RWBY
===
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<!-- .slide: data-transition="zoom" -->
Reader/Writer/Close :dizzy:
> [name=郭學聰 Hsueh-Tsung Kuo] [time=Thu, 27 Dec 2018] [color=red]
---
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## who am I?
----
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* programmer from Rayark, a game company in Taiwan
* backend engineer, focus on common service
* usually develop something related to my work in Python, Ruby, Golang, C#
* built almost entire VOEZ game server by myself only
* supported Sdorica backend development
---
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## outline
----
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* Reader/Writer spec & definition
* Reader/Writer interface variation
* why ioutil?
* defer & Close()
* customize Reader/Writer/Closer
* from block cipher to streaming
---
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## RWBY?
----
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---
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## Reader/Writer spec & definition
* Reader https://golang.org/pkg/io/#Reader
* Writer https://golang.org/pkg/io/#Writer
* Closer https://golang.org/pkg/io/#Closer
----
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### Reader
```go=
type Reader interface {
Read(p []byte) (n int, err error)
}
```
----
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### Reader
Key point:
* Callers should always process the `n > 0` bytes returned before considering the error err.
----
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:o:
```go=
func ReadAtLeast(r Reader, buf []byte, min int) (n int, err error) {
if len(buf) < min {
return 0, ErrShortBuffer
}
for n < min && err == nil {
var nn int
nn, err = r.Read(buf[n:])
n += nn
}
if n >= min {
err = nil
} else if n > 0 && err == EOF {
err = ErrUnexpectedEOF
}
return
}
```
https://golang.org/src/io/io.go?s=10817:10883#L294
----
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:o:
```go=
func (b *Buffer) ReadFrom(r io.Reader) (n int64, err error) {
b.lastRead = opInvalid
for {
i := b.grow(MinRead)
b.buf = b.buf[:i]
m, e := r.Read(b.buf[i:cap(b.buf)])
if m < 0 {
panic(errNegativeRead)
}
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
}
}
}
```
https://golang.org/src/bytes/buffer.go?s=6926:6985#L191
----
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### Writer
```go=
type Writer interface {
Write(p []byte) (n int, err error)
}
```
----
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### Writer
Key point:
* Write must return a non-nil error if it returns `n < len(p)`.
----
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### Closer
```go=
type Closer interface {
Close() error
}
```
----
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### Closer
Key point:
* The behavior of Close after the first call is undefined.
---
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## Reader/Writer interface variation
See: https://golang.org/pkg/io/
----
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### ReaderAt
```go=
type ReaderAt interface {
ReadAt(p []byte, off int64) (n int, err error)
}
```
----
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### ReaderAt
Key point:
* When ReadAt returns `n < len(p)`, it returns a non-nil error explaining why more bytes were not returned.
https://golang.org/pkg/io/#ReaderAt
---
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## why io & io/ioutil?
* Key point of Reader
----
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to ensure Read() fill buf
```go=
func ReadAtLeast(r Reader, buf []byte, min int) (n int, err error)
func ReadFull(r Reader, buf []byte) (n int, err error)
```
https://golang.org/pkg/io/#ReadAtLeast
https://golang.org/pkg/io/#ReadFull
----
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or use utility function to read/write all
```go=
var Discard io.Writer = devNull(0)
func NopCloser(r io.Reader) io.ReadCloser
func ReadAll(r io.Reader) ([]byte, error)
func ReadDir(dirname string) ([]os.FileInfo, error)
func ReadFile(filename string) ([]byte, error)
func TempDir(dir, prefix string) (name string, err error)
func TempFile(dir, pattern string) (f *os.File, err error)
func WriteFile(filename string, data []byte, perm os.FileMode) error
```
https://golang.org/pkg/io/ioutil/
---
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## defer & Close()
----
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### may miss error
```go=
func RWBY() (err error) {
defer f.Close()
}
```
----
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### don't miss any error
```go=
func RWBY() (err error) {
defer func() {
cerr := f.Close()
if err == nil {
err = cerr
}
}()
}
```
----
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### don't miss any error
```go=
func safeClose(c io.Closer, err *error) {
cerr := c.Close()
if *err == nil {
*err = cerr
}
}
func RWBY() (err error) {
defer safeClose(f, &err)
}
```
---
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## customize Reader/Writer/Closer
----
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### WriterCounter
```go=
type WriterCounter struct {
writer io.Writer
count int64
}
func (wc *WriterCounter) Write(p []byte) (n int, err error) {
n, err = wc.writer.Write(p)
atomic.AddInt64(&wc.count, int64(n))
return
}
func (wc *WriterCounter) Count() int64 {
return atomic.LoadInt64(&wc.count)
}
func NewWriterCounter(w io.Writer) *WriterCounter {
return &WriterCounter{writer: w}
}
```
----
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### WriteNopCloser
```go=
type writeNopCloser struct {
io.Writer
}
func (_ writeNopCloser) Close() error {
return nil
}
func WriteNopCloser(w io.Writer) io.WriteCloser {
return writeNopCloser{Writer: w}
}
```
----
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### ChainCloseReadCloser
```go=
type ChainCloseReadCloser struct {
io.ReadCloser
underlyingReadClosers []io.ReadCloser
}
func (chainCloseReadCloser *ChainCloseReadCloser) Close() error {
err := chainCloseReadCloser.ReadCloser.Close()
for _, underlyingReadCloser := range chainCloseReadCloser.underlyingReadClosers {
cerr := underlyingReadCloser.Close()
// always return first close error
if err == nil {
err = cerr
}
}
return err
}
func NewChainCloseReadCloser(readCloser io.ReadCloser, underlyingReadClosers ...io.ReadCloser) io.ReadCloser {
return &ChainCloseReadCloser{ReadCloser: readCloser, underlyingReadClosers: underlyingReadClosers}
}
```
---
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## from block cipher to streaming
----
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* ???
---
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## conclusion
----
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> :hash: Reader/Writer/Closer 博大精深
> [name=Hsueh-Tsung Kuo] [time=Thu, 26 Apr 2018] [color=red]
----
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### special thanks
* komod
---
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## Q&A
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