io.Pipe() causes WaitGroup to get stuck - go

Question

I am processing a huge data file which is approx. 100 GB. Each line in that huge file is a JSON piece of data which I'd like to read, compress, and store in an in memory database.
var wg sync.WaitGroup
for {
line, err := reader.ReadString('\n')
if err != nil {
break
}
go func(index int) {
wg.Add(1)
pr, pw := io.Pipe()
zw := lzw.NewWriter(pw, lzw.LSB, 8)
_, err := io.Copy(zw, strings.NewReader(line))
pw.Close()
zw.Close()
if err != nil {
fmt.Println(err.Error())
}
b, err := io.ReadAll(pr)
if err != nil {
fmt.Println(err.Error())
}
client.Set(ctx, fmt.Sprintf("%d", index), base64.StdEncoding.EncodeToString(b), time.Hour*1000)
pr.Close()
wg.Done()
}(index)
if index%10000 == 0 {
fmt.Println(index)
wg.Wait()
}
index += 1
}
However, this code stops after processing the first 10000 lines. When I move down the wg.Add(1) after the zw.Close() it keeps on processing the rest of the line (but becomes instable). Without the lzw and io.Pipe() when I try to store the exact values in an uncompressed way, then everything works without any issue.
I am not sure whether I am not using the WaitGroup correctly or there is something associated with the io.Pipe() which I am not aware of yet.

Answer 1

TLDR:
1- Removing pr, pw := io.Pipe() makes the code more simple, since it is superfluous,
try this:
line, err := reader.ReadString('\n')
if err == io.EOF {
wg.Wait()
break
}
if err != nil {
log.Fatal(err)
}
wg.Add(1)
go func(index int) {
var buf bytes.Buffer
{ // lexical scoping (static scoping)
zw := lzw.NewWriter(&buf, lzw.LSB, 8)
n, err := zw.Write([]byte(line)) // n, err := io.Copy(zw, strings.NewReader(line))
if err != nil {
log.Fatal(err)
}
if int(n) != len(line) {
log.Fatal(n, len(line))
}
// It is the caller's responsibility to call Close on the WriteCloser when finished writing.
if err = zw.Close(); err != nil {
log.Fatal(err)
}
}
ctx, cancelFunc := context.WithTimeout(context.Background(), 100*time.Millisecond)
client.Set(ctx, fmt.Sprintf("%d", index), base64.StdEncoding.EncodeToString(buf.Bytes()), 1000*time.Hour)
cancelFunc()
wg.Done()
}(index)
if index%tenThousand == 0 {
wg.Wait()
}
2- You need to put the wg.Add(1) before go func(index int) {:
wg.Add(1)
go func(index int) {
3- The wg.Wait() logic:
if index%10000 == 0 {
fmt.Println(index)
wg.Wait()
}
What happens for the last iteration if index%10000 != 0.
So here when err == io.EOF you need to wg.Wait() for all goroutines to join:
if err == io.EOF {
wg.Wait()
fmt.Println("\n**** All done **** index =", index)
break
}
4- You may use lexical scoping (static scoping) to limit some variables scope and make the code more manageable - and to know when to Close the lzw.NewWriter :
{ // lexical scoping (static scoping)
zw := lzw.NewWriter(bufio.NewWriter(&buf), lzw.LSB, 8)
n, err := io.Copy(zw, strings.NewReader(line))
if err != nil {
log.Fatal(err)
}
if int(n) != len(line) {
log.Fatal(n, len(line))
}
// It is the caller's responsibility to call Close on the WriteCloser when finished writing.
if err = zw.Close(); err != nil {
log.Fatal(err)
}
}
5- Always check the errors, e.g.:
if err = zw.Close(); err != nil {
log.Fatal(err)
}
This is the working version close to your code - try this just to experiment with concurrency logic to see what happens (not recommended since this has superfluous goroutines and io.Pipe - just working:
package main
import (
"bufio"
"compress/lzw"
"context"
"encoding/base64"
"fmt"
"io"
"log"
"strings"
"sync"
"time"
)
func main() {
index := 0
client := &myClient{}
reader := bufio.NewReader(file)
// your code:
var wg sync.WaitGroup
for {
index++
line, err := reader.ReadString('\n')
if err != nil {
msg <- fmt.Sprint(index, " Done not waiting with err: ", err, time.Now())
wg.Wait() // break waiting // if index%tenThousand != 0
break
}
wg.Add(1)
go func(i int) {
msg <- fmt.Sprint(i, " Enter running ... ", time.Now())
asyncReader, asyncWriter := io.Pipe() // make it async to read and write
zipWriter := lzw.NewWriter(asyncWriter, lzw.LSB, 8)
go func() { // async
_, err := io.Copy(zipWriter, strings.NewReader(line))
if err != nil {
log.Fatal(err)
}
_ = zipWriter.Close()
_ = asyncWriter.Close() // for io.ReadAll
}()
b, err := io.ReadAll(asyncReader)
if err != nil {
log.Fatal(err)
}
client.Set(context.Background(), fmt.Sprintf("%d", i), base64.StdEncoding.EncodeToString(b), time.Hour*1000)
asyncReader.Close()
time.Sleep(1 * time.Second)
msg <- fmt.Sprint(i, " Exit running ... ", time.Now())
wg.Done()
}(index)
msg <- fmt.Sprint(index, " ", index%tenThousand == 0, " after go call")
if index%tenThousand == 0 {
wg.Wait()
msg <- fmt.Sprint("..", index, " Done waiting after go call. ", time.Now())
}
}
msg <- "Bye forever."
wg.Wait()
close(msg)
wgMsg.Wait()
}
// just for the Go Playground:
const tenThousand = 2
type myClient struct {
}
func (p *myClient) Set(ctx context.Context, a, b string, t time.Duration) {
// fmt.Println("a =", a, ", b =", b, ", t =", t)
if ctx.Err() != nil {
fmt.Println(ctx.Err())
}
}
var file, myw = io.Pipe()
func init() {
go func() {
for i := 1; i <= tenThousand+1; i++ {
fmt.Fprintf(myw, "%d text to compress aaaaaaaaaaaaaa\n", i)
}
myw.Close()
}()
wgMsg.Add(1)
go func() {
defer wgMsg.Done()
for s := range msg {
fmt.Println(s)
}
}()
}
var msg = make(chan string, 100)
var wgMsg sync.WaitGroup
Output:
1 false after go call
2 true after go call
1 Enter running ... 2009-11-10 23:00:00 +0000 UTC m=+0.000000001
2 Enter running ... 2009-11-10 23:00:00 +0000 UTC m=+0.000000001
1 Exit running ... 2009-11-10 23:00:01 +0000 UTC m=+1.000000001
2 Exit running ... 2009-11-10 23:00:01 +0000 UTC m=+1.000000001
..2 Done waiting after go call. 2009-11-10 23:00:01 +0000 UTC m=+1.000000001
3 false after go call
3 Enter running ... 2009-11-10 23:00:01 +0000 UTC m=+1.000000001
4 Done not waiting with err: EOF 2009-11-10 23:00:01 +0000 UTC m=+1.000000001
3 Exit running ... 2009-11-10 23:00:02 +0000 UTC m=+2.000000001
Bye forever.