buf.go

/*
Copyright 2018 Gravitational, Inc.

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
*/

package utils

import (
	"bytes"
	"io"
)

// NewSyncBuffer returns new in memory buffer
func NewSyncBuffer() *SyncBuffer {
	reader, writer := io.Pipe()
	buf := &bytes.Buffer{}
	copyDone := make(chan struct{})
	go func() {
		defer close(copyDone)
		io.Copy(buf, reader)
	}()
	return &SyncBuffer{
		reader:   reader,
		writer:   writer,
		buf:      buf,
		copyDone: copyDone,
	}
}

// SyncBuffer is in memory bytes buffer that is
// safe for concurrent writes
type SyncBuffer struct {
	reader *io.PipeReader
	writer *io.PipeWriter
	buf    *bytes.Buffer
	// copyDone makes SyncBuffer.Close block until io.Copy goroutine finishes.
	// This prevents data races between io.Copy and SyncBuffer.String/Bytes.
	copyDone chan struct{}
}

func (b *SyncBuffer) Write(data []byte) (n int, err error) {
	return b.writer.Write(data)
}

// String returns contents of the buffer
// after this call, all writes will fail
func (b *SyncBuffer) String() string {
	b.Close()
	return b.buf.String()
}

// Bytes returns contents of the buffer
// after this call, all writes will fail
func (b *SyncBuffer) Bytes() []byte {
	b.Close()
	return b.buf.Bytes()
}

// Close closes reads and writes on the buffer
func (b *SyncBuffer) Close() error {
	err := b.reader.Close()
	err2 := b.writer.Close()

	// Explicitly wait for io.Copy goroutine to finish.
	<-b.copyDone

	if err != nil {
		return err
	}
	return err2
}