maps: maps.Clone reference semantics when cloning a map with large value types

Go version

go version go1.21.4 linux/amd64

What operating system and processor architecture are you using (`go env`)?

GO111MODULE=''
GOARCH='amd64'
GOBIN=''
GOCACHE='/home/tdog/.cache/go-build'
GOENV='/home/tdog/.config/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFLAGS=''
GOHOSTARCH='amd64'
GOHOSTOS='linux'
GOINSECURE=''
GOMODCACHE='/home/tdog/.asdf/installs/golang/1.21.4/packages/pkg/mod'
GONOPROXY=''
GONOSUMDB=''
GOOS='linux'
GOPATH='/home/tdog/.asdf/installs/golang/1.21.4/packages'
GOPRIVATE=''
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/home/tdog/.asdf/installs/golang/1.21.4/go'
GOSUMDB='sum.golang.org'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='/home/tdog/.asdf/installs/golang/1.21.4/go/pkg/tool/linux_amd64'
GOVCS=''
GOVERSION='go1.21.4'
GCCGO='gccgo'
GOAMD64='v1'
AR='ar'
CC='gcc'
CXX='g++'
CGO_ENABLED='1'
GOMOD='/home/tdog/dev/maps-clone/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 -m64 -pthread -Wl,--no-gc-sections -fmessage-length=0 -ffile-prefix-map=/tmp/go-build1194712858=/tmp/go-build -gno-record-gcc-switches'

What did you do?

https://go.dev/play/p/vkz2JOzn6zV

package main

import (
	"fmt"
	"maps"
)

type Value struct {
	Val     int
	padding [100]int
}

func main() {
	const key = "key"

	m := make(map[string]Value)

	m[key] = Value{Val: 1}

	c := maps.Clone(m)

	v := m[key]
	v.Val = 2
	m[key] = v

	fmt.Println(m[key].Val, c[key].Val)
}

Possibly related to https://go-review.googlesource.com/c/go/+/471400

What did you expect to see?

2 1

What did you see instead?

2 2

@gopherbot

@gopherbot, please backport to Go 1.21. This appears to cause subtle aliasing bugs in cloned maps.

Backport issue(s) opened: #64475 (for 1.21).

Remember to create the cherry-pick CL(s) as soon as the patch is submitted to master, according to https://go.dev/wiki/MinorReleases.

(I made a minor correction to the original report, changing the expectation to 2 1 (from 1 2)).

I can confirm this behavior changed after CL 471400 moved the Clone implementation to the runtime.

@cuiweixie

@cuiweixie

The bug triggers this branch of mapclone2 https://go.googlesource.com/go/+/refs/heads/master/src/runtime/map.go#1512

	if src.B == 0 {
		dst.buckets = newobject(t.Bucket)
		dst.count = src.count
		typedmemmove(t.Bucket, dst.buckets, src.buckets)
		return dst
	}

Change https://go.dev/cl/546515 mentions this issue: maps: fix aliasing problems with Clone

maps: fix aliasing problems with Clone

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maps: maps.Clone reference semantics when cloning a map with large value types #64474

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maps: maps.Clone reference semantics when cloning a map with large value types #64474

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Go version

What operating system and processor architecture are you using (go env)?

What did you do?

What did you expect to see?

What did you see instead?

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bcmills commented on Nov 30, 2023

gopherbot commented on Nov 30, 2023

findleyr commented on Nov 30, 2023

mauri870 commented on Nov 30, 2023

randall77 commented on Nov 30, 2023

mauri870 commented on Nov 30, 2023

gopherbot commented on Dec 1, 2023

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What operating system and processor architecture are you using (`go env`)?