Why does it not create many threads when many goroutines are blocked in writing file in golang?

Question

As we know in go, a thread may be created when the goroutine has to perform a blocking call, such as a system call, or a call to a C library via cgo. Some test code:

Answer 1

The issue 4056 discusses how to limit the number of actual threads (not goroutine) created.

Go 1.2 introduced that thread limit management in commit 665feee.

You can see a test to check if the number of thread created is actually reached or not in pkg/runtime/crash_test.go#L128-L134:

func TestThreadExhaustion(t *testing.T) {
    output := executeTest(t, threadExhaustionSource, nil)
    want := "runtime: program exceeds 10-thread limit\nfatal error: thread exhaustion"
    if !strings.HasPrefix(output, want) {
        t.Fatalf("output does not start with %q:\n%s", want, output)
    }
}

That same file has an example to create an actual thread (for a given goroutine), using runtime.LockOSThread():

func testInNewThread(name string) {
    c := make(chan bool)
    go func() {
        runtime.LockOSThread()
        test(name)
        c <- true
    }()
    <-c
}