And as Axel's own reproducer shows, even having two threads reading and
writing the same *variable* which points to a string can result in
indeterminate behaviour, since a string is really a struct containing two
parts (a pointer and a len). You're not mutating the string itself, but
you are updating a variable at the same time as it's being read.
In this regard, Go is no more thread-safe than C or C++, unless you make
use of the concurrency features it provides (i.e. channels) instead of
concurrently reading and writing the same variables.
On Monday 25 March 2024 at 13:18:15 UTC Axel Wagner wrote:
> TBQH the word "mutable" doesn't make a lot of sense in Go (even though the
> spec also calls strings "immutable").
> Arguably, *all* values in Go are immutable. It's just that all *pointers*
> in Go allow to modify the referenced variables - and some types allow you
> to get a pointer to a shared variable, which strings don't.
>
> That is, a `[]byte` is immutable - you have to write `x = append(x, v)`
> specifically because `append` creates a new slice value and overwrites the
> variable `x` with it.
> However, a `[]byte` refers to an underlying array and `&b[0]` allows you
> to obtain a pointer to that underlying array. So a `[]byte` represents a
> reference and that reference allows to mutate the referenced storage
> location. The same goes for a `*T`, a `map[K]V`, or a `type S struct{ X
> int; P *int }` - `S` itself is immutable, but `S.X` is a reference to some
> potentially shared variable.
>
> A `string` meanwhile, does not allow you to obtain a pointer to the
> underlying storage and that's what makes it "immutable". And that does
> indeed mean that if you pass a `string` value around, that can't lead to
> data races, while passing a `[]byte` around *might*.
>
> But for this case, it doesn't really matter whether or not the field is a
> `string` or a `[]byte` or an `int`: Because the "mutable" type is the
> `*URL`. Which represents a reference to some underlying `URL` variable,
> that you can then mutate. The race happens because you have a method on a
> pointer that mutates a field - *regardless* of the type of that field.
>
> I don't know if that helps, it's a bit subtle.
>
> On Mon, Mar 25, 2024 at 1:35 PM 'Lirong Wang' via golang-nuts <
> golan...@googlegroups.com> wrote:
>
>> Wow, i am from other language and i thought `string` is immutable or
>> something like that, so thread-safe for this operation. learned
>> something new!!! Thanks
>> On Thursday, March 21, 2024 at 11:42:24 PM UTC+8 Ethan Reesor wrote:
>>
>>> 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 <
>>> golan...@googlegroups.com> wrote:
>>>
>>>> On Thu, Mar 21, 2024 at 2:48 PM 王李荣 <wanglir...@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).
>>>>>>>
>>>>>>> --
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>>>>>>> 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>
>>>>>>> .
>>>>>>>
>>>>>> --
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>>>>>
>>>>> <https://groups.google.com/d/msgid/golang-nuts/31f77ff2-cf11-4b3e-9b14-874b6cc41da3n%40googlegroups.com?utm_medium=email&utm_source=footer>
>>>>> .
>>>>>
>>>> --
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>>>>
>>>> <https://groups.google.com/d/msgid/golang-nuts/CAEkBMfG8v0qO_NP4PipEBL%3Dd_Ase9ntWi4EL1dQE_6ubeZQnww%40mail.gmail.com?utm_medium=email&utm_source=footer>
>>>> .
>>>>
>>> --
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>>
> To view this discussion on the web visit
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>>
>> <https://groups.google.com/d/msgid/golang-nuts/65400cce-b60c-4bb0-97a7-a963c6621098n%40googlegroups.com?utm_medium=email&utm_source=footer>
>> .
>>
>
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