fix: adapt multipart stream to support go1.27 io.EOF change

.golangci.yml

@@ -5,6 +5,7 @@ linters:
     - depguard
     - err113 # disabled temporarily: there are just too many issues to address
     - exhaustruct
+    - exhaustruct_v5
     - funlen
     - gochecknoglobals
     - gochecknoinits

multipart_stream.go

@@ -342,6 +342,10 @@ func (s *MultipartFormStream) Drain() error {
 // particular, a net/http server request body may discard a limited amount of
 // unread data to allow connection reuse, so Close is not guaranteed to return
 // immediately. Call Drain when trailing form fields must be collected.
+//
+// Close returns the error reported by the request body, except io.EOF: a
+// net/http server body reports io.EOF once it discards that unread remainder,
+// which tells us the body ended, not that closing it failed.
 func (s *MultipartFormStream) Close() error {
 	if s == nil || s.closed {
 		return nil
@@ -357,7 +361,14 @@ func (s *MultipartFormStream) Close() error {
 		return nil
 	}
 
-	return s.request.Body.Close()
+	// Since go1.27.0, net/http (*body).Close returns io.EOF when it discards
+	// the unread remainder of the request itself. Reaching the end of the body
+	// is not a close failure.
+	if err := s.request.Body.Close(); err != nil && !stderrors.Is(err, io.EOF) {
+		return err
+	}
+
+	return nil
 }
 
 func supportsMultipartFormStream(method string) bool {

multipart_stream_test.go

@@ -325,6 +325,44 @@ func TestMultipartFormStreamDrainCollectsTrailingFieldsAndClosesBody(t *testing.
 	require.ErrorIs(t, err, io.EOF)
 }
 
+func TestMultipartFormStreamCloseIgnoresBodyCloseEOF(t *testing.T) {
+	// Since go1.27.0, a net/http server request body reports io.EOF from Close
+	// when it discards the unread remainder of the request. The stream must not
+	// pass that on as a failure.
+	body, contentType := orderedMultipartBody(t,
+		orderedFile{field: testFieldFile, filename: streamedFilename, content: "payload"},
+		orderedField{name: "after", value: "value"},
+	)
+	trackedBody := &observableReadCloser{Reader: body, closeErr: io.EOF}
+	request := httptest.NewRequestWithContext(t.Context(), http.MethodPost, testUploadPath, nil)
+	request.Body = trackedBody
+	request.Header.Set(HeaderContentType, contentType)
+	stream, err := NewMultipartFormStream(request)
+	require.NoError(t, err)
+
+	_, err = stream.NextFile()
+	require.NoError(t, err)
+	require.NoError(t, stream.Drain())
+	assert.EqualT(t, "value", request.Form.Get("after"))
+	assert.TrueT(t, trackedBody.Closed())
+}
+
+func TestMultipartFormStreamCloseReportsBodyCloseError(t *testing.T) {
+	body, contentType := orderedMultipartBody(t,
+		orderedFile{field: testFieldFile, filename: streamedFilename, content: "payload"},
+	)
+	closeErr := stderrors.New("close failed")
+	trackedBody := &observableReadCloser{Reader: body, closeErr: closeErr}
+	request := httptest.NewRequestWithContext(t.Context(), http.MethodPost, testUploadPath, nil)
+	request.Body = trackedBody
+	request.Header.Set(HeaderContentType, contentType)
+	stream, err := NewMultipartFormStream(request)
+	require.NoError(t, err)
+
+	require.ErrorIs(t, stream.Close(), closeErr)
+	assert.TrueT(t, trackedBody.Closed())
+}
+
 func TestMultipartFormStreamCloseAbortsWithoutDraining(t *testing.T) {
 	body, contentType := orderedMultipartBody(t,
 		orderedFile{field: testFieldFile, filename: streamedFilename, content: "payload"},
@@ -626,6 +664,8 @@ func orderedMultipartBody(t *testing.T, parts ...orderedMultipartPart) (*bytes.B
 type observableReadCloser struct {
 	io.Reader
 
+	closeErr error
+
 	mu     sync.Mutex
 	closed bool
 }
@@ -635,7 +675,7 @@ func (r *observableReadCloser) Close() error {
 	defer r.mu.Unlock()
 	r.closed = true
 
-	return nil
+	return r.closeErr
 }
 
 func (r *observableReadCloser) Closed() bool {