If you comment out the read method then all threads will block. That is the the behavior of an unbuffered channel - a writer blocks until a reader is ready. Which is why you always need a valid reader running. Unless the channel is closed and then the writer will panic.
The code I provided is valid. > On Aug 31, 2019, at 2:40 AM, T L <tapir....@gmail.com> wrote: > > > >> On Friday, August 30, 2019 at 1:40:33 PM UTC-4, Robert Engels wrote: >> You changed the Read() method incorrectly - it should be using the Read >> lock, not the Write lock. >> >> Still, as I pointed out when I posted it, Play has a problem where it aborts >> if all routines are sleeping (not just blocked), so you need to run it >> locally. > > My fault. But it doesn't matter, for the Read method is never called (I > commented it off). > It also crash locally for all goroutines are blocked. > >> -----Original Message----- >> From: T L >> Sent: Aug 30, 2019 12:05 PM >> To: golang-nuts >> Subject: Re: [go-nuts] An old problem: lack of priority select cases >> >> >> >>> On Friday, August 30, 2019 at 12:39:41 PM UTC-4, Robert Engels wrote: >>> >>> Makes no difference in the code I posted.... as long as they all use the >>> same MultiWriterChannel. In fact, others can be late started, as they will >>> fail fast if the channel is already closed. >> >> https://play.golang.org/p/pcwIu2w8ZRb >> >> All go routines are blocked in the modified version. >> >>> -----Original Message----- >>> From: T L >>> Sent: Aug 30, 2019 11:13 AM >>> To: golang-nuts >>> Subject: Re: [go-nuts] An old problem: lack of priority select cases >>> >>> >>> >>>> On Friday, August 30, 2019 at 10:35:29 AM UTC-4, Robert Engels wrote: >>>> I don't think so. Why do you think that is the case? The RWLock is "fair" >>>> in the sense that once the 'closer' attempts to get the lock, it is >>>> guaranteed to get it (as the code is structured) - the subsequent readers >>>> will queue behind the "writer = closer". >>> >>> How about unknown/random number of senders and readers? >>> >>>> >>>> -----Original Message----- >>>> From: T L >>>> Sent: Aug 30, 2019 8:50 AM >>>> To: golang-nuts >>>> Subject: Re: [go-nuts] An old problem: lack of priority select cases >>>> >>>> @Robert >>>> I think there is a difference between the code of @Leo and you. >>>> In you code, the Wirte/Read/Close are all possible to block for ever. >>>> >>>>> On Thursday, August 29, 2019 at 8:59:10 PM UTC-4, Robert Engels wrote: >>>>> >>>>> Oops. You are right. The original used two different methods Closed() and >>>>> Read() and when I refactored I forgot to add the Read lock to the Read(). >>>>> That's why you always have code reviews... >>>>> -----Original Message----- >>>>> From: T L >>>>> Sent: Aug 29, 2019 6:25 PM >>>>> To: golang-nuts >>>>> Subject: Re: [go-nuts] An old problem: lack of priority select cases >>>>> >>>>> >>>>> >>>>>> On Wednesday, August 28, 2019 at 10:05:06 PM UTC-4, robert engels wrote: >>>>>> Here is a version using RWLock https://play.golang.org/p/YOwuYFiqtlf >>>>> >>>>> Doesn't the Read method need to be guarded by the reader lock? >>>>> >>>>> >>>>>> >>>>>> It won’t run correctly in the playground because it terminates when all >>>>>> routines are asleep - which happens during the test (not sure why it >>>>>> does this, as sleeping is different than a deadlock). >>>>>> >>>>>> It is probably less efficient, and less orderly than the other example >>>>>> using WaitGroup but you get the idea I hope. It forcibly terminates the >>>>>> writers before they complete by design. >>>>>> >>>>>>> On Aug 28, 2019, at 4:09 PM, Michel Levieux <m.le...@capitaldata.fr> >>>>>>> wrote: >>>>>>> >>>>>>> One should also be careful regarding the conceptual demands he or she >>>>>>> is making. >>>>>>> Having a shared resource (that is complex enough that it cannot be >>>>>>> atomically accessed or modified) means essentially that "having >>>>>>> multiple writers being transparent to the readers", fundamentally, is >>>>>>> not possible. >>>>>>> >>>>>>> From the moment itself when such a resource is shared, there must be >>>>>>> some sort of mecanism (that one using resources atomically usable) that >>>>>>> ensures the integrity of it. >>>>>>> Maybe what you're talking about is having it transparent in terms of >>>>>>> code, in which case we both agree, but if you're looking for something >>>>>>> transparent in essence, as in performance, logical construction and all >>>>>>> the rest, I think there is a misunderstanding here: even if it was >>>>>>> added in the language, there would be many many things going on under >>>>>>> the hood, as it is already (and cannot really be otherwise) for channel >>>>>>> use alone. >>>>>>> >>>>>>> As for the priority using selects, I think it's more of something to be >>>>>>> dealt with on the "user-side". There are many kinds of priority in >>>>>>> general, and trying to implement something in the language itself would >>>>>>> IMO either be too specific compared to the nessecary time to do so or >>>>>>> it would probably have a huge overhead on the "classical' use case of >>>>>>> the select construct. >>>>>>> + the fact that it is apparently already possible using RWMutexes. >>>>>>> >>>>>>>> Le mer. 28 août 2019 à 22:37, Marcin Romaszewicz <mar...@gmail.com> a >>>>>>>> écrit : >>>>>>>> Think of a channel as existing for the lifetime of a particular data >>>>>>>> stream, and not have it be associated with either producer or >>>>>>>> consumer. Here's an example: >>>>>>>> >>>>>>>> https://play.golang.org/p/aEAXXtz2X1g >>>>>>>> >>>>>>>> The channel here is closed after all producers have exited, and all >>>>>>>> consumers continue to run until the channel is drained of data. >>>>>>>> >>>>>>>> The producers are managed by something somewhere in your code - and >>>>>>>> that is the scope at which it makes sense to create channel ownership. >>>>>>>> I've used a waitgroup to ensure that the channel is closed after all >>>>>>>> producers exit, but you can use whatever barrier construct you want. >>>>>>>> >>>>>>>> Even if you must have a channel per producer, you can safely close the >>>>>>>> producer side, without notifying the downstream about this. The >>>>>>>> example early in the thread uses multiple channels, with one channel >>>>>>>> being used to signal that the producers should exit. Channels aren't >>>>>>>> really the right model for this, you want a thread safe flag of some >>>>>>>> sort. For example: >>>>>>>> >>>>>>>> var exitFlag uint64 >>>>>>>> func producer(chan data int, wg *sync.WaitGroup) { >>>>>>>> defer wg.Done() >>>>>>>> for { >>>>>>>> shouldExit := atomic.LoadUint64(&exitFlag) >>>>>>>> if shouldExit == 1 { >>>>>>>> return >>>>>>>> } >>>>>>>> chan <- rand.Intn(100) >>>>>>>> } >>>>>>>> } >>>>>>>> >>>>>>>> Here's 10 producers and 3 consumers sharing a channel and closing it >>>>>>>> safely upon receiving an exit flag: >>>>>>>> https://play.golang.org/p/RiKi1PGVSvF >>>>>>>> >>>>>>>> -- Marcin >>>>>>>> >>>>>>>>> On Wed, Aug 28, 2019 at 11:29 AM Leo Lara <l...@leopoldolara.com> >>>>>>>>> wrote: >>>>>>>>> I do not think priority select is *necessary*, it could be a nice >>>>>>>>> addition if the performance does not change. >>>>>>>>> >>>>>>>>>> On Wednesday, August 28, 2019 at 8:27:36 PM UTC+2, Leo Lara wrote: >>>>>>>>>> Hi Robert, >>>>>>>>>> >>>>>>>>>> From the article: """To bound more the problem, in my case, you >>>>>>>>>> control the writers but not the readers""" >>>>>>>>>> >>>>>>>>>> So what I was trying to do was to be able to close, with mutiple >>>>>>>>>> writers, while being transparent for the readers. The readers only >>>>>>>>>> need to read as usual form the channel. >>>>>>>>>> >>>>>>>>>> For example, if you want to write a library where the user just >>>>>>>>>> reads from a channel, this is an approach I found where the user of >>>>>>>>>> the lirbary deos nto have to do anything special. Of course, there >>>>>>>>>> might be another solution, but if you need to modify the reader we >>>>>>>>>> are talking about a different problem. >>>>>>>>>> >>>>>>>>>> Cheers!! >>>>>>>>>> >>>>>>>>>>> On Wednesday, August 28, 2019 at 7:17:24 PM UTC+2, Robert Engels >>>>>>>>>>> wrote: >>>>>>>>>>> A better solution is to wrap the writes using a RWLock, grab the >>>>>>>>>>> read lock for writing, and the Write lock for closing. Pretty >>>>>>>>>>> simple. >>>>>>>>>>> >>>>>>>>>>> Just encapsulate it all in a MultiWriterChannel struct - generics >>>>>>>>>>> would help here :) >>>>>>>>>>> >>>>>>>>>>> -----Original Message----- >>>>>>>>>>> From: Leo Lara >>>>>>>>>>> Sent: Aug 28, 2019 11:24 AM >>>>>>>>>>> To: golang-nuts >>>>>>>>>>> Subject: [go-nuts] Re: An old problem: lack of priority select >>>>>>>>>>> cases >>>>>>>>>>> >>>>>>>>>>> This is connected with my article: >>>>>>>>>>> https://dev.to/leolara/closing-a-go-channel-written-by-several-goroutines-52j2 >>>>>>>>>>> >>>>>>>>>>> I think there I show it is possible to workaround that limitation >>>>>>>>>>> using standard Go tools. Of course, the code would be simple with >>>>>>>>>>> priority select, but also perhaps select would become less >>>>>>>>>>> efficient. >>>>>>>>>>> >>>>>>>>>>>> On Wednesday, August 28, 2019 at 6:06:33 PM UTC+2, T L wrote: >>>>>>>>>>>> The old thread: >>>>>>>>>>>> https://groups.google.com/forum/#!topic/golang-nuts/ZrVIhHCrR9o >>>>>>>>>>>> >>>>>>>>>>>> Go channels are flexible, but in practice, I often encountered >>>>>>>>>>>> some situations in which channel are hard to use. >>>>>>>>>>>> Given an example: >>>>>>>>>>>> >>>>>>>>>>>> import "math/rand" >>>>>>>>>>>> >>>>>>>>>>>> type Producer struct { >>>>>>>>>>>> data chan int >>>>>>>>>>>> closed chan struct{} >>>>>>>>>>>> } >>>>>>>>>>>> >>>>>>>>>>>> func NewProducer() *Producer { >>>>>>>>>>>> p := &Producer { >>>>>>>>>>>> data: make(chan int), >>>>>>>>>>>> closed: make(chan struct{}), >>>>>>>>>>>> } >>>>>>>>>>>> >>>>>>>>>>>> go p.run() >>>>>>>>>>>> >>>>>>>>>>>> return p >>>>>>>>>>>> } >>>>>>>>>>>> >>>>>>>>>>>> func (p *Produce) Stream() chan int { >>>>>>>>>>>> return p.data >>>>>>>>>>>> } >>>>>>>>>>>> >>>>>>>>>>>> func (p *Producer) run() { >>>>>>>>>>>> for { >>>>>>>>>>>> // If non-blocking cases are selected by their appearance >>>>>>>>>>>> order, >>>>>>>>>>>> // then the following slect block is a perfect use. >>>>>>>>>>>> select { >>>>>>>>>>>> case(0) <-p.closed: return >>>>>>>>>>>> case p.data <- rand.Int(): >>>>>>>>>>>> } >>>>>>>>>>>> } >>>>>>>>>>>> } >>>>>>>>>>>> >>>>>>>>>>>> func (p *Produce) Clsoe() { >>>>>>>>>>>> close(p.closed) >>>>>>>>>>>> close(p.data) >>>>>>>>>>>> } >>>>>>>>>>>> >>>>>>>>>>>> func main() { >>>>>>>>>>>> p := NewProducer() >>>>>>>>>>>> for n := p.Stream() { >>>>>>>>>>>> // use n ... >>>>>>>>>>>> } >>>>>>>>>>>> } >>>>>>>>>>>> >>>>>>>>>>>> >>>>>>>>>>>> If the first case in the select block in the above example has a >>>>>>>>>>>> higher priority than the second one, >>>>>>>>>>>> then coding will be much happier for the use cases like the above >>>>>>>>>>>> one. >>>>>>>>>>>> >>>>>>>>>>>> In short, the above use case requires: >>>>>>>>>>>> * for receivers, data streaming end is notified by the close of a >>>>>>>>>>>> channel. >>>>>>>>>>>> * for senders, data will never be sent to closed channel. >>>>>>>>>>>> >>>>>>>>>>>> But, as Go 1 doesn't support priority select cases, it is much >>>>>>>>>>>> tedious to implement the code >>>>>>>>>>>> satisfying the above listed requirements. The final implementation >>>>>>>>>>>> is often very ugly and inefficient. >>>>>>>>>>>> >>>>>>>>>>>> Does anyone else also experience the pain? >>>>>>>>>>> >>>>>>>>>>> >>>>>>>>>>> -- >>>>>>>>>>> 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 golan...@googlegroups.com. >>>>>>>>>>> To view this discussion on the web visit >>>>>>>>>>> https://groups.google.com/d/msgid/golang-nuts/b284f880-034a-4721-8686-ef48d3e2c14c%40googlegroups.com. >>>>>>>>>>> >>>>>>>>>>> >>>>>>>>>>> >>>>>>>>> >>>>>>>>> >>>>>>>>> -- >>>>>>>>> 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 golan...@googlegroups.com. >>>>>>>>> To view this discussion on the web visit >>>>>>>>> https://groups.google.com/d/msgid/golang-nuts/aeb38a0a-8268-42d7-a8eb-ce5ef01c5380%40googlegroups.com. >>>>>>>> >>>>>>>> >>>>>>>> -- >>>>>>>> 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 golan...@googlegroups.com. >>>>>>>> To view this discussion on the web visit >>>>>>>> https://groups.google.com/d/msgid/golang-nuts/CA%2Bv29LvcUhUvrZb_8AKYWj0A%2Bqd5LKBPmbz-RVBb%3DJn_gNZE6w%40mail.gmail.com. >>>>>>> >>>>>>> >>>>>>> -- >>>>>>> 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 golan...@googlegroups.com. >>>>>>> To view this discussion on the web visit >>>>>>> https://groups.google.com/d/msgid/golang-nuts/CANgi337s1Low95QvqJUAOTsqcVji7uMQ_jr%3DFftpt2uMz5_XSQ%40mail.gmail.com. >>>>>> >>>>> >>>>> -- >>>>> 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 golan...@googlegroups.com. >>>>> To view this discussion on the web visit >>>>> https://groups.google.com/d/msgid/golang-nuts/a5cff3f8-cc1c-4719-9f2f-7b9c31086f6a%40googlegroups.com. >>>>> >>>>> >>>>> >>>> >>>> -- >>>> 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 golan...@googlegroups.com. >>>> To view this discussion on the web visit >>>> https://groups.google.com/d/msgid/golang-nuts/4bf95bb2-33ed-46ed-9436-48df1072914f%40googlegroups.com. >>>> >>>> >>>> >>> >>> -- >>> 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 golan...@googlegroups.com. >>> To view this discussion on the web visit >>> https://groups.google.com/d/msgid/golang-nuts/d155dcf6-7c01-4f7e-b408-eef9903cd837%40googlegroups.com. >>> >>> >>> >> >> -- >> 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 golan...@googlegroups.com. >> To view this discussion on the web visit >> https://groups.google.com/d/msgid/golang-nuts/74653e49-f374-4ac8-998e-fd874cdf6bd4%40googlegroups.com. >> >> >> > > -- > 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/3d75a129-efee-402f-aafa-9fe76af4e789%40googlegroups.com. -- You received this message because you are subscribed to the Google Groups "golang-nuts" group. 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