writer.go

package stackless

import (
	"errors"
	"fmt"
	"io"
	"sync"

	"github.com/valyala/bytebufferpool"
)

// Writer is an interface stackless writer must conform to.
//
// The interface contains common subset for Writers from compress/* packages.
type Writer interface {
	Write(p []byte) (int, error)
	Flush() error
	Close() error
	Reset(w io.Writer)
}

// NewWriterFunc must return new writer that will be wrapped into
// stackless writer.
type NewWriterFunc func(w io.Writer) Writer

// NewWriter creates a stackless writer around a writer returned
// from newWriter.
//
// The returned writer writes data to dstW.
//
// Writers that use a lot of stack space may be wrapped into stackless writer,
// thus saving stack space for high number of concurrently running goroutines.
func NewWriter(dstW io.Writer, newWriter NewWriterFunc) Writer {
	w := &writer{
		dstW: dstW,
	}
	w.zw = newWriter(&w.xw)
	return w
}

type writer struct {
	dstW io.Writer
	zw   Writer
	xw   xWriter

	err error
	n   int

	p  []byte
	op op
}

type op int

const (
	opWrite op = iota
	opFlush
	opClose
	opReset
)

func (w *writer) Write(p []byte) (int, error) {
	w.p = p
	err := w.do(opWrite)
	w.p = nil
	return w.n, err
}

func (w *writer) Flush() error {
	return w.do(opFlush)
}

func (w *writer) Close() error {
	return w.do(opClose)
}

func (w *writer) Reset(dstW io.Writer) {
	w.xw.Reset()
	w.do(opReset) //nolint:errcheck
	w.dstW = dstW
}

func (w *writer) do(op op) error {
	w.op = op
	if !stacklessWriterFunc(w) {
		return errHighLoad
	}
	err := w.err
	if err != nil {
		return err
	}
	if w.xw.bb != nil && len(w.xw.bb.B) > 0 {
		_, err = w.dstW.Write(w.xw.bb.B)
	}
	w.xw.Reset()

	return err
}

var errHighLoad = errors.New("cannot compress data due to high load")

var (
	stacklessWriterFuncOnce sync.Once
	stacklessWriterFuncFunc func(ctx any) bool
)

func stacklessWriterFunc(ctx any) bool {
	stacklessWriterFuncOnce.Do(func() {
		stacklessWriterFuncFunc = NewFunc(writerFunc)
	})
	return stacklessWriterFuncFunc(ctx)
}

func writerFunc(ctx any) {
	w := ctx.(*writer)
	switch w.op {
	case opWrite:
		w.n, w.err = w.zw.Write(w.p)
	case opFlush:
		w.err = w.zw.Flush()
	case opClose:
		w.err = w.zw.Close()
	case opReset:
		w.zw.Reset(&w.xw)
		w.err = nil
	default:
		panic(fmt.Sprintf("BUG: unexpected op: %d", w.op))
	}
}

type xWriter struct {
	bb *bytebufferpool.ByteBuffer
}

func (w *xWriter) Write(p []byte) (int, error) {
	if w.bb == nil {
		w.bb = bufferPool.Get()
	}
	return w.bb.Write(p)
}

func (w *xWriter) Reset() {
	if w.bb != nil {
		bufferPool.Put(w.bb)
		w.bb = nil
	}
}

var bufferPool bytebufferpool.Pool