Go Fiber v2 Buffer Reuse: When Request Data Becomes Empty or Corrupted After Handler Return

Recognizable condition

You read c.Body(), c.Query(), or c.Params() inside a handler and get the expected values. After the handler returns—especially in a deferred function, a goroutine, or the next middleware—the same accessor returns an empty string or stale data from a different request.

Why it happens

Fiber is built on fasthttp, which reuses underlying byte buffers across requests for performance. The strings returned by context accessors are valid only until the handler completes. Retaining a reference without copying is a use-after-free-style bug at the Go level.

Diagnostic table

Symptom	Likely cause
c.Body() returns "" after handler	Buffer recycled before deferred code runs
c.Query("id") differs between concurrent requests	Shared buffer interleaving under load
Handler logs correct data, but later middleware sees empty	Context pointer points to pooled buffer

Ordered checks

1. Confirm the data is correct inside the handler by printing it immediately.
2. Add a deferred function after the handler returns and access the same accessor—does it vanish? That confirms the buffer lifecycle issue.
3. Check if any middleware or goroutine stores *fiber.Ctx or its derived strings beyond the handler scope.
4. Run a concurrent load test (e.g., hey -n 200 -c 50 https://localhost:8080/api) with distinct payloads. Interleaved or empty results confirm buffer reuse.

Fixes tied to findings

Targeted copy (preferred for hot paths)

Extract the value you need once, early in the handler, and work with the copy:

// Inside handler
body := c.Body()
_ = body // use body; don't return c.Body() later

If you must retain it, copy explicitly:

safeBody := string(c.Body())

Enable immutable mode (global safeguard)

Create the app with fiber.Config{Immutable: true}. This makes all context accessors return fresh copies per request, eliminating the window of vulnerability:

app := fiber.New(fiber.Config{
  Immutable: true,
  // other config
})

Trade-off: Each request incurs one extra allocation per retained string. On ultra-high-throughput paths this can reduce throughput by a few percent. Use targeted copies when the hot path serves simple JSON payloads, and enable Immutable for handlers that retain request data across goroutines or deferred calls.

Escalation criteria

1. If copying all retained values stops the corruption, you’re done—no framework change needed.
2. If corruption persists, inspect custom middleware that stores *fiber.Ctx or keeps references to the context itself. Immutable does not protect middleware that holds the context pointer.
3. Only then check the fasthttp version pinned in go.mod or open a framework bug report with a reproducible test case.

Verification you can run

Write a test that fires concurrent requests with unique bodies, stores c.Body() results in a slice, and asserts each stored value matches its request after all handlers complete. Example skeleton:

func TestBufferReuse(t *testing.T) {
  app := fiber.New(fiber.Config{})
  client := http.Client{Timeout: 5 * time.Second}
  var wg sync.WaitGroup
  results := make([]string, 10)
  for i := 0; i < 10; i++ {
    wg.Add(1)
    go func(idx int) {
      defer wg.Done()
      req, _ := http.NewRequest("POST", "http://localhost:8080", bytes.NewReader([]byte(`{"id":" + strconv.Itoa(idx) + `}`)))
      res, err := client.Do(req)
      if err != nil {
        t.Error(err)
        return
      }
      defer res.Body.Close()
      body, _ := io.ReadAll(res.Body)
      results[idx] = string(body)
    }(i)
  }
  wg.Wait()
  // assert all 10 bodies are distinct
  for i, expected := range results {
    if results[i] != expected {
      t.Errorf("request %d: expected %q, got %q", i, expected, results[i])
    }
  }
}

Run with go test -race and a load generator to surface unsafe retention of context-derived strings. Toggle Immutable: true and confirm the corruption disappears, then reintroduce targeted copies to restore performance.

Cautions

• Immutable: true does not protect values stored by third-party middleware that keep references to the context itself.
• Never pass *fiber.Ctx into goroutines; extract needed values first, as the context is pooled and reset.
• This behavior applies to Fiber v2. Fiber v3 may change APIs and defaults, so confirm the major version in go.mod before applying fixes.