runtime: mark map bucket slots as empty during map clear

So iterators that are in progress can know entries have been deleted and
terminate the iterator properly.

Update #55002
Update #56351
Fixes #59411

Change-Id: I924f16a00fe4ed6564f730a677348a6011d3fb67
Reviewed-on: https://go-review.googlesource.com/c/go/+/481935
Reviewed-by: Keith Randall <khr@golang.org>
Auto-Submit: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Keith Randall <khr@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/runtime/map.go

@@ -1008,6 +1008,22 @@ func mapclear(t *maptype, h *hmap) {
 
 	h.flags ^= hashWriting
 
+	// Mark buckets empty, so existing iterators can be terminated, see issue #59411.
+	markBucketsEmpty := func(bucket unsafe.Pointer, mask uintptr) {
+		for i := uintptr(0); i <= mask; i++ {
+			b := (*bmap)(add(bucket, i*uintptr(t.bucketsize)))
+			for ; b != nil; b = b.overflow(t) {
+				for i := uintptr(0); i < bucketCnt; i++ {
+					b.tophash[i] = emptyRest
+				}
+			}
+		}
+	}
+	markBucketsEmpty(h.buckets, bucketMask(h.B))
+	if oldBuckets := h.oldbuckets; oldBuckets != nil {
+		markBucketsEmpty(oldBuckets, h.oldbucketmask())
+	}
+
 	h.flags &^= sameSizeGrow
 	h.oldbuckets = nil
 	h.nevacuate = 0

test/fixedbugs/issue59411.go

@@ -0,0 +1,77 @@
+// run
+
+// Copyright 2023 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+import (
+	"math"
+	"reflect"
+)
+
+func main() {
+	for i := 0; i < 100; i++ {
+		f()
+		g()
+	}
+}
+
+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 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.
+			panic("BAD")
+		}
+		i++
+	}
+	if len(m) != 0 {
+		panic("clear did not empty the map")
+	}
+}
+
+func g() {
+	// 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
+	v := reflect.ValueOf(m)
+	iter := v.MapRange()
+	for iter.Next() {
+		if i == 6 {
+			// Partway through iteration, clear the map.
+			v.Clear()
+		} else if i > 6 {
+			// If we advance to the next iteration, that's a bug.
+			panic("BAD")
+		}
+		i++
+	}
+	if v.Len() != 0 {
+		panic("clear did not empty the map")
+	}
+}