The reason it checks for known implementations is because it “knows the implementation”, so it can be sure it has the correct semantics.
This is the problem with Go “duck typing”. The real solution would be to declare a new interface called BodyContent, that had methods of io.Reader, and a method ContentLength() -which could be delegated to Len() - not re-use a generic method like Len(). but even then that is not foolproof because of the “duck typing”. I know most do not agree, and see the Go “duck typing” as a plus, but IMO it is a huge problem for larger code sets with proper maintainable designs. > On Feb 6, 2019, at 5:05 PM, Burak Serdar <bser...@ieee.org> wrote: > > On Wed, Feb 6, 2019 at 2:50 PM robert engels <reng...@ix.netcom.com > <mailto:reng...@ix.netcom.com>> wrote: >> >> I am not sure what that has to do with the discussion. My point was that for >> it to be applicable here, it needs to be defined as part of io.Reader, since >> that is what Body is declared as. It is not, so using in the manner outlined >> is not correct IMO. > > Len() cannot be part of Reader. Not all readers can support it. Len() > can be a part of the implementation that can be set as Body. > Currently, the http implementation is checking for known Reader > implementations to figure out the length of data. That's a hack. > Checking if the reader supports Len() would fix that without any API > changes. > > > >> >>> On Feb 6, 2019, at 3:37 PM, Dan Kortschak <d...@kortschak.io> wrote: >>> >>> The generalised semantics of Len are that it returns the number of >>> available elements in the collection, being a cognate of the len built- >>> in. This means that as you consume elements from a buffer, the Len >>> value reduces. This is directly equivalent to >>> >>> for len(buf) != 0 { >>> println(buf[0]) >>> buf = buf[1:] >>> } >>> >>> On Wed, 2019-02-06 at 08:56 -0600, Robert Engels wrote: >>>> But is it really? If you read the description for Len() on >>>> bytes.Buffer it is the length of unread portion. But that doesn’t >>>> mean the buffer isn’t just a portion of the entire body - it can be a >>>> chunk which is continually reloaded. >>>> >>>> This is the danger in using private APIs publically based upon the >>>> existence of a method - it leads to very brittle code - and there are >>>> almost certainly better ways to design it to avoid these issues. If >>>> the core api is not expressive enough then it will be more >>>> difficult. >>>> >>>>> >>>>> On Feb 6, 2019, at 8:30 AM, Burak Serdar <bser...@ieee.org> wrote: >>>>> >>>>>> >>>>>> On Wed, Feb 6, 2019 at 5:15 AM Robert Engels <rengels@ix.netcom.c >>>>>> om> wrote: >>>>>> >>>>>> I see now, but if that can be the case, shouldn’t the Body be >>>>>> documented that the Reader may be a ReaderWithLen, and the >>>>>> consumer is free to type check/cast? If not, you are using >>>>>> internal details that you should not be. >>>>> Yes, the documentation should say if the reader has a Len() method >>>>> it >>>>> would be used to set the ContentLength. Len is no longer an >>>>> internal >>>>> detail then. >>>>> >>>>>> >>>>>> >>>>>> This is a problem with Go in general. Because the returned object >>>>>> “implements” some interface because it happens to have the >>>>>> required method, doesn’t mean it was designed to be used that >>>>>> way, or that it has the required semantics - unless documented to >>>>>> have them. >>>>> I agree with you there. Len() is straight forward, but in general >>>>> just >>>>> because a function is named something doesn't mean it'll do the >>>>> same >>>>> thing for all implementations. On the other end of the spectrum is >>>>> Java-like interfaces where you want explicit inheritance of a >>>>> specific >>>>> interface. I don't know if there's anything in between, but I like >>>>> Go's approach much better. >>>>> >>>>>> >>>>>> >>>>>> On Feb 6, 2019, at 2:22 AM, Matteo Biagetti <matteo.biagetti@gmai >>>>>> l.com> wrote: >>>>>> >>>>>> Make sense, thanks for explanation >>>>>> >>>>>> >>>>>> >>>>>> Il giorno mercoledì 6 febbraio 2019 07:28:54 UTC+1, Burak Serdar >>>>>> ha scritto: >>>>>>> >>>>>>> >>>>>>> On Tue, Feb 5, 2019 at 8:13 PM robert engels <ren...@ix.netcom. >>>>>>> com> wrote: >>>>>>>> >>>>>>>> >>>>>>>> That’s what I was trying to point out. Your design is not >>>>>>>> correct. The Body is a Reader, not a Buffer - the length of >>>>>>>> the request/body may be indeterminate - that is, a stream. >>>>>>>> Attempting to get the length of an underlying buffer is not >>>>>>>> only probably not possible, but not correct in many >>>>>>>> situations. >>>>>>> The length of the body *may* be indeterminate, and if that's >>>>>>> the case, >>>>>>> the underlying Reader will not have a Len method. The design is >>>>>>> to >>>>>>> handle the case where the underlying Reader is a Buffer with a >>>>>>> Len >>>>>>> method. If the Reader has Len, then the NopCloser derived from >>>>>>> that >>>>>>> will also have a Len, and NewRequest can set the content >>>>>>> length. If >>>>>>> the Reader does not have Len, then the content length is >>>>>>> unknown. >>>>>>> >>>>>>>> >>>>>>>> >>>>>>>> There is a reason the Body is a ReaderCloser and not a >>>>>>>> buffer. It is part of the http specification. >>>>>>>> >>>>>>>> On Feb 5, 2019, at 9:00 PM, Burak Serdar <bse...@ieee.org> >>>>>>>> wrote: >>>>>>>> >>>>>>>> On Tue, Feb 5, 2019 at 7:00 PM Robert Engels <ren...@ix.netco >>>>>>>> m.com> wrote: >>>>>>>> >>>>>>>> >>>>>>>> Shouldn’t you just be taking the content length from the >>>>>>>> header if forwarding the same body. There is no need for the >>>>>>>> length of the body. >>>>>>>> >>>>>>>> >>>>>>>> True. What I was suggesting is a fix for the general case. >>>>>>>> >>>>>>>> >>>>>>>> On Feb 5, 2019, at 6:53 PM, Burak Serdar <bse...@ieee.org> >>>>>>>> wrote: >>>>>>>> >>>>>>>> On Tue, Feb 5, 2019 at 5:18 PM Dan Kortschak <d...@kortschak. >>>>>>>> io> wrote: >>>>>>>> >>>>>>>> Personally, I think this is a bug in the behaviour of >>>>>>>> NewRequest. See h >>>>>>>> ttps://github.com/golang/go/issues/18117 for some additional >>>>>>>> context. >>>>>>>> >>>>>>>> >>>>>>>> Agreed. One solution could be to have: >>>>>>>> >>>>>>>> type HasLen interface { >>>>>>>> int Len() >>>>>>>> } >>>>>>>> >>>>>>>> Then have NopCloser return a nopCloser with len if the >>>>>>>> underlying >>>>>>>> implementation has len, with the obvious changes to >>>>>>>> NewRequest.Ugly, >>>>>>>> but can be done without API changes. >>>>>>>> >>>>>>>> >>>>>>>> >>>>>>>> >>>>>>>> On Tue, 2019-02-05 at 05:18 -0800, matteo....@gmail.com >>>>>>>> wrote: >>>>>>>> I've the following situation: >>>>>>>> I proxy a request to another server and when I made a POST >>>>>>>> and create >>>>>>>> a new >>>>>>>> request, the contentLength is zero: >>>>>>>> >>>>>>>> req2, _ := http.NewRequest(req.Method, newApiUrl , >>>>>>>> req.Body) >>>>>>>> fmt.Println("New request from body:", >>>>>>>> req2.ContentLength) // >>>>>>>> print 0 >>>>>>>> >>>>>>>> Checking in the source code of the NewRequest func Body don't >>>>>>>> respect >>>>>>>> some >>>>>>>> interface and populate the ContentLength field. >>>>>>>> >>>>>>>> Could be a bug? Which could be a valid approach in order to >>>>>>>> create a >>>>>>>> new >>>>>>>> request from an existing one and correct set the Body length? >>>>>>>> >>>>>>>> A working example here: >>>>>>>> >>>>>>>> https://play.golang.org/p/SvCDLj0NrXb >>>>>>>> >>>>>>>> Thanks! >>>>>>>> >>>>>>>> >>>>>>>> -- >>>>>>>> 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. >>>>>>>> For more options, visit https://groups.google.com/d/optout. >>>>>>>> >>>>>>>> >>>>>>>> -- >>>>>>>> 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. >>>>>>>> For more options, visit https://groups.google.com/d/optout. >>>>>>>> >>>>>>>> >>>>>>>> -- >>>>>>>> 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. >>>>>>>> For more options, visit https://groups.google.com/d/optout. >>>>>>>> >>>>>>>> >>>>>>>> -- >>>>>>>> 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. >>>>>>>> For more options, visit https://groups.google.com/d/optout. >>>>>>>> >>>>>>>> >>>>>> -- >>>>>> 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. >>>>>> For more options, visit https://groups.google.com/d/optout. >>>>>> >>>>>> -- >>>>>> 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. >>>>>> For more options, visit https://groups.google.com/d/optout. >> >> -- >> 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 >> <mailto:golang-nuts+unsubscr...@googlegroups.com>. >> For more options, visit https://groups.google.com/d/optout >> <https://groups.google.com/d/optout>. > > -- > 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 > <mailto:golang-nuts+unsubscr...@googlegroups.com>. > For more options, visit https://groups.google.com/d/optout > <https://groups.google.com/d/optout>. -- 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. For more options, visit https://groups.google.com/d/optout.
