I hadn't used the race detector before. I do see a race warning for (*URL).String() among an embarrassing number of other results. I'm going to update (*URL).String() to use atomic.Pointer to remove the race.
Thanks, Ethan On Thu, Mar 21, 2024 at 8:59 AM 'Axel Wagner' via golang-nuts < golang-nuts@googlegroups.com> wrote: > On Thu, Mar 21, 2024 at 2:48 PM 王李荣 <wanglirong1...@gmail.com> wrote: > >> hi Axel, >> >> is not modifying `u.memoize.str` thread-safe? the len and the data >> point should become visible at same time? >> > > What makes you think that? To be clear, there are no benign data races. > Even a data-race on a variable smaller than a word is still a data-race, > unless you do it holding a lock or using atomic instructions. But strings > are *larger* than single words. > > To demonstrate that the effect I am talking about is real, look at this > code: https://go.dev/play/p/LzRq9-OH-Xb > > >> >> 在2024年3月16日星期六 UTC+8 06:29:06<Axel Wagner> 写道： >> >>> Have you tried running the code with the race detector enabled? I >>> suspect that you are concurrently modifying `u.memoize.str` by calling >>> `u.String()` from multiple goroutines. And the non-zero length of the >>> string header written by one goroutine becomes visible to the other one, >>> before the modification to the data pointer. >>> >>> On Fri, Mar 15, 2024 at 11:15 PM Ethan Reesor <ethan....@gmail.com> >>> wrote: >>> >>>> From this CI job >>>> <https://gitlab.com/accumulatenetwork/accumulate/-/jobs/6398114923>: >>>> >>>> panic: runtime error: invalid memory address or nil pointer dereference >>>> [signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x51d8b7] >>>> goroutine 1589381 [running]: >>>> strings.EqualFold({0xc000beec20?, 0x0?}, {0x0?, 0xacace7?}) >>>> /usr/local/go/src/strings/strings.go:1111 +0x37 >>>> >>>> gitlab.com/accumulatenetwork/accumulate/pkg/url.(*URL).Equal(0xc000a74e40?, >>>> 0xc00094c540) >>>> /builds/accumulatenetwork/accumulate/pkg/url/url.go:472 +0x10c >>>> >>>> This is in a docker container based on the go:1.22 image, so the panic >>>> appears to be happening here: >>>> >>>> func EqualFold(s, t string) bool { >>>> // ASCII fast path >>>> i := 0 >>>> for ; i < len(s) && i < len(t); i++ { >>>> sr := s[i] >>>> tr := t[i] // <-- line 1111 >>>> >>>> (*URL).Equal >>>> <https://gitlab.com/accumulatenetwork/accumulate/-/blob/5b1cb612d76d4163a101303e51a6fd352224cdab/pkg/url/url.go#L465> >>>> : >>>> >>>> func (u *URL) Equal(v *URL) bool { >>>> if u == v { >>>> return true >>>> } >>>> if u == nil || v == nil { >>>> return false >>>> } >>>> return strings.EqualFold(u.String(), v.String()) >>>> } >>>> >>>> (*URL).String >>>> <https://gitlab.com/accumulatenetwork/accumulate/-/blob/5b1cb612d76d4163a101303e51a6fd352224cdab/pkg/url/url.go#L240> >>>> : >>>> >>>> func (u *URL) String() string { >>>> if u.memoize.str != "" { >>>> return u.memoize.str >>>> } >>>> >>>> u.memoize.str = u.format(nil, true) >>>> return u.memoize.str >>>> } >>>> >>>> (*URL).format >>>> <https://gitlab.com/accumulatenetwork/accumulate/-/blob/5b1cb612d76d4163a101303e51a6fd352224cdab/pkg/url/url.go#L189> >>>> : >>>> >>>> func (u *URL) format(txid []byte, encode bool) string { >>>> var buf strings.Builder >>>> // ... write to the builder >>>> return buf.String() >>>> } >>>> >>>> How is this possible? Based on `addr=0x0` in the panic I think this is >>>> a nil pointer panic, as opposed to some other kind of segfault. The only >>>> way I can reproduce panic-on-string-index is with >>>> `(*reflect.StringHeader)(unsafe.Pointer(&s)).Data >>>> = 0`, but I don't see how that can be happening here. I'm saving the string >>>> but I'm not doing anything weird with it. And the string header is a value >>>> type so code that manipulates the returned string shouldn't modify the >>>> original. And I'm definitely not doing any kind of unsafe string >>>> manipulation like that in my code, anywhere. The only reference to unsafe >>>> anywhere in my code is for parameters for calling GetDiskFreeSpaceExW >>>> (Windows kernel32.dll call). >>>> >>>> -- >>>> You received this message because you are subscribed to the Google >>>> Groups "golang-nuts" group. >>>> To unsubscribe from this group and stop receiving emails from it, send >>>> an email to golang-nuts...@googlegroups.com. >>>> To view this discussion on the web visit >>>> https://groups.google.com/d/msgid/golang-nuts/d6f6bb75-45e9-4a38-9bbd-d332e7f3e57cn%40googlegroups.com >>>> <https://groups.google.com/d/msgid/golang-nuts/d6f6bb75-45e9-4a38-9bbd-d332e7f3e57cn%40googlegroups.com?utm_medium=email&utm_source=footer> >>>> . >>>> >>> -- >> You received this message because you are subscribed to the Google Groups >> "golang-nuts" group. >> To unsubscribe from this group and stop receiving emails from it, send an >> email to golang-nuts+unsubscr...@googlegroups.com. >> To view this discussion on the web visit >> https://groups.google.com/d/msgid/golang-nuts/31f77ff2-cf11-4b3e-9b14-874b6cc41da3n%40googlegroups.com >> <https://groups.google.com/d/msgid/golang-nuts/31f77ff2-cf11-4b3e-9b14-874b6cc41da3n%40googlegroups.com?utm_medium=email&utm_source=footer> >> . >> > -- > You received this message because you are subscribed to a topic in the > Google Groups "golang-nuts" group. > To unsubscribe from this topic, visit > https://groups.google.com/d/topic/golang-nuts/Dgy0fyb4Shw/unsubscribe. > To unsubscribe from this group and all its topics, send an email to > golang-nuts+unsubscr...@googlegroups.com. > To view this discussion on the web visit > https://groups.google.com/d/msgid/golang-nuts/CAEkBMfG8v0qO_NP4PipEBL%3Dd_Ase9ntWi4EL1dQE_6ubeZQnww%40mail.gmail.com > <https://groups.google.com/d/msgid/golang-nuts/CAEkBMfG8v0qO_NP4PipEBL%3Dd_Ase9ntWi4EL1dQE_6ubeZQnww%40mail.gmail.com?utm_medium=email&utm_source=footer> > . > -- You received this message because you are subscribed to the Google Groups "golang-nuts" group. 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