Bug? taskPool.map() with bufSize and writeln() gets stuck

[Martin Nowak]

On Sat, 11 Feb 2012 02:31:29 +0100, Ali Çehreli <acehreli at yahoo.com> wrote:

> Sorry for the double-post; I have asked the same question on D.learn  
> earlier but I think this is more of a question to this forum.
>
> Tested on Ubuntu 11.10 64-bit dmd.
>
> The following program gets stuck during the writeln() call.
>
> - Note that the foo() call alone works fine.
>
> - Also note that the program works fine when there is no writeln() call  
> nor foo() call. All elements get processed in that case and the results  
> are ignored.
>
> Am I using taskPool.map incorrectly or is this a bug? Can you help  
> identify where the problem may be? How is writeln() using the range  
> differently than foo() to cause this behavior?
>
> import std.stdio;
> import std.parallelism;
> import core.thread;
>
> int func(int i)
> {
>      writeln("processing ", i);
>      return i;
> }
>
> void main()
> {
>      auto results = taskPool.map!func([1,2,3,4,5,6,7,8], 2);
>
>      writeln(results);  // <-- Gets stuck HERE
>
>      foo(results); // this works fine
> }
>
> void foo(R)(R range)
> {
>      for ( ; !range.empty; range.popFront()) {
>          writeln(range.front);
>      }
> }
>
> Thank you,
> Ali

Yeah, you have a deadlock in there, it's somewhat hidden though.
The issue is that writeln will take lock on stdout once. This will deadlock
with the lazy processing of the map range.