limit:      limitBytes,
 		remaining:  limitBytes,
 	}
 }
 
-type limitedReader struct {
+// maxBytesReader is very similar to the http.MaxBytesReader but support
+// rereads and doesn't try to prematurely close the related response
+// to allow consequent middlewares to operate correctly.
+type maxBytesReader struct {
 	io.ReadCloser
 	limit     int64
 	remaining int64
+	stickyErr error
 }
 
-func (r *limitedReader) Read(b []byte) (int, error) {
-	if r.remaining <= 0 {
-		return 0, ErrRequestEntityTooLarge
+func (r *maxBytesReader) Read(b []byte) (int, error) {
+	if r.stickyErr != nil {
+		return 0, r.stickyErr
 	}
 
-	if int64(len(b)) > r.remaining {
-		b = b[0:r.remaining]
+	if len(b) == 0 {
+		return 0, nil
+	}
+
+	// if possible no need to read the entire chunk since
+	// remaining+1 is enough to determine whether it exceed the limit
+	if int64(len(b))-1 > r.remaining {
+		b = b[:r.remaining+1]
 	}
 
 	n, err := r.ReadCloser.Read(b)
 
-	r.remaining -= int64(n)
+	if int64(n) <= r.remaining {
+		r.remaining -= int64(n)
+		r.stickyErr = err
+		return n, err
+	}
 
-	return n, err
+	n = int(r.remaining)
+
+	r.remaining = 0
+	r.stickyErr = ErrRequestEntityTooLarge
+
+	return n, r.stickyErr
 }
 
 // explicit casts to ensure that the main struct methods will be invoked
 // (extra precautions in case of nested interface wrapping erasure)
 // ---
 
-func (r *limitedReader) Reread() {
+func (r *maxBytesReader) Reread() {
 	rereader, ok := r.ReadCloser.(router.Rereader)
 	if ok {
 		rereader.Reread()
 		r.remaining = r.limit
+		r.stickyErr = nil
 	}
 }
 
-func (r *limitedReader) Close() error {
-	closer, ok := r.ReadCloser.(io.Closer)
-	if ok {
-		return closer.Close()
-	}
-	return nil
+func (r *maxBytesReader) Close() error {
+	return r.ReadCloser.Close()
 }
diff --git a/apis/middlewares_body_limit_test.go b/apis/middlewares_body_limit_test.go
index abfd48b0..f4ac9ae7 100644
--- a/apis/middlewares_body_limit_test.go
+++ b/apis/middlewares_body_limit_test.go
@@ -1,6 +1,7 @@
 package apis_test
 
 import (
+	"io"
 	"net/http"
 	"net/http/httptest"
 	"strings"
@@ -39,6 +40,15 @@ func TestBodyLimitMiddleware(t *testing.T) {
 
 	pbRouter.POST("/a", testHandler) // default global BodyLimit check
 	pbRouter.POST("/b", testHandler).Bind(apis.BodyLimit(customLimit))
+	pbRouter.POST("/iof", func(e *core.RequestEvent) error {
+		// ensure that normal io methods still operate correctly
+		b, err := io.ReadAll(e.Request.Body)
+		if err != nil {
+			return err
+		}
+
+		return e.String(http.StatusOK, string(b))
+	}).Bind(apis.BodyLimit(customLimit))
 
 	mux, err := pbRouter.BuildMux()
 	if err != nil {
@@ -109,6 +119,14 @@ func TestBodyLimitMiddleware(t *testing.T) {
 			true,
 			http.StatusRequestEntityTooLarge,
 		},
+		// ---
+		{
+			"io.ReadAll io.EOF exact limit check",
+			"/iof",
+			`"` + strings.Repeat("a", customLimit-2) + `"`,
+			true,
+			http.StatusOK,
+		},
 	}
 
 	for _, s := range scenarios {