Re: Re: How to notice abrupt tcp connection losses in server/client?

[Zelphir Kaltstahl]

On 22.06.2018 18:00, address@hidden wrote:
> Message: 2
> Date: Fri, 22 Jun 2018 00:09:16 +0100
> From: Chris Vine <address@hidden>
> To: address@hidden
> Subject: Re: How to notice abrupt tcp connection losses in
>       server/client?
> Message-ID: <address@hidden>
> Content-Type: text/plain; charset=UTF-8
>
> On Fri, 22 Jun 2018 00:19:49 +0200
> Zelphir Kaltstahl <address@hidden> wrote:
>> On 21.06.2018 18:00, address@hidden wrote:
>>> From: Chris Vine <address@hidden>
> [snip]
>>> The POSIX recv() function returns 0 on end of file (connection closed)
>>> so I expect the scheme recv! procedure does the same.  So on end-of-file
>>> your code appears to be producing an endless supply of empty strings.
>> I actually tried to (break) the loop when the received message is
>> (eof-object? ...), but it might be that I used this predicate at the
>> wrong place (directly inside the loop when the message was already
>> copied from the receive-buffer, instead of when recv!, I believe), now
>> that I think about it ? and maybe that is why (break)ing the loop did
>> not work. Need to investigate more.
> It looks as if recv! returns 0 on end-of-file, not an eof-object, so
> that may be your problem.  That is not the case with get-line or
> get-bytevector-n.  Note that recv! may also return less than the
> requested number of bytes - it does not guarantee to block until the
> entire request has been met or end-of-file is reached, as it is just a
> wrapper for POSIX/windows recv().
I think you are exactly right. If it returns 0 for end of file (what is
the difference between this and end of file object?), then 0 bytes will
be copied to the receive byte vector. When converting that to utf8
string, it will be the empty string and that is printed. However, since
there is still no new message (client connection is no more) there is
still only EOF and the same will happen over and over again.

> I think you will need to use recv! with windows, because you cannot
> read from sockets in windows using POSIX read().  With other OS's it
> would be more normal to use the R5RS/R6RS port procedures, because they
> are easier to use and are buffered.
>
> For asynchronous non-blocking ports, I am not sure whether recv! is safe
> with guile-2.2's suspendable ports or not.  The get-line and
> get-bytevector-n procedures are.  The get-bytevector-n! procedure is
> not.  I suspect recv! may not be also.
>
>>> More generally, you will find it easier to use something like the
>>> get-line procedure to read text, or something like get-bytevector-n or
>>> get-bytevector-n! for reading binary records.  recv! only really becomes
>>> important when the flags argument is meaningful.
>> Ah ok, thanks for the advice! I thought recv! was the way to go, as it
>> worked so nicely so far.
> A call to unix read() on a socket is equivalent to a call to recv()
> with default flags.  At the end of it, the buffered guile port reading
> procedures generally involve a call to read().
>
>>> Rather than starting a new thread for each connection, you will get much
>>> better scalability if you use something like fibers
>>> ( https://github.com/wingo/fibers/ ), guile-a-sync2
>>> ( https://github.com/ChrisVine/guile-a-sync2/ ) or 8sync
>>> ( https://www.gnu.org/software/8sync/ ).
>> Yep =) I am aware. I already wrote a comment inside the code noting that
>> this new thread thing is probably not so good. Not sure I should
>> introduce fibers into this example code yet though. And looking at
>> fibers and 8sync is on my to-do list ; )
>>> Chris
>> I will try these things and see how it goes. Thanks!
> The guile-a-sync/guile-a-sync2 libraries (of which I am the author) have
> an example of a client and server in the doc directory, which might be
> of interest to you.  fibers is definitely worth a look also, but is
> linux only, and also guile-2.2 only.  I think it also has some examples
> (not sure).
>
> Chris

I now use get-bytevector-n with the count which was the size of the
receive-buffer and it does not have the endless looping behavior. All
seems fine now. Thanks!