go 1.5.1 linux/amd64 deadlock detection failed

[wudizhanche1000]

package main

import (
    "log"
    "net/http"
)
//delete this function, it would perform well
func useless_func(address string) []byte {
    http.Get("https://www.google.com")
    return nil
}
func test_a(test_channel chan int) {
    test_channel <- 1
    return
}

func test() {
    test_channel := make(chan int)
    for i := 0; i < 10; i++ {
        go test_a(test_channel)
    }
    for {
        log.Println(<-test_channel)
    }
}
func main() {
    test()
}

this code won't break because of clearly deadlock, it only happened under Linux with go 1.5.1. but if i delete useless_func and run it again, it would raise a deadlock error as expected.

[bradfitz]

Importing the net package disables the deadlock detector. It has nothing to do with net/http or that func. For example, this also doesn't catch the deadlock:

package main

import "log"
import _ "net"

func test_a(test_channel chan int) {
        test_channel <- 1
        return
}

func test() {
        test_channel := make(chan int)
        for i := 0; i < 10; i++ {
                go test_a(test_channel)
        }
        for {
                log.Println(<-test_channel)
        }
}
func main() {
        test()
}

The deadlock detector makes no guarantees. It basically only catches things with the most trivial programs.

[dominikh]

To expand: The issue really lies with using cgo (which net uses, ignoring the details). When using cgo, the Go deadlock detection cannot function properly, because C world might call Go functions at any time, so in theory no deadlock exists; we might just be waiting for an external function call indefinitely.