unique: panic for Make(struct{}{})

[haruyama480]

Go version

go version go1.23.1 darwin/arm64

Output of go env in your module/workspace:

GO111MODULE=''
GOARCH='arm64'
GOBIN=''
GOCACHE='/Users/xxx/Library/Caches/go-build'
GOENV='/Users/xxx/Library/Application Support/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFLAGS=''
GOHOSTARCH='arm64'
GOHOSTOS='darwin'
GOINSECURE=''
GOMODCACHE='/Users/xxx/go/pkg/mod'
GONOPROXY=''
GONOSUMDB=''
GOOS='darwin'
GOPATH='/Users/xxx/go'
GOPRIVATE=''
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/Users/xxx/go/pkg/mod/golang.org/toolchain@v0.0.1-go1.23.1.darwin-arm64'
GOSUMDB='sum.golang.org'
GOTMPDIR=''
GOTOOLCHAIN='go1.23.1'
GOTOOLDIR='/Users/xxx/go/pkg/mod/golang.org/toolchain@v0.0.1-go1.23.1.darwin-arm64/pkg/tool/darwin_arm64'
GOVCS=''
GOVERSION='go1.23.1'
GODEBUG=''
GOTELEMETRY='local'
GOTELEMETRYDIR='/Users/xxx/Library/Application Support/go/telemetry'
GCCGO='gccgo'
GOARM64='v8.0'
AR='ar'
CC='clang'
CXX='clang++'
CGO_ENABLED='1'
GOMOD='/Users/xxx/ghq/github.com/xxx/go-learning/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/7c/vn1g1x4n4nqclvb5dnjkf8q80000gn/T/go-build85467120=/tmp/go-build -gno-record-gcc-switches -fno-common'

What did you do?

https://go.dev/play/p/esaCHP90Kzk

output

fatal error: getWeakHandle on invalid pointer

goroutine 1 gp=0xc0000061c0 m=0 mp=0x5003e0 [running]:
runtime.throw({0x4857e3?, 0x500301?})
	/usr/local/go-faketime/src/runtime/panic.go:1067 +0x48 fp=0xc0000485c0 sp=0xc000048590 pc=0x45d948
runtime.getWeakHandle(0x51eec0)
	/usr/local/go-faketime/src/runtime/mheap.go:2135 +0x113 fp=0xc000048608 sp=0xc0000485c0 pc=0x422f93
runtime.getOrAddWeakHandle(0x51eec0)
	/usr/local/go-faketime/src/runtime/mheap.go:2083 +0x1c fp=0xc000048650 sp=0xc000048608 pc=0x422cbc
internal/weak.runtime_registerWeakPointer(0x0?)
	/usr/local/go-faketime/src/runtime/mheap.go:2045 +0x13 fp=0xc000048668 sp=0xc000048650 pc=0x45d233
internal/weak.Make[...](0x51eec0?)
	/usr/local/go-faketime/src/internal/weak/pointer.go:62 +0x5d fp=0xc000048680 sp=0xc000048668 pc=0x46a63d
unique.Make[...].func1()
	/usr/local/go-faketime/src/unique/handle.go:57 +0x88 fp=0xc0000486b8 sp=0xc000048680 pc=0x46a588
unique.Make[...]({})
	/usr/local/go-faketime/src/unique/handle.go:67 +0x135 fp=0xc000048738 sp=0xc0000486b8 pc=0x46a475
main.main()
	/tmp/sandbox2127609540/prog.go:8 +0x1a fp=0xc000048750 sp=0xc000048738 pc=0x46941a
runtime.main()
	/usr/local/go-faketime/src/runtime/proc.go:272 +0x28b fp=0xc0000487e0 sp=0xc000048750 pc=0x4315eb
runtime.goexit({})
	/usr/local/go-faketime/src/runtime/asm_amd64.s:1700 +0x1 fp=0xc0000487e8 sp=0xc0000487e0 pc=0x463ea1

goroutine 2 gp=0xc000006700 m=nil [force gc (idle)]:
runtime.gopark(0x0?, 0x0?, 0x0?, 0x0?, 0x0?)
	/usr/local/go-faketime/src/runtime/proc.go:424 +0xce fp=0xc000048fa8 sp=0xc000048f88 pc=0x45da2e
runtime.goparkunlock(...)
	/usr/local/go-faketime/src/runtime/proc.go:430
runtime.forcegchelper()
	/usr/local/go-faketime/src/runtime/proc.go:337 +0xa5 fp=0xc000048fe0 sp=0xc000048fa8 pc=0x431925
runtime.goexit({})
	/usr/local/go-faketime/src/runtime/asm_amd64.s:1700 +0x1 fp=0xc000048fe8 sp=0xc000048fe0 pc=0x463ea1
created by runtime.init.7 in goroutine 1
	/usr/local/go-faketime/src/runtime/proc.go:325 +0x1a

goroutine 3 gp=0xc000006c40 m=nil [GC sweep wait]:
runtime.gopark(0x0?, 0x0?, 0x0?, 0x0?, 0x0?)
	/usr/local/go-faketime/src/runtime/proc.go:424 +0xce fp=0xc000049780 sp=0xc000049760 pc=0x45da2e
runtime.goparkunlock(...)
	/usr/local/go-faketime/src/runtime/proc.go:430
runtime.bgsweep(0xc00006a000)
	/usr/local/go-faketime/src/runtime/mgcsweep.go:277 +0x94 fp=0xc0000497c8 sp=0xc000049780 pc=0x41d814
runtime.gcenable.gowrap1()
	/usr/local/go-faketime/src/runtime/mgc.go:203 +0x25 fp=0xc0000497e0 sp=0xc0000497c8 pc=0x412285
runtime.goexit({})
	/usr/local/go-faketime/src/runtime/asm_amd64.s:1700 +0x1 fp=0xc0000497e8 sp=0xc0000497e0 pc=0x463ea1
created by runtime.gcenable in goroutine 1
	/usr/local/go-faketime/src/runtime/mgc.go:203 +0x66

goroutine 4 gp=0xc000006e00 m=nil [GC scavenge wait]:
runtime.gopark(0xc00006a000?, 0x49eab8?, 0x1?, 0x0?, 0xc000006e00?)
	/usr/local/go-faketime/src/runtime/proc.go:424 +0xce fp=0xc000049f78 sp=0xc000049f58 pc=0x45da2e
runtime.goparkunlock(...)
	/usr/local/go-faketime/src/runtime/proc.go:430
runtime.(*scavengerState).park(0x4ff640)
	/usr/local/go-faketime/src/runtime/mgcscavenge.go:425 +0x49 fp=0xc000049fa8 sp=0xc000049f78 pc=0x41b289
runtime.bgscavenge(0xc00006a000)
	/usr/local/go-faketime/src/runtime/mgcscavenge.go:653 +0x3c fp=0xc000049fc8 sp=0xc000049fa8 pc=0x41b7bc
runtime.gcenable.gowrap2()
	/usr/local/go-faketime/src/runtime/mgc.go:204 +0x25 fp=0xc000049fe0 sp=0xc000049fc8 pc=0x412225
runtime.goexit({})
	/usr/local/go-faketime/src/runtime/asm_amd64.s:1700 +0x1 fp=0xc000049fe8 sp=0xc000049fe0 pc=0x463ea1
created by runtime.gcenable in goroutine 1
	/usr/local/go-faketime/src/runtime/mgc.go:204 +0xa5

goroutine 17 gp=0xc000084380 m=nil [chan receive]:
runtime.gopark(0x0?, 0x0?, 0x0?, 0x0?, 0x0?)
	/usr/local/go-faketime/src/runtime/proc.go:424 +0xce fp=0xc000044718 sp=0xc0000446f8 pc=0x45da2e
runtime.chanrecv(0xc000098070, 0x0, 0x1)
	/usr/local/go-faketime/src/runtime/chan.go:639 +0x41c fp=0xc000044790 sp=0xc000044718 pc=0x40587c
runtime.chanrecv1(0x0?, 0x0?)
	/usr/local/go-faketime/src/runtime/chan.go:489 +0x12 fp=0xc0000447b8 sp=0xc000044790 pc=0x405452
runtime.unique_runtime_registerUniqueMapCleanup.func1(...)
	/usr/local/go-faketime/src/runtime/mgc.go:1732
runtime.unique_runtime_registerUniqueMapCleanup.gowrap1()
	/usr/local/go-faketime/src/runtime/mgc.go:1735 +0x2f fp=0xc0000447e0 sp=0xc0000447b8 pc=0x414fef
runtime.goexit({})
	/usr/local/go-faketime/src/runtime/asm_amd64.s:1700 +0x1 fp=0xc0000447e8 sp=0xc0000447e0 pc=0x463ea1
created by unique.runtime_registerUniqueMapCleanup in goroutine 1
	/usr/local/go-faketime/src/runtime/mgc.go:1730 +0x96

Program exited.

What did you see happen?

fatal error: getWeakHandle on invalid pointer

What did you expect to see?

no panic

[gabyhelp]

Related Issues and Documentation

• go build #52634 (closed)
• cmd/go: panic: runtime error: invalid memory address or nil pointer dereference during type checking in `go generate` #66670 (closed)
• cmd/compile: go:linkname causes "internal compiler error: panic: runtime error: invalid memory address or nil pointer dereference" #59621
• cmd/compile/internal/types2: unify.go:449: assertion failed, panic #67872 (closed)
• sb #53220 (closed)
• cmd/go: panic when building a `main` package in GOPATH mode when a go.work file exists #58767 (closed)
• cmd/go: bootstrap with Go 1.21.x fail at tip, panic: godebug: Value of name not listed in godebugs.All: gotypesalias #64106 (closed)
• cmd/cgo: cgo build fails with panic building trivial code on M1 Mac (darwin/arm64) #53193 (closed)
• Bugs-Report #62338 (closed)
• clear #57553 (closed)

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

[haruyama480]

struct{} has no value, so there may be no way to implement unique.Make(sturct{}{}).
However, a runtime panic may be somewhat surprising.

[ianlancetaylor]

CC @mknyszek

[cuonglm]

The address of *T where T is zero-size type will be the address of runtime.zerobase, which does not belong to heap address, thus the runtime panic.

It seems to me that we need a special treatment for zerobase.

[haruyama480]

It seems to me that we need a special treatment for zerobase.
yes.

I think a traditional approach is treat zerobase as a null pointer in Handle[T], though I don't know Go can treat a null pointer.
To apply this handling for all types, we can treat zero value too, to omit insertion of default value.

[mknyszek]

I think we do need to handle zero-sized types specially, but I don't think the unique package needs to be aware of runtime.zerobase.

In theory we can have all zero-sized types use the same pointer under the hood, because you can't compare two Handle[T] where T differs anyway, and all zero-sized values of the same type compare equal with each other (really, there's only one value for each type).

package unique

var zero uintptr

func Make[T comparable](value T) Handle[T] {
    if unsafe.Sizeof(*new(T)) == 0 {
        return Handle[T]{(*T)(unsafe.Pointer(&zero))}
    }
    // Rest of Make.
    ...
}

func (h Handle[T]) Value() T {
    if unsafe.Sizeof(*new(T)) == 0 {
        return *new(T)
    }
    return *h.value
}

I don't really see the use-case for this, so it doesn't seem urgent, but I can send a patch for this.

[mknyszek]

Actually @haruyama480 is right, we don't need var zero uintptr and &zero, we can just say nil. That works too.

[gopherbot]

Change https://go.dev/cl/613397 mentions this issue: unique: handle zero-size types