reflect: ptrAlign panic when accessing Method

[mark-rushakoff]

What version of Go are you using (go version)?

$ go version
go version devel +e9c994954f Sat Oct 19 04:45:10 2019 +0000 darwin/amd64

What did you do?

When running with -gcflags=all=-d=checkptr against an existing codebase, I saw many panics that seemed to originate from go-cmp. Upon further investigation and minimizing the repro case, I have this near-minimal example:

package main

import (
	"fmt"
	"reflect"
)

type Foo struct{}

func (Foo) Bar() {}

func main() {
	t := reflect.TypeOf(Foo{})
	m, ok := t.MethodByName("Bar")
	fmt.Println(m, ok)
}

When run with gotip run -gcflags=all=-d=checkptr ., the following panic occurs:

panic: (runtime.ptrAlign) (0x10e45e0,0xc0000701f0)

goroutine 1 [running]:
reflect.(*funcType).in(...)
	/Users/mr/gotip/src/github.com/golang/go/src/reflect/type.go:1011
reflect.funcStr(0xc000090120, 0x10d38c0, 0xc00007aa64)
	/Users/mr/gotip/src/github.com/golang/go/src/reflect/type.go:2040 +0x9b4
reflect.FuncOf(0xc0000701e0, 0x1, 0x1, 0x11f4ef0, 0x0, 0x0, 0x100c700, 0x0, 0x0)
	/Users/mr/gotip/src/github.com/golang/go/src/reflect/type.go:2023 +0x6d1
reflect.(*rtype).Method(0x10def60, 0x0, 0x10c38db, 0x3, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, ...)
	/Users/mr/gotip/src/github.com/golang/go/src/reflect/type.go:823 +0x54e
reflect.(*rtype).MethodByName(0x10def60, 0x10fe7b9, 0x3, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, ...)
	/Users/mr/gotip/src/github.com/golang/go/src/reflect/type.go:845 +0x1a7
main.main()
	/tmp/eeee/main.go:14 +0xc0
exit status 2

When run with go version go1.12.9 darwin/amd64, this program outputs {Bar func(main.Foo) <func(main.Foo) Value> 0} true.

/cc @mdempsky

[mdempsky]

Thanks, this is a known issue. In general, the standard library still has a few issues with -d=checkptr (see #34972).

For this issue in particular, I have a fix at https://go-review.googlesource.com/c/go/+/201839 if you want to try that.

[mdempsky]

This should be fixed now.