[Rd] Question on non-blocking socket

[Ben Engbers]

Hi

Op 15-02-2023 om 14:38 schreef Tomas Kalibera:
> On 2/15/23 01:24, Ben Engbers wrote:
>> Hi,
>>
>> December 27, 2021 I started a thread asking for help troubleshooting 
>> non-blocking sockets.
..

>> I have two questions.
>> The first is where I can find R documentation on proper use of 
>> non-blocking sockets and on the proper use of the socketSelect function?
> 
> In addition to the demos I sent to you in that 2021 thread on 
> R-pkg-devel, you could also have a look at how it is used in R itself, 
> in the parallel package, in snowSOCK.R, to set up the snow cluster in 
> parallel. Some hints may be also found in the blog post 
> https://blog.r-project.org/2020/03/17/socket-connections-update/. But, 
> in principle, R API is just a thin layer on top of what the OS provides, 
> so general literature and tutorials on sockets should help, there should 
> be even textbooks used at CS universities in networking classes.
Thanks for the suggestions!

> Basically select() can tell you when data is ready (on input), when the 
> socket interface is able to accept more data (on output) or when there 
> is an incoming connection. In practice, you should not need any delays 
> to be inserted in your program to make it work - if that is needed, it 
> means that is an error in it (a race condition). If the program is 
> polling (checking in a loop whether something has already happened, and 
> then sleeping for a short while), the duration of the sleep may indeed 
> influence latency, but should not affect correctness - if it does, there 
> is an error.

In RBaseX I first calculate an MD5 hash that is send to the server and 
then I check the status byte that is returned by the server.

writeBin(auth, private$conn)
socketSelect(list(conn))
Accepted <- readBin(conn, what = "raw", n = 1) == 0

Without the second line, 'Accepted' is always FALSE. With this line it 
is TRUE.

BaseX provides example API's in several languages. I've looked at 
several but indeed none uses any form of delay.
All API's follow the same pattern, calculate a MD5, send it to the 
server and check the status byte. So the server is not likely to enforce 
a delay. So there is nothing left but to look for that racing condition ;-(

Ben