Root config: Add env config (#6400)

https://github.com/XTLS/Xray-core/pull/6400#issuecomment-4880198100
https://github.com/XTLS/Xray-core/pull/6400#issuecomment-4916892425
https://github.com/XTLS/Xray-core/pull/6400#issuecomment-4929136670

common/buf/io.go

@@ -122,7 +122,7 @@ func NewReader(reader io.Reader) Reader {
 	}
 
 	_, isFile := reader.(*os.File)
-	if !isFile && useReadv {
+	if !isFile && useReadV() {
 		if sc, ok := reader.(syscall.Conn); ok {
 			rawConn, err := sc.SyscallConn()
 			if err != nil {

common/buf/readv_reader.go

@@ -5,6 +5,7 @@ package buf
 
 import (
 	"io"
+	"sync/atomic"
 	"syscall"
 
 	"github.com/xtls/xray-core/common/platform"
@@ -143,13 +144,24 @@ func (r *ReadVReader) ReadMultiBuffer() (MultiBuffer, error) {
 	return mb, nil
 }
 
-var useReadv bool
+var useReadv atomic.Bool
 
-func init() {
+func useReadV() bool {
+	return useReadv.Load()
+}
+
+func reloadEnvSettings() error {
 	const defaultFlagValue = "NOT_DEFINED_AT_ALL"
 	value := platform.NewEnvFlag(platform.UseReadV).GetValue(func() string { return defaultFlagValue })
+	enabled := false
 	switch value {
 	case defaultFlagValue, "auto", "enable":
-		useReadv = true
+		enabled = true
 	}
+	useReadv.Store(enabled)
+	return nil
+}
+
+func init() {
+	platform.RegisterEnvReload(reloadEnvSettings)
 }

common/buf/readv_reader_stub.go

@@ -10,7 +10,9 @@ import (
 	"github.com/xtls/xray-core/features/stats"
 )
 
-const useReadv = false
+func useReadV() bool {
+	return false
+}
 
 func NewReadVReader(reader io.Reader, rawConn syscall.RawConn, counter stats.Counter) Reader {
 	panic("not implemented")