CODEWITHSUNDEEP
goconcurrency
Dev chimp
Dev chimp

Reputation: 57

golang used waitGroup and still ended up with a deadlock error

I'm trying to implement the bridge pattern following Go Concurrency book

func bridge_impl() {
    done := make(chan interface{})
    defer close(done)
    var wg sync.WaitGroup
    bridge := func(
        done <-chan interface{},
        chanStream <-chan <-chan interface{},
    ) <-chan interface{} {
        valStream := make(chan interface{})
        go func() {
            wg.Add(1)
            defer close(valStream)
            for {
                var stream <-chan interface{}
                select {
                case maybeStream, ok := <-chanStream:
                    fmt.Println("works")
                    if ok == false {
                        return
                    }
                    stream = maybeStream
                case <-done:
                    return
                }
                for val := range stream {
                    select {
                    case valStream <- val:
                    case <-done:
                    }
                }
            }
        }()
        return valStream
    }
    genVals := func() <-chan <-chan interface{} {
        chanStream := make(chan (<-chan interface{}))
        go func() {
            wg.Add(1)
            defer close(chanStream)
            for i := 0; i < 10; i++ {
                stream := make(chan interface{})
                stream <- i
                close(stream)
                chanStream <- stream
            }
        }()
        return chanStream
    }
    for v := range bridge(done, genVals()) {
        fmt.Printf("%v ", v)
    }
    wg.Wait()
}

However I'm receiving a deadlock errorall goroutines are asleep - deadlock! at first I thought I should add a waitgroup even though it wasn't implemented in the book example but I ended up with the same error

Upvotes: 0

Views: 473

Answers (2)

mkopriva
mkopriva

Reputation: 38213

From what I understand you do not need a WaitGroup at all, you just need to re-order the statements in the genVals function's loop:

for i := 0; i < 10; i++ {
    stream := make(chan interface{})
    chanStream <- stream
    stream <- i
    close(stream)
}

https://go.dev/play/p/7D9OzrsvZyi

Upvotes: 2

medasx
medasx

Reputation: 634

There are two main issues.
Working example

First issue:

for i := 0; i < 10; i++ {
    stream := make(chan interface{})
    stream <- i
    close(stream)
    chanStream <- stream
}

writing to unbuffered channel after creation with no goroutine reading. Use buffered channel or another goroutine.

stream := make(chan interface{}, 1) // buffer size 1 to not block `stream <- i`

Second issue:
Using wg.Add(1) without wg.Done().
You can use defer in both cases.

wg.Add(1)
defer wg.Done()

Upvotes: 2

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