runtime: iterator returns map entries after clear (pre-swissmap)

[randall77]

Go version

tip

Output of go env in your module/workspace:

GO111MODULE=''
GOARCH='arm64'
GOAUTH='netrc'
GOBIN=''
GOCACHE='/Users/khr/Library/Caches/go-build'
GOENV='/Users/khr/Library/Application Support/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFLAGS=''
GOHOSTARCH='arm64'
GOHOSTOS='darwin'
GOINSECURE=''
GOMODCACHE='/Users/khr/gopath/pkg/mod'
GONOPROXY=''
GONOSUMDB=''
GOOS='darwin'
GOPATH='/Users/khr/gopath'
GOPRIVATE=''
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/Users/khr/sandbox/ro5'
GOSUMDB='sum.golang.org'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='/Users/khr/sandbox/ro5/pkg/tool/darwin_arm64'
GOVCS=''
GOVERSION='devel go1.24-fb5dcc1526 Mon Nov 4 12:50:55 2024 -0800'
GODEBUG=''
GOTELEMETRY='local'
GOTELEMETRYDIR='/Users/khr/Library/Application Support/go/telemetry'
GCCGO='gccgo'
GOARM64='v8.0'
AR='ar'
CC='clang'
CXX='clang++'
CGO_ENABLED='1'
GOMOD='/Users/khr/sandbox/ro5/src/go.mod'
GOWORK=''
CGO_CFLAGS='-O2 -g'
CGO_CPPFLAGS=''
CGO_CXXFLAGS='-O2 -g'
CGO_FFLAGS='-O2 -g'
CGO_LDFLAGS='-O2 -g'
PKG_CONFIG='pkg-config'
GOGCCFLAGS='-fPIC -arch arm64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -ffile-prefix-map=/var/folders/z9/dty110711l9cr9w3ktv1_2380000gn/T/go-build1711499111=/tmp/go-build -gno-record-gcc-switches -fno-common'

What did you do?

package main

func nan() float64 {
	var x, y float64
	return x / y
}

func main() {
	m := map[float64]int{}

	// Make a small map with nan keys
	for i := 0; i < 8; i++ {
		m[nan()] = i
	}

	// Start iterating on it.
	start := true
	for k, v := range m {
		if start {
			// Add some more elements.
			for i := 0; i < 10; i++ {
				m[float64(i)] = i
			}
			// Now clear the map.
			clear(m)
			start = false
		} else {
			// This should never print anything.
			println(k, v)
		}
	}
}

When run, this should never print anything. The map is cleared during the first iteration. No map elements should survive to cause a second iteration.

There is no bug now that we've switched by default to swiss maps. The old maps, which you can get with GOEXPERIMENT=noswissmap, have this problem. This problem is present in 1.22.6 and 1.23.2. Presumably it has been there since clear was introduced in 1.21.

Seems related to #59411. Somehow the fix for that issue doesn't help here.

@prattmic

What did you see happen?

NaN 3
NaN 4
NaN 5
NaN 6
NaN 7
NaN 0
NaN 1

What did you expect to see?

nothing printed

[gabyhelp]

Related Issues and Documentation

• runtime: clear() on map doesn't correctly clear existing iterators #59411 (closed)
• Runtime: Deleting the key in the map has a probability of unexpected results #48051 (closed)
• cmd/compile: maps with type params doesn't recognize map-clearing range idiom #51699 (closed)

_{(Emoji vote if this was helpful or unhelpful; more detailed feedback welcome in this discussion.)}

[randall77]

Looks like the fix in #59411 is only a partial one. In this case the oldbuckets field is already zeroed (because growth is done) before clear is called. I'm not sure how we'd fix it to make it complete, other than just by copying what swissmap does with a counter incremented on each clear.

[fengyoulin]

I haven't found a better way, and I'm trying to implement it according to your idea.

[gopherbot]

Change https://go.dev/cl/625275 mentions this issue: runtime: fix iterator returns map entries after clear (pre-swissmap)

[jianfengtony]

As i test, this issue also affect go1.22.10, so why not backport it to this version? thanks.

[ianlancetaylor]

@jianfengtony Our backport guidelines can be found at https://go.dev/wiki/MinorReleases. Do you think this meets the guidelines for backporting? If so, that wiki page shows how to request a backport. Thanks.