runtime timers stop firing when GOMAXPROCS is reduced

stuck-timer-repro.go

package main

import (
        "fmt"
        "runtime"
        "sync"
        "time"
)

func main() {
        for i := 0; i < 20; i++ {
                fmt.Println("attempt", i)
                run(10, 100)
        }
}

func run(gomaxprocs, goroutines int) {
        runtime.GOMAXPROCS(gomaxprocs)

        var wg sync.WaitGroup

        // Crate tickers in background goroutines (spread them across Ps)
        tickers := make([]*time.Ticker, goroutines)
        for i := 0; i < goroutines; i++ {
                wg.Add(1)
                go func(i int) {
                        t := time.NewTicker(time.Millisecond)
                        tickers[i] = t
                        wg.Done()
                }(i)
        }

        // Reduce GOMAXPROCS to try and trigger the bug.
        for i := runtime.GOMAXPROCS(0); i > 0; i-- {
                runtime.Gosched()
                runtime.GOMAXPROCS(i)
        }
        runtime.GOMAXPROCS(1)

        // Wait for all tickers to be scheduled
        wg.Wait()

        // Verify all tickers are working
        for i := 0; i < 10; i++ {
                for ti, t := range tickers {
                        select {
                        case <-t.C:
                        case <-time.After(10 * time.Second):
                                fmt.Printf("========= Likely bug =======\ntimer %v timed out after 10s of waiting %v\n", ti, i)                                                                            

                                waitStart := time.Now()
                                <-t.C
                                fmt.Println("Timer eventually fired after", time.Since(waitStart))
                        }
                }
        }
}