From: Robert Klemme <shortcutter@...>
Date: 2009-03-16T07:17:39+09:00
Subject: Re: does IO.read block?

On 15.03.2009 23:02, Michael Malone wrote:
> I have a setup where I am writing to a pipe in one process and reading 
> in another.  I am closing the end I'm not using etc, but I have a new 
> problem.  Having recently read that I was just getting lucky when it 
> came to my IO.write completing each time (probably due to Ruby's green 
> thread implementation in v1.8 and 'nice' scheduling) but now that the OS 
> has taken over scheduling, it tends to interrupt things when it damn 
> well feels like it, so I believe some of my write calls aren't 
> completing, so I've set up a loop like this:
> 
> rescue Exception => error
>   ex_string = Marshal.dump(error)
>   ex_size = ex_string.bytesize
>   bytes_written = 0
>   while bytes_written < ex_size
>     bytes_written += write_end.write(ex_string.slice!(bytes_written))
>   end
>   ensure
>     write_end.close

You do not seem to open the stream in this context, why do you close it 
here?  Or do you have a pattern like

write_end = ... # open
begin
   ...
rescue
   ...
ensure
   write_end.close
end

>   end
> end

IMHO you can simplify that do

rescue Exception => error
   Marshal.dump(error, write_end)
end

Which should also be more efficient since the looping is done in C code.

> and in the other process, I currently make just one call to 
> read_end.read() so my question is, is this guaranteed to get all of the 
> bytes written through multiple calls to write or do I need to set up a 
> similar loop on the read end?  like
> 
> while !(read_end.eof?)
>     string += read_end.read
> end
> 
> Or does read_end.read block until it finds EOF?

Yes, that's what it does:
http://www.ruby-doc.org/core/classes/IO.html#M002295

But again, I would just do

obj = Marshal.read(read_end)

Which, again is more efficient since you do not need the additional 
buffer and looping is done in C code.

> Can anyone tell me what happens when read_end.read is re-scheduled 
> partway through?  Or is that guaranteed to finish?

I am not sure what you mean by "rescheduled".  Even if the OS preempts 
execution of this process or thread it will continue at the same 
location of the stream and semantics of the methods do not change.

Kind regards

	robert