runtime: clear() on map doesn't correctly clear existing iterators

[randall77]

This program shouldn't print anything:

package main

import "math"

func main() {
	for i := 0; i < 100; i++ {
		f()
	}
}
func f() {
	// Allocate map.
	m := map[float64]int{}
	// Fill to just before a growth trigger.
	const N = 13 << 4 // 6.5 * 2 * 2^k
	for i := 0; i < N; i++ {
		m[math.NaN()] = i
	}
	// Trigger growth.
	m[math.NaN()] = N

	// Iterate through map.
	i := 0
	for _, v := range m {
		if i == 6 {
			// Partway through iteration, clear the map.
			clear(m)
		} else if i > 6 {
			// If we advance to the next iteration, that's a bug.
			println("BAD", i, v)
		}
		i++
	}
}

It does (at tip).

For maps whose keys have NaNs in them, iterators that are in progress can't tell that some entries have been deleted by the clear builtin. This is because where those NaNs should move to in the larger bucket array isn't computable. Pre clear bultin, this didn't matter because the NaN entries couldn't be deleted.

@cuonglm

Update #55002, #56351

[cuonglm]

@randall77 Could we mark all bucket slot as emptyOne?

[gopherbot]

Change https://go.dev/cl/481935 mentions this issue: runtime: mark map bucket slots as empty during map clear