Does runtime.LockOSThread allow child goroutines to run in same OS thread?

Question

I understand that in Go, runtime.LockOSThread() will bind a goroutine to one OS thread and not allow other goroutines to execute in that thread. Is this also true for child goroutines?

For example:

runtime.LockOSThread()
go func() {
    go func() {
        // Do something
    }()
    // Do something
}()

Do both of these goroutines execute in a single and exclusive OS thread or only the first one?

Answer 1

The documentation for runtime.LockOSThread says:

LockOSThread wires the calling goroutine to its current operating system thread. Until the calling goroutine exits or calls UnlockOSThread, it will always execute in that thread, and no other goroutine can.

^{(emphasis mine)}

This means that if a certain implementation of Go did what you're asking, it would be faulty.

To clarify: if a goroutine had reserved a thread and another goroutine executed on that same thread; that's what would be wrong.

Answer 2

We can check it using pthread.h's pthread_self:

package main

// #include <pthread.h>
import "C"
import (
    "fmt"
    "runtime"
)

func main() {
    runtime.GOMAXPROCS(runtime.NumCPU())
    ch1 := make(chan bool)
    ch2 := make(chan bool)
    fmt.Println("main", C.pthread_self())
    go func() {
        runtime.LockOSThread()
        fmt.Println("locked", C.pthread_self())
        go func() {
            fmt.Println("locked child", C.pthread_self())
            ch1 <- true
        }()
        ch2 <- true
    }()
    <-ch1
    <-ch2
}

On my machine it prints something like this, main and locked always being sometimes the same, but sometimes different:

main 139711253194560
locked 139711219787520
locked child 139711236572928

EDIT I forgot about GOMAXPROCS. Added, the results are varying now.