Deadlock while waiting for pipe io.Copy in Goroutine

Deadlock when waiting for pipe io.Copy in Goroutine is a common problem. When we wait for the completion of io.Copy in a Goroutine, if the pipe is not closed properly, it will cause a deadlock. In this case, Goroutine will keep waiting for data and cannot continue execution. The solution to this problem is to manually close the pipe after the io.Copy completes to ensure that the Goroutine can end properly. PHP editor Apple has introduced the causes and solutions to this problem in detail, hoping to help you better deal with this type of deadlock situation.

Question content

In the code below, the call to io.copy never returns; it just blocks indefinitely, causing a deadlock. This behavior only occurs when io.reader is connected to os.stdout io.writer using io.pipe. However, I need to use pipes because in my full code I use io.multiwriter and io.pipes to connect an io.reader to Many functions require io.reader.

func main() {
    read := strings.newreader("abcdefghij")
    pipereader, pipewriter := io.pipe()

    var wg sync.waitgroup
    wg.add(1)
    go func() {
        println("start copy")
        _, err := io.copy(os.stdout, pipereader)
        if err != nil {
            println(err.error())
        }
        println("end copy")
        wg.done()
    }()

    _, err := io.copy(pipewriter, read)
    if err != nil {
        println(err.error())
    }

    wg.wait()
}

Output:

Start copy
abcdefghij
fatal error: all goroutines are asleep - deadlock!

goroutine 1 [semacquire]:
sync.runtime_Semacquire(0xc0000b0018?)
    /usr/local/go-faketime/src/runtime/sema.go:62 +0x25
sync.(*WaitGroup).Wait(0x4969c8?)
    /usr/local/go-faketime/src/sync/waitgroup.go:139 +0x52
main.main()
    /tmp/sandbox4108076976/prog.go:31 +0x23c

goroutine 18 [select]:
io.(*pipe).read(0xc0000a6120, {0xc0000b6000, 0x8000, 0xc00009e101?})
    /usr/local/go-faketime/src/io/pipe.go:57 +0xb1
io.(*PipeReader).Read(0x10?, {0xc0000b6000?, 0xc00009e1e0?, 0x4f75a0?})
    /usr/local/go-faketime/src/io/pipe.go:136 +0x25
io.copyBuffer({0x496aa8, 0xc00009e1e0}, {0x4969a8, 0xc0000a4018}, {0x0, 0x0, 0x0})
    /usr/local/go-faketime/src/io/io.go:427 +0x1b2
io.Copy(...)
    /usr/local/go-faketime/src/io/io.go:386
os.genericReadFrom(0xb000000006018ab?, {0x4969a8, 0xc0000a4018})
    /usr/local/go-faketime/src/os/file.go:162 +0x67
os.(*File).ReadFrom(0xc0000a4008, {0x4969a8, 0xc0000a4018})
    /usr/local/go-faketime/src/os/file.go:156 +0x1b0
io.copyBuffer({0x496a28, 0xc0000a4008}, {0x4969a8, 0xc0000a4018}, {0x0, 0x0, 0x0})
    /usr/local/go-faketime/src/io/io.go:413 +0x14b
io.Copy(...)
    /usr/local/go-faketime/src/io/io.go:386
main.main.func1()
    /tmp/sandbox4108076976/prog.go:18 +0x71
created by main.main
    /tmp/sandbox4108076976/prog.go:16 +0x1d3

Here is the playground link to the code: https://goplay.tools/snippet/70ubgiz8ftv

Is there a way to avoid deadlock while preserving io.pipe?

Solution

Close the writing end of the pipe after completion:

_, err := io.Copy(pipeWriter, read)
pipeWriter.Close()
if err != nil {
    println(err.Error())
}

Otherwise, the reader will wait indefinitely.

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