You are trying to prove something nobody actually doubted. Meanwhile, you
seem to be aggressively ignoring what I actually wrote. I find that pretty
rude.
On Tue, 8 Oct 2024 at 01:15, robert engels <reng...@ix.netcom.com> wrote:
> Here is a slightly easier version to see the race between the mutation and
> the copy for the value method:
>
> package main
>
> import (
> "log"
> "sync"
> )
>
> type S struct {
> lock *sync.Mutex
> index int
> values [128]int
> }
>
> func (s *S) mutate() {
> s.lock.Lock();
> defer s.lock.Unlock();
> s.index++;
> for i:=0; i< 128; i++ {
> s.values[i]=s.index;
> }
> }
>
> func (s S) validate() {
> for i:=0;i<128;i++ {
> if s.values[i]!=s.index {
> log.Fatal("mismatch error")
> }
> }
> }
>
> func doit(s *S) {
> for {
> s.mutate()
> s.validate()
> }
> }
>
> func main() {
> var s S
> var lock sync.Mutex
> s.lock = &lock
> var wg sync.WaitGroup
> wg.Add(1)
> for i:=0;i<64;i++ {
> go doit(&s)
> }
> wg.Wait()
> }
>
>
> On Oct 7, 2024, at 6:06 PM, robert engels <reng...@ix.netcom.com> wrote:
>
> I wrote a simple test. Sure enough it fails, and it reports a data race.
>
> package main
>
> import (
> "log"
> "sync"
> )
>
> type S struct {
> sync.Mutex
> index int
> values [128]int
> }
>
> func (s *S) mutate() {
> s.Lock();
> defer s.Unlock();
> s.index++;
> for i:=0; i< 128; i++ {
> s.values[i]=s.index;
> }
> }
>
> func (s S) validate() {
> for i:=0;i<128;i++ {
> if s.values[i]!=s.index {
> log.Fatal("mismatch error")
> }
> }
> }
>
> func doit(s *S) {
> for {
> s.mutate()
> s.validate()
> }
> }
>
> func main() {
> var s S
> var wg sync.WaitGroup
> wg.Add(1)
> for i:=0;i<64;i++ {
> go doit(&s)
> }
> wg.Wait()
> }
>
> In fact, you get a linter warning, because of the copy of the mutex in
> calling the value method - since it knows it should be a reference.
>
>
> On Oct 7, 2024, at 5:30 PM, Robert Engels <reng...@ix.netcom.com> wrote:
>
> I am fairly certain if you mix pointer and receiver methods and the
> receiver methods mutate - even if you synchronize those you will get a data
> race calling the value methods. It must afaik as the runtime/compiler has
> no implicit synchronization when creating the copies. That is a data race.
>
> On Oct 7, 2024, at 5:10 PM, Axel Wagner <axel.wagner...@googlemail.com>
> wrote:
>
>
> My argument had nothing to do with synchronization.
>
> FTR I find the synchronization argument also extremely dubious. By that
> argument, you also can't pass the address to a local variable to another
> function, when using it as a value elsewhere. It's a weird argument to
> make. time.Time uses a mix of pointer- and value receivers and IMO no one
> can make a serious argument that this would expose programs to risks of
> data races.
>
> But to repeat my actual argument in favour of (sometimes) mixing receiver
> kinds:
> 1. It is totally reasonable to use some types as values.
> 2. Such types, intended to be used as values, will need to use
> value-receivers for some methods, as otherwise their value-version does not
> implement certain interfaces (methods are not promoted from pointer to
> value types). Like fmt.Stringer, for example. And
> 3. such types still need to sometimes use pointer-receivers, to implement
> functionalities like unmarshalling.
>
> time.Time is a standard library example of such a type. I also provided an
> example for an "enum-like" type implementing flag.Value.
>
> On Mon, 7 Oct 2024 at 23:57, Robert Engels <reng...@ix.netcom.com> wrote:
>
>> I am pretty sure it is immaterial. If the object isn’t immutable any copy
>> or mutation operation needs to be synchronized.
>>
>> But the problem afaik is that you can’t control synchronization when the
>> object is copied for a value receiver - which means you cant properly
>> synchronize when you have pointer and value receivers unless you do it
>> externally (which is a huge pain to do everywhere).
>>
>> On Oct 7, 2024, at 4:43 PM, 'Axel Wagner' via golang-nuts <
>> golang-nuts@googlegroups.com> wrote:
>>
>>
>> No offence, but I made an argument. You don't have to agree with the
>> argument and it might be wrong. But to convince me, at least, that argument
>> would need to actually be referenced.
>>
>> I gave reasons why, in my opinion, *not* mixing value and pointer
>> receivers sometimes leads to incorrect code. So as far as I'm concerned
>> (until someone tells me my reasons are wrong) Goland's linter simply
>> encourages you to write bad code. It would not be the first time that I
>> strongly disagree with the recommendations of an IDE. Goland in particular
>> has a history of making, in my opinion, pretty questionable decisions.
>>
>> On Mon, 7 Oct 2024 at 22:39, Cleberson Pedreira Pauluci <
>> pauluci.cleber...@gmail.com> wrote:
>>
>>> Many places and books I've read generally say: If a function needs to
>>> update a variable, or if an argument is so large that we want to avoid
>>> copying it, we should pass the pointer. Same for methods (pointer
>>> receiver). (The Go programming language book).
>>>
>>> About mixing "value receiver" and "pointer receiver". Even the IDE
>>> complains about this and recommends following the Go documentation. (Goland)
>>>
>>>
>>> Em segunda-feira, 7 de outubro de 2024 às 15:15:25 UTC-3, burak serdar
>>> escreveu:
>>>
>>>> Mixing pointer and value receivers can be race-prone, because of the
>>>> copying involved in passing value receivers.
>>>>
>>>> On Mon, Oct 7, 2024 at 12:03 PM 'Axel Wagner' via golang-nuts
>>>> <golan...@googlegroups.com> wrote:
>>>> >
>>>> > To be honest, I always found this recommendation a little bit
>>>> strange, personally.
>>>> >
>>>> > I'll note that the standard library does not really keep to this
>>>> either. For example, time.Time.UnmarshalText (obviously) has a
>>>> pointer-receiver, while almost all other methods on time.Time have a value
>>>> receiver.
>>>> > And if you implement flag.Value, the Set method obviously needs a
>>>> pointer receiver, but if the String method has one as well, it won't print
>>>> properly when used as a value. In basically every implementation of
>>>> flag.Value I've ever written, String needed a value receiver, while Set
>>>> needed a pointer receiver.
>>>> >
>>>> > I understand the basic idea of the advice, that if a type keeps state
>>>> that is manipulated via methods, then it should generally be passed around
>>>> as a pointer, so giving all the methods a pointer-receiver works well. But
>>>> if a type *is* intended to be used as a value (like time.Time or Enum in my
>>>> example) then you will almost certainly end up with a mix of receiver kinds
>>>> - as soon as you want to add any form of de-serialization to it. So "don't
>>>> mix receiver kinds" seems like misleading advice to me.
>>>> >
>>>> > On Mon, 7 Oct 2024 at 19:44, Ian Lance Taylor <ia...@golang.org>
>>>> wrote:
>>>> >>
>>>> >> On Mon, Oct 7, 2024 at 10:29 AM Ken Lee <ken.lee....@gmail.com>
>>>> wrote:
>>>> >> >
>>>> >> > ---
>>>> >> > There is a consideration to make, though: historically it has been
>>>> considered bad form in Go to give a type a mix of value and pointer
>>>> receivers in methods without a very specific reason for doing so.
>>>> >> > ---
>>>> >> >
>>>> >> > Is this still the case now? As in 2024.
>>>> >>
>>>> >> As a general guideline, yes.
>>>> >>
>>>> >> https://go.dev/wiki/CodeReviewComments#receiver-type
>>>> >>
>>>> >> Ian
>>>> >>
>>>> >>
>>>> >>
>>>> >> > On Sunday 13 January 2013 at 7:03:29 am UTC+8 Kevin Gillette
>>>> wrote:
>>>> >> >>
>>>> >> >> Indeed. In addition to implicit dereferencing for value
>>>> receivers, the reverse also works as well: anything that is addressable
>>>> (including 'value' variables on the stack, or a field of element of
>>>> anything that's addressable) will implicitly be addressed when a
>>>> pointer-receiver method is called on them (though you must explicitly use
>>>> the address operator when you need to pass value variables as pointers).
>>>> >> >>
>>>> >> >> There is a consideration to make, though: historically it has
>>>> been considered bad form in Go to give a type a mix of value and pointer
>>>> receivers in methods without a very specific reason for doing so. The
>>>> typical justification is that a small struct in a getter method might as
>>>> well have a value receiver even though the corresponding setter method uses
>>>> a pointer receiver; this, however, can lead to confusion on the part of the
>>>> app programmer if they start out using only the read-only methods upon what
>>>> turns out to be a value-copy of the original (but hey, it compiled and
>>>> seems to work, so it must be correct) -- when use of pointer-receiver
>>>> methods don't seem to produce the documented changes in the original, it
>>>> can be difficult to debug.
>>>> >> >>
>>>> >> >>
>>>> >> >> On Saturday, January 12, 2013 3:17:16 PM UTC-7, Dave Collins
>>>> wrote:
>>>> >> >>>
>>>> >> >>> On Saturday, January 12, 2013 3:52:35 PM UTC-6, Taric Mirza
>>>> wrote:
>>>> >> >>>>
>>>> >> >>>> Thanks! Works like a charm and is helping cleaning up my code a
>>>> ton.
>>>> >> >>>>
>>>> >> >>>> One other question, this is really more about coding style:
>>>> >> >>>>
>>>> >> >>>> In the case where you manipulate members of the struct, then
>>>> using
>>>> >> >>>> pointers as in your example is the way to go.
>>>> >> >>>>
>>>> >> >>>> But, you have a choice for functions that just read values from
>>>> the
>>>> >> >>>> struct instead of manipulating it. Is there a best practice
>>>> coding
>>>> >> >>>> style here, between dereferencing the struct and then using
>>>> that, or
>>>> >> >>>> dereferencing each member of the struct as you go? eg:
>>>> >> >>>>
>>>> >> >>>> // A:
>>>> >> >>>>
>>>> >> >>>> laser := worldobj.(*Laser)
>>>> >> >>>> fmt.Printf("%0.4f,%0.4f", (*laser).x, (*laser).y)
>>>> >> >>>>
>>>> >> >>>> versus
>>>> >> >>>>
>>>> >> >>>> // B:
>>>> >> >>>>
>>>> >> >>>> laser := *(worldobj.(*Laser))
>>>> >> >>>> fmt.Printf("%0.4f,%0.4f", laser.x, laser.y)
>>>> >> >>>>
>>>> >> >>>>
>>>> >> >>>> I'm kind of torn. I would imagine A) has slightly better
>>>> >> >>>> performance, and doesn't require any code-rework if you later
>>>> on need
>>>> >> >>>> to manipulate the struct.
>>>> >> >>>>
>>>> >> >>>> On the other hand, B) is more readable since you don't have to
>>>> look at
>>>> >> >>>> pointers all over the place, just on one line.
>>>> >> >>>
>>>> >> >>>
>>>> >> >>> Actually, you don't need to dereference at all. Go automatically
>>>> handles this for you.
>>>> >> >>>
>>>> >> >>> See this example: http://play.golang.org/p/ANaKaFSQLn
>>>> >> >>>
>>>> >> > --
>>>> >> > 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/03df7dce-5c48-44a3-bc3c-851ded2a1f08n%40googlegroups.com.
>>>>
>>>> >>
>>>> >> --
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>>>> >> To unsubscribe from this group and stop receiving emails from it,
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>>>> >> To view this discussion on the web visit
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>>>>
>>>> >
>>>> > --
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>>>>
>>>>
>>>
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>>>
>>
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>>
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