我如何使用通道知道循环启动的所有goroutine何时完成

Question

I'm currently trying to range over a map to do concurrent database requests rather than synchronously, obviously because of the speed boost.

My problem is I have something like this:

var mainthreads = make(chan *mainthread)
var mainthreadsFetched = make(chan struct{})
for containerid := range containers {
    go func() {
        rows, err := db.Query("SELECT thread_id, belongs_to, thread_name, access_level FROM forum_mainthread WHERE belongs_to = ?", containerid)
        defer rows.Close()
        if err != nil {
            log.Println(err)
        }
        for rows.Next() {
            mainthread := &MainThread{}
            err := rows.Scan(&mainthread.MainThreadID, &mainthread.BelongsTo, &mainthread.ThreadName, &mainthread.AccessLevel)
            if err != nil {
                log.Println(err)
            }
            mainthreads <- mainthread
        }
    }()
    mainthreadsFetched <- struct{}{}
}

// Get all mainthreads
<-mainthreadsFetched
// Do other stuff after complete

Obviously mainthreadsFetched <- struct{}{} is being called almost instantly because the loop finishes faster than you can blink, how can I create a new channel per loop that will not block each new goroutine from starting, but rather let the loop start all goroutines and then send out on a channel when every goroutine is done.

Answer 1

Using sync.WaitGroup is a great solution, and the one usually used.

Alternatively, you can receive on mainthreadsFetched len(containers) times, instead of just 1. That will guarantee that all go routines have completed. You'll need to move the send line to the end of the go routine (or, better, into a defer).

Also, since containerid is in the for loop, its value changes. You need to pass it as a parameter to the go routine closure.