[Golang io.Pipe and io.TeeReader combined] #go #golang #io #pipe #tee #reader #writer

1. let's start with a simple io.Pipe example.go

package main

import (
	"fmt"
	"io"
	"io/ioutil"
	"sync"
)

func readFrom(pr *io.PipeReader, wg *sync.WaitGroup) {
	defer wg.Done()
	b, _ := ioutil.ReadAll(pr)
	fmt.Println(string(b), ".")
}

func main() {
	wg := sync.WaitGroup{}
	wg.Add(2)

	pr, pw := io.Pipe()

	go readFrom(pr, &wg) // this will consume all three writes in one single read.
  	go readFrom(pr, &wg) // this secondary read needs pw.Close() to unblock it!
  						 // specifically, the pipe reader will keep going util the
  						 // writer has signified an EOF, which wont happen without
						 // the call to Close(). 
  						 //
  						 // this problem wouldn't exist if we had used a different
  						 // read method, but as we used ioutil.ReadAll it is designed
  						 // around the expectation of an EOF.

	pw.Write([]byte("foo"))
	pw.Write([]byte("-"))
	pw.Write([]byte("bar"))

  	pw.Close() // do before Wait() otherwise deadlock (see comment above)

	wg.Wait()
}

io.Pipe and io.TeeReader combined.go

package main

import (
	"fmt"
	"io"
	"io/ioutil"
	"strings"
	"sync"
)

func main() {
	wg := sync.WaitGroup{}
	wg.Add(1)

	s := strings.NewReader("Test")
	r, w := io.Pipe()
	tee := io.TeeReader(s, w)

	go func() {
		defer wg.Done()

		fmt.Println("about to block thread waiting for a write to io.Pipe's reader")
		content, _ := ioutil.ReadAll(r)
		fmt.Println("1: ", content)
	}()

	fmt.Println("about to block main thread until write to io.TeeReader's configured writer is complete")
	content, _ := ioutil.ReadAll(tee) // this will force a write to io.TeeReader's writer
	fmt.Println("2: ", content)

	w.Close() // close the io.Pipe first before waiting for thread to complete (otherwise get a deadlock)

	wg.Wait()
}