wait for mpuid until got in waitForCreateMPU

[kungf]

if mpuId is nil in mpuPart, will deadlock, because fh.mpuWG.Add(1)
called before mpuPart, it can not wait itself until defer mpuWG.Done().
we can wait mpuid in waitForCreateMPU until got it.

[kahing]

sorry I don't understand the issue here or the change you are trying to make. does it address a real issue or a perceived code quality issue?

[kungf]

hi @kahing
In the code below,

1. fh.mpuWG.Add(1)
2. then go fh.mpuPart()
3. if fh.mpuId is nil, then fh.mpuWG.Wait(), here will deadlock if it first mpuWG add(1) then wait() mpuWG to be zero, before Done() release

func (fh *FileHandle) uploadCurrentBuf(parallel bool) (err error) {
	err = fh.waitForCreateMPU()
        ......
	if parallel {
		fh.mpuWG.Add(1)
		go fh.mpuPart(buf, part, fh.nextWriteOffset)
                 /--- this is part of mpuPart func
	              if fh.mpuId == nil {
		              fh.mpuWG.Wait()
                ---/
	} else {
        ......
	}
	return
}

there is a simple test case:

package main
import (
	"fmt"
	"sync"
)

var wg sync.WaitGroup
func test() {
	wg.Wait()
	fmt.Println("test")
	wg.Done()
}

func main() {
	wg.Add(1)
	go test()
	wg.Wait()
}