runtime/debug: SetMaxThreads(maxInt) crashes program with "-1-thread limit"

[bcmills]

$ go version
go version go1.6.1 linux/amd64

According to https://golang.org/pkg/runtime/debug/#SetMaxThreads, this program should be able to spawn essentially as many threads as it wants:

$ cat maxthreads/maxthreads.go
package main

import (
        "fmt"
        "runtime/debug"
)

func main() {
        const maxInt = int(^uint(0) >> 1)
        fmt.Println(maxInt)
        debug.SetMaxThreads(maxInt)
}

Instead, the runtime kills it immediately:

$ go run maxthreads/maxthreads.go
9223372036854775807
runtime: program exceeds -1-thread limit
fatal error: thread exhaustion

goroutine 1 [running]:
runtime.throw(0x518650, 0x11)
        /usr/lib/google-golang/src/runtime/panic.go:550 +0x99 fp=0xc820043e90 sp=0xc820043e78 pc=0x4289c9
runtime.checkmcount()
        /usr/lib/google-golang/src/runtime/proc.go:483 +0x94 fp=0xc820043eb0 sp=0xc820043e90 pc=0x42b5a4
runtime/debug.setMaxThreads(0x7fffffffffffffff, 0x2710)
        /usr/lib/google-golang/src/runtime/proc.go:4180 +0x46 fp=0xc820043ec0 sp=0xc820043eb0 pc=0x435b96
runtime/debug.SetMaxThreads(0x7fffffffffffffff, 0x1)
        /usr/lib/google-golang/src/runtime/debug/garbage.go:140 +0x21 fp=0xc820043ed8 sp=0xc820043ec0 pc=0x469581
main.main()
        /usr/local/google/home/bcmills/src/maxthreads/maxthreads.go:11 +0xd9 fp=0xc820043f50 sp=0xc820043ed8 pc=0x4010d9
runtime.main()
        /usr/lib/google-golang/src/runtime/proc.go:188 +0x2b0 fp=0xc820043fa0 sp=0xc820043f50 pc=0x42a160
runtime.goexit()
        /usr/lib/google-golang/src/runtime/asm_amd64.s:2002 +0x1 fp=0xc820043fa8 sp=0xc820043fa0 pc=0x4577b1
exit status 2

[bcmills]

(See also issue #4056.)

[bcmills]

The same program works fine with int(^uint32(0) >> 1) as a workaround, but the arbitrary 32-bit limit is surprising when I'm trying to write a call that says "let me use as many threads as I want".

[ianlancetaylor]

This is because of this line in proc.go:

sched.maxmcount = int32(in)

We could change all the variables from int32 to int, or, probably just as good in practice, use the maximum int32 value here if the argument to setMaxThreads is larger.

[gopherbot]

CL https://golang.org/cl/31516 mentions this issue.