前往频道：如何使其不受阻碍？

Question

I have a script which is selecting some data from a database and sending this to a channel to be processed by multiple goroutines, with the results then sent back to the main thread to be updated on the database.

However, it is hanging (probably blocking) on sending the data to first channel.

The channels are created globally with:

var chin = make(chan in)
var chout = make(chan out)

in and out are both structs

First the goroutines are started:

for i:=0; i<5; i++ {
     go worker()
}

The code to load the channels is then:

        if verbose {
            fmt.Println(`Getting nextbatch2 and sending to workers`)
        }

        rows, err = nextbatch2.Query()
        if err != nil {
            panic(err)
        }

        var numtodo int
        for rows.Next() {
            err = rows.Scan(&id, &data)
            if err != nil {
                rows.Close()
                panic(err)
            }

            // Start
            var vin in
            vin.id = id
            vin.data = data
            chin <- vin
            numtodo++
        }
        rows.Close()

Then immediately after this:

        if verbose {
            fmt.Println(`Processing out channel from workers`)
        }

        for res := range chout {
            update5.Exec(res.data, res.id)
            if numtodo--; numtodo == 0 {
                break
            }
        }

And in the background are multiple worker() goroutines running:

func worker() {
      for res := range chin {
            var v out
            v.id = res.id
            v.data = process(res.data)
            chout <- v
      }
}

This code hangs after printing Getting nextbatch2 and sending to workers. It never gets to Processing out channel from workers. So it's hanging somewhere inside the rows.Next() loop, for which I can't figure out the reason as the chin channel should be non-blocking - even if the worker() goroutines were not processing it should still at least finish that loop.

Any ideas?

EDIT:

By adding on fmt.Println(" on", numtodo) at the end of the rows.Next() loop I can see that it blocks after 5, which I don't understand as it should be non-blocking, right?

EDIT 2:

By changing the channels to make(chan in/out, 100) it now will block after 105.

Answer 1

Receivers always block until there is data to receive. If the channel is unbuffered, the sender blocks until the receiver has received the value.

https://golang.org/doc/effective_go.html#channels

So you can rewrite your consumer-code to something like:

go func(){
    for res := range chout {
        update5.Exec(res.data, res.id)
    }
}()

Also you need to close(chin) and close(chout) for proper usage of range statement.