From bb7e649369355e6606115aa3492c57f0d514842f Mon Sep 17 00:00:00 2001
From: bensynapse <118375461+bensynapse@users.noreply.github.com>
Date: Tue, 6 Oct 2026 01:47:04 +0300
Subject: [PATCH] fix: preserve captured response prefixes across writes

---
 dump.go      |   9 +---
 dump_test.go | 115 +++++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 117 insertions(+), 7 deletions(-)

diff --git a/dump.go b/dump.go
index 6a52a1e..ba0fac1 100644
--- a/dump.go
+++ b/dump.go
@@ -38,13 +38,8 @@ type bodyWriter struct {
 func (w *bodyWriter) Write(b []byte) (int, error) {
 	length := len(b)
 
-	if w.body != nil {
-		if w.body.Len()+length > w.maxSize {
-			w.body.Truncate(min(w.maxSize, length, w.body.Len()))
-			w.body.Write(b[:min(w.maxSize-w.body.Len(), length)])
-		} else {
-			w.body.Write(b)
-		}
+	if w.body != nil && w.body.Len() < w.maxSize {
+		w.body.Write(b[:min(w.maxSize-w.body.Len(), length)])
 	}
 	w.bytes += length //nolint:staticcheck
 	return w.ResponseWriter.Write(b)
diff --git a/dump_test.go b/dump_test.go
index ba2d180..1365106 100644
--- a/dump_test.go
+++ b/dump_test.go
@@ -1,7 +1,11 @@
 package sloghttp
 
 import (
+	"bytes"
+	"encoding/json"
 	"io"
+	"log/slog"
+	"net/http"
 	"net/http/httptest"
 	"strings"
 	"testing"
@@ -50,3 +54,114 @@ func TestBodyWriter_ReadFrom(t *testing.T) {
 type simpleReader struct {
 	io.Reader
 }
+
+func TestBodyWriter_CapturedPrefix(t *testing.T) {
+	tests := []struct {
+		name    string
+		maxSize int
+		chunks  []string
+		want    string
+	}{
+		{"empty", 4, nil, ""},
+		{"empty writes", 4, []string{"", "abc", ""}, "abc"},
+		{"below limit", 4, []string{"ab", "c"}, "abc"},
+		{"single overflow", 4, []string{"abcdef"}, "abcd"},
+		{"exact limit", 4, []string{"ab", "cd"}, "abcd"},
+		{"short crossing chunk", 4, []string{"abc", "de"}, "abcd"},
+		{"long crossing chunk", 4, []string{"ab", "cdef"}, "abcd"},
+		{"short write after limit", 4, []string{"abcd", "e"}, "abcd"},
+		{"long write after limit", 4, []string{"abcd", "efghij"}, "abcd"},
+		{"empty write after limit", 4, []string{"abcd", "", "e"}, "abcd"},
+		{"repeated short writes", 4, []string{"a", "b", "c", "d", "e", "f"}, "abcd"},
+		{"short write after overflow", 4, []string{"abcdef", "g"}, "abcd"},
+		{"zero limit", 0, []string{"ab", "c", ""}, ""},
+	}
+
+	for _, tt := range tests {
+		for _, readFrom := range []bool{false, true} {
+			method := "Write"
+			if readFrom {
+				method = "ReadFrom"
+			}
+			t.Run(tt.name+"/"+method, func(t *testing.T) {
+				response := httptest.NewRecorder()
+				writer := newBodyWriter(response, tt.maxSize, true)
+				content := strings.Join(tt.chunks, "")
+				if readFrom {
+					reader := &bodyChunksReader{chunks: append([]string(nil), tt.chunks...)}
+					n, err := writer.ReadFrom(reader)
+					if err != nil || n != int64(len(content)) {
+						t.Fatalf("ReadFrom() = %d, %v, want %d, nil", n, err, len(content))
+					}
+				} else {
+					for _, chunk := range tt.chunks {
+						n, err := writer.Write([]byte(chunk))
+						if err != nil || n != len(chunk) {
+							t.Fatalf("Write() = %d, %v, want %d, nil", n, err, len(chunk))
+						}
+					}
+				}
+				if got := string(writer.Body()); got != tt.want {
+					t.Errorf("captured body = %q, want %q", got, tt.want)
+				}
+				if got := response.Body.String(); got != content {
+					t.Errorf("response body = %q, want %q", got, content)
+				}
+				if got := writer.BytesWritten(); got != len(content) {
+					t.Errorf("bytes written = %d, want %d", got, len(content))
+				}
+			})
+		}
+	}
+}
+
+type bodyChunksReader struct {
+	chunks []string
+}
+
+func (r *bodyChunksReader) Read(p []byte) (int, error) {
+	if len(r.chunks) == 0 {
+		return 0, io.EOF
+	}
+	n := copy(p, r.chunks[0])
+	r.chunks[0] = r.chunks[0][n:]
+	if r.chunks[0] == "" {
+		r.chunks = r.chunks[1:]
+	}
+	return n, nil
+}
+
+func TestMiddleware_CapturedResponsePrefix(t *testing.T) {
+	var output bytes.Buffer
+	logger := slog.New(slog.NewJSONHandler(&output, nil))
+	config := DefaultConfig()
+	config.WithResponseBody = true
+	content := strings.Repeat("a", ResponseBodyMaxSize) + "bc"
+	handler := NewWithConfig(logger, config)(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		for _, chunk := range []string{content[:ResponseBodyMaxSize], "b", "c"} {
+			if _, err := io.WriteString(w, chunk); err != nil {
+				t.Fatalf("write response: %v", err)
+			}
+		}
+	}))
+	response := httptest.NewRecorder()
+	handler.ServeHTTP(response, httptest.NewRequest(http.MethodGet, "/", nil))
+	if got := response.Body.String(); got != content {
+		t.Fatalf("response length = %d, want %d", len(got), len(content))
+	}
+	var entry struct {
+		Response struct {
+			Body   string
+			Length int
+		}
+	}
+	if err := json.Unmarshal(output.Bytes(), &entry); err != nil {
+		t.Fatalf("decode log: %v", err)
+	}
+	if got := entry.Response.Body; got != content[:ResponseBodyMaxSize] {
+		t.Errorf("logged body length = %d, want prefix length %d", len(got), ResponseBodyMaxSize)
+	}
+	if got := entry.Response.Length; got != len(content) {
+		t.Errorf("logged response length = %d, want %d", got, len(content))
+	}
+}