concurrentjsonRW.go

package main

import (
	"encoding/json"
	"fmt"
	"os"
	"strconv"
	"sync"
	"time"
)

var wg sync.WaitGroup

var mut sync.Mutex

func Writer(file *os.File) {
	enc := json.NewEncoder(file)
	for i := 0; i < 5; i++ {
		fmt.Println("Writer")
		// mut.Lock()
		enc.Encode("Line" + strconv.Itoa(i))
		// mut.Unlock()
		time.Sleep(500 * time.Millisecond)
	}

	wg.Done()
}

func Reader(file *os.File) {
	enc := json.NewDecoder(file)
	var str string
	for i := 0; i < 5; i++ {
		fmt.Println("Reader")
		// mut.Lock()
		enc.Decode(&str)
		// mut.Unlock()
		fmt.Println(str)
		time.Sleep(500 * time.Millisecond)
	}

	wg.Done()
}

func main() {
	wg.Add(2)

	file1, _ := os.Create("test.txt")
	file2, _ := os.Open("test.txt")

	go Writer(file1)

	// Need to ensure that Reader starts reading only after atleast one line has been written.
	time.Sleep(500 * time.Millisecond)
	go Reader(file2)

	wg.Wait()
}

For some reason if Reader starts reading before Writer has written the first line, then the Reader is never able to read any lines at all. Even if Writer manages to catch up and write more lines than the Reader has currently read.
To understand this I may need to properly understand how the encoding/json package reads from files. It probably keeps incrementing SEEK_CUR (or equivalent) which makes Reader unable to read anything if it starts first, but that doesn't explain why it doesn't read anything even when writer has eventually moved forward.
Maybe they somehow end up working on different files?