Go基本库阅读：io库

package main

import (
	"bufio"
	"fmt"
	"io"
	"os"
)

func main() {
	inputFile, inputError := os.Open("input.dat")
	if inputError != nil {
		fmt.Printf("An error occurred on opening the inputfile\n" +
			"Does the file exist?\n" +
			"Have you got acces to it?\n")
		return // exit the function on error
	}
	defer inputFile.Close()

	inputReader := bufio.NewReader(inputFile)
	for {
		inputString, readerError := inputReader.ReadString('\n')
		if readerError == io.EOF {
			return
		}
		fmt.Printf("The input was: %s", inputString)
	}
}

//import package
import "io/ioutil"

//code snippet
    bodyBytes, err := ioutil.ReadAll(resp.Body)
    
 //---------- optioninal ---------------------
    //handling Errors
    if err != nil {
        log.Fatal(err)
    }
    
    //print result
    bodyString := string(bodyBytes)
    log.Info(bodyString)

/*
方法从数组p中读取最多len(p)个bytes，返回实际读取的bytes数量n和返回的错误err。Reader不会阻塞去进行等待，如果数据没有准备好（实际数据与len(p)不匹配），它会直接返回
实现细节：
1. Read方法读取到最后一个bytes是返回n,EOF/nil，之后再调用则应该返回0,EOF
2. 不鼓励在读取到0个bytes后返回nil，除非len(p)==0。一般情况下0和nil不能同时返回，而且它绝不能暗示EOF。
3. 该方法的实现绝对不能保留数组p
调用者：
* 调用者应该先处理读到的bytes后，再考虑err的问题，从而将正常的错误与EOF同等对待。
*/
type Reader interface {
	Read(p []byte) (n int, err error)
}

// net包中conn有对应的实现，conn的Read依赖于netfd的Read，而后者依赖于bufferedPipe的Read
// 位置：src/net/net_fake.go:186
func (p *bufferedPipe) Read(b []byte) (int, error) {
	p.mu.Lock()
	defer p.mu.Unlock()
  // 加锁后直接循环
  // 与官方实现说的有点不太一样，Read实际上是可以实现等待的，只是需要自行设置DDL
	for { 
		if p.closed && len(p.buf) == 0 {
			return 0, io.EOF // 管道关闭，或者缓冲为0时返回。
		}
		if !p.rDeadline.IsZero() { // 如果存在DDL，则执行对应的逻辑
			d := time.Until(p.rDeadline)
			if d <= 0 { // DDL到了，则返回对应的信号
				return 0, syscall.EAGAIN
			}
			time.AfterFunc(d, p.rCond.Broadcast) // 到时间后，BroadCast会唤醒p.rCond中的所有协程
		}
		if len(p.buf) > 0 { // 缓存区中有数据，则直接弹出
			break
		}
		p.rCond.Wait()
	}

	n := copy(b, p.buf) // 直接将一个slice的元素拷贝给另外一个slice，n表示实际复制成功的slice元素个数
	p.buf = p.buf[n:] // 进行截断，避免重复
	p.wCond.Broadcast() // 唤醒所有的wCond
	return n, nil
}

// Read的位置为src/bytes/reader.go:39
// A Reader implements the io.Reader, io.ReaderAt, io.WriterTo, io.Seeker,
// io.ByteScanner, and io.RuneScanner interfaces by reading from
// a byte slice.
// Unlike a Buffer, a Reader is read-only and supports seeking.
// The zero value for Reader operates like a Reader of an empty slice.
type Reader struct {
	s        []byte
	i        int64 // current reading index
	prevRune int   // index of previous rune; or < 0
}

// Read implements the io.Reader interface.
func (r *Reader) Read(b []byte) (n int, err error) {
	if r.i >= int64(len(r.s)) { // 如果当前的index大于所读取的数据数组s，则EOF
		return 0, io.EOF // 这里与之前的文档一致，0和nil不同时出现，0应该跟着EOF，除非len(p)==0
	}
	r.prevRune = -1 // 这里应该是单纯的用不上
// 简单的使用copy
	n = copy(b, r.s[r.i:])
	r.i += int64(n)
	return
}

/*
与Read相比，ReadAt会在输入源的index==off的位置开始读取
如果n<len(p)，ReadAt必须返回错误来告知为什么没有读取完整。如果数据源可用但长度非len(p)，ReadAt会一直Block到出结果或者err
PS: 从前面的网络包实现中可以发现，实际实现中并没有严格遵循这一点
与Read不同的另外一点在于，ReadAt的Client要能够同时对同一个输入源进行操作。与之相对，尽管没有明说，但是Read的网络包实现中是加了锁的
*/
type ReaderAt interface {
	ReadAt(p []byte, off int64) (n int, err error)
}

// src/bytes/reader.go中同样实现了ReadAt的方法
// ReadAt implements the io.ReaderAt interface.（原注释，我对此的理解是因为要支持并发操作，因此如果对状态进行修改将会产生问题）
func (r *Reader) ReadAt(b []byte, off int64) (n int, err error) {
	// cannot modify state - see io.ReaderAt
// legal check
	if off < 0 {
		return 0, errors.New("bytes.Reader.ReadAt: negative offset")
	}
	if off >= int64(len(r.s)) {
		return 0, io.EOF
	}

	n = copy(b, r.s[off:])
	if n < len(b) {
		err = io.EOF // 比Read多的地方，如果n<len(b)时会额外增加这个错误
	}
	return
}

/*
ReadFrom的目标是从Reader中一直读取数据直到读到其他错误或者EOF
*/
type ReaderFrom interface {
	ReadFrom(r Reader) (n int64, err error)
}
// src/bytes/buffer.go:197中实现了这一方法，会一直从某个reader中读取数据直到读完为止
// ReadFrom reads data from r until EOF and appends it to the buffer, growing
// the buffer as needed. The return value n is the number of bytes read. Any
// error except io.EOF encountered during the read is also returned. If the
// buffer becomes too large, ReadFrom will panic with ErrTooLarge.
func (b *Buffer) ReadFrom(r io.Reader) (n int64, err error) {
	b.lastRead = opInvalid
	for {
		i := b.grow(MinRead) // MinRead=512，是单个Read调用的最小的slice size。而grow则会扩展slice从而保证额外的MinRead大小的数据能够被写入
		b.buf = b.buf[:i] // 确保len正确，从切片中新建切片不会改变cap
		m, e := r.Read(b.buf[i:cap(b.buf)])
		if m < 0 {
			panic(errNegativeRead)
		}
		// 读到了m个数据。这里体现了一个特点，即Read的err的处理逻辑是先处理数据再处理错误
		b.buf = b.buf[:i+m]
		n += int64(m)
		if e == io.EOF {
			return n, nil // e is EOF, so return nil explicitly
		}
		if e != nil {
			return n, e
		}
	}
}

/*
SectionReawder可以指定读取某一个范围的数据，该范围为从off之后n个点的数据，在读完n个点之后返回EOF
*/
type SectionReader struct {
	r     ReaderAt
	base  int64
	off   int64
	limit int64
}

func NewSectionReader(r ReaderAt, off int64, n int64) *SectionReader

package main

import (
	"fmt"
	"io"
	"log"
	"strings"
)

func main() {
	r := strings.NewReader("some io.Reader stream to be read\n")
	s := io.NewSectionReader(r, 5, 17)

	buf := make([]byte, 9)
	if _, err := s.Read(buf); err != nil {
		log.Fatal(err)
	}

	fmt.Printf("%s\n", buf)

}

/*
与Read方法相对，write方法会从数组p中读取至多len(p)的数据写入到基本数据流中。
其中实现并不一定要包含p，比如Discard就没有使用到p数组。
问题：基本数据流该怎么调用？它是结构体的成员之一吗
*/
type Writer interface {
	Write(p []byte) (n int, err error)
}
// src/bytes/buffer.go
// Write appends the contents of p to the buffer, growing the buffer as
// needed. The return value n is the length of p; err is always nil. If the
// buffer becomes too large, Write will panic with ErrTooLarge.
func (b *Buffer) Write(p []byte) (n int, err error) {
	b.lastRead = opInvalid
	m, ok := b.tryGrowByReslice(len(p))
	if !ok {
		m = b.grow(len(p))
	}
// 直接通过拷贝来实现写入
	return copy(b.buf[m:], p), nil
}

// src/bytes/buffer.go:249
// WriteTo writes data to w until the buffer is drained or an error occurs.
// The return value n is the number of bytes written; it always fits into an
// int, but it is int64 to match the io.WriterTo interface. Any error
// encountered during the write is also returned.
func (b *Buffer) WriteTo(w io.Writer) (n int64, err error) {
// 函数将b.buf的数据写入到w中
	b.lastRead = opInvalid
	if nBytes := b.Len(); nBytes > 0 {
		m, e := w.Write(b.buf[b.off:]) // 尝试向w中写入
		if m > nBytes { // 写入异常
			panic("bytes.Buffer.WriteTo: invalid Write count")
		}
		b.off += m
		n = int64(m)
		if e != nil {
			return n, e
		}
		// all bytes should have been written, by definition of
		// Write method in io.Writer
		if m != nBytes {
			return n, io.ErrShortWrite
		}
	}
	// Buffer is now empty; reset.
	b.Reset()
// 由于采用了if，所以这里使用的是直接对n进行赋值的方法，而不是如ReadFrom一样的循环累加
	return n, nil
}

// src/os/file.go:200
// WriteAt writes len(b) bytes to the File starting at byte offset off.
// It returns the number of bytes written and an error, if any.
// WriteAt returns a non-nil error when n != len(b).
//
// If file was opened with the O_APPEND flag, WriteAt returns an error.
func (f *File) WriteAt(b []byte, off int64) (n int, err error) {
	if err := f.checkValid("write"); err != nil {
		return 0, err
	}
	if f.appendMode {
		return 0, errWriteAtInAppendMode
	}

	if off < 0 {
		return 0, &PathError{Op: "writeat", Path: f.name, Err: errors.New("negative offset")}
	}

	for len(b) > 0 {
		m, e := f.pwrite(b, off)
		if e != nil {
			err = f.wrapErr("write", e)
			break
		}
		n += m
		b = b[m:]
		off += int64(m)
	}
	return
}

/*
Seek方法将下一个Read或者Write的index设置为offset，而这个offset的解释则根据whence进行。
SeekStart与开始相关，SeekCurrent与当前相关，SeekEnd与结尾相关
Seek返回处理后得到的新的offset以及对应的错误
*/
type Seeker interface {
	Seek(offset int64, whence int) (int64, error)
}
// 实现示范，src/io/io.go
func (s *SectionReader) Seek(offset int64, whence int) (int64, error) {
	switch whence {
	default:
		return 0, errWhence
	case SeekStart:
		offset += s.base
	case SeekCurrent:
		offset += s.off
	case SeekEnd:
		offset += s.limit
	}
	if offset < s.base {
		return 0, errOffset
	}
	s.off = offset
	return offset - s.base, nil
}

// src/io/pipe.go:199
func Pipe() (*PipeReader, *PipeWriter) {
	p := &pipe{
		wrCh: make(chan []byte), // write channel
		rdCh: make(chan int), // read channel
		done: make(chan struct{}), // done channel
	}
	return &PipeReader{p}, &PipeWriter{p}
}

func (p *pipe) read(b []byte) (n int, err error) {
	// 从管道中读取到b
	select {
	case <-p.done: // 如果完成，则关闭
		return 0, p.readCloseError()
	default:
	}

	select {
	case bw := <-p.wrCh: // 从writechannel中读数据
		nr := copy(b, bw)
		p.rdCh <- nr
		return nr, nil
	case <-p.done:
		return 0, p.readCloseError()
	}
}

func (p *pipe) write(b []byte) (n int, err error) {
  // 从b中写入到管道
	select {
	case <-p.done:
		return 0, p.writeCloseError()
	default:
		p.wrMu.Lock()
		defer p.wrMu.Unlock()
	}

	for once := true; once || len(b) > 0; once = false {
		select {
		case p.wrCh <- b:
			nw := <-p.rdCh
			b = b[nw:]
			n += nw
		case <-p.done:
			return n, p.writeCloseError()
		}
	}
	return n, nil
}

// Copy copies from src to dst until either EOF is reached
// on src or an error occurs. It returns the number of bytes
// copied and the first error encountered while copying, if any.
//
// A successful Copy returns err == nil, not err == EOF.
// Because Copy is defined to read from src until EOF, it does
// not treat an EOF from Read as an error to be reported.
//
// If src implements the WriterTo interface,
// the copy is implemented by calling src.WriteTo(dst).
// Otherwise, if dst implements the ReaderFrom interface,
// the copy is implemented by calling dst.ReadFrom(src).
func Copy(dst Writer, src Reader) (written int64, err error) {
	return copyBuffer(dst, src, nil)
}

// CopyBuffer is identical to Copy except that it stages through the
// provided buffer (if one is required) rather than allocating a
// temporary one. If buf is nil, one is allocated; otherwise if it has
// zero length, CopyBuffer panics.
//
// If either src implements WriterTo or dst implements ReaderFrom,
// buf will not be used to perform the copy.
func CopyBuffer(dst Writer, src Reader, buf []byte) (written int64, err error) {
	if buf != nil && len(buf) == 0 {
		panic("empty buffer in CopyBuffer")
	}
	return copyBuffer(dst, src, buf)
}

// copyBuffer is the actual implementation of Copy and CopyBuffer.
// if buf is nil, one is allocated.
func copyBuffer(dst Writer, src Reader, buf []byte) (written int64, err error) {
	// If the reader has a WriteTo method, use it to do the copy.
	// Avoids an allocation and a copy.
	if wt, ok := src.(WriterTo); ok {
		return wt.WriteTo(dst)
	}
	// Similarly, if the writer has a ReadFrom method, use it to do the copy.
	if rt, ok := dst.(ReaderFrom); ok {
		return rt.ReadFrom(src)
	}
	if buf == nil {
		size := 32 * 1024
		if l, ok := src.(*LimitedReader); ok && int64(size) > l.N {
			if l.N < 1 {
				size = 1
			} else {
				size = int(l.N)
			}
		}
		buf = make([]byte, size)
	}
	for {
		nr, er := src.Read(buf)
		if nr > 0 {
			nw, ew := dst.Write(buf[0:nr])
			if nw < 0 || nr < nw {
				nw = 0
				if ew == nil {
					ew = errInvalidWrite
				}
			}
			written += int64(nw)
			if ew != nil {
				err = ew
				break
			}
			if nr != nw {
				err = ErrShortWrite
				break
			}
		}
		if er != nil {
			if er != EOF {
				err = er
			}
			break
		}
	}
	return written, err
}