Make sure removed/resetted user valeus get garbage collected

Since our userData slice isn't shrunk when we delete values, it can
still keep pointing to things that have been removed. Set these pointers
to nil so that the key and value can be garbage collected.

Fixes #1812

userdata.go

@@ -77,6 +77,8 @@ func (d *userData) Reset() {
 		if vc, ok := v.(io.Closer); ok {
 			vc.Close()
 		}
+		(*d)[i].value = nil
+		(*d)[i].key = nil
 	}
 	*d = (*d)[:0]
 }
@@ -92,6 +94,7 @@ func (d *userData) Remove(key any) {
 		if kv.key == key {
 			n--
 			args[i], args[n] = args[n], args[i]
+			args[n].key = nil
 			args[n].value = nil
 			args = args[:n]
 			*d = args

userdata_test.go

@@ -3,7 +3,9 @@ package fasthttp
 import (
 	"fmt"
 	"reflect"
+	"runtime"
 	"testing"
+	"time"
 )
 
 func TestUserData(t *testing.T) {
@@ -118,3 +120,32 @@ func TestUserDataSetAndRemove(t *testing.T) {
 	testUserDataGet(t, &u, []byte(shortKey), "")
 	testUserDataGet(t, &u, []byte(longKey), "")
 }
+
+func TestUserData_GC(t *testing.T) {
+	t.Parallel()
+
+	var u userData
+	key := "foo"
+	final := make(chan struct{})
+
+	func() {
+		val := &RequestHeader{}
+		runtime.SetFinalizer(val, func(v *RequestHeader) {
+			close(final)
+		})
+
+		u.Set(key, val)
+	}()
+
+	u.Reset()
+	runtime.GC()
+
+	select {
+	case <-final:
+	case <-time.After(time.Second):
+		t.Fatalf("value is garbage collected")
+	}
+
+	// Keep u alive, otherwise val will always get garbage collected.
+	u.Set("bar", 1)
+}