std.concurrency and immutable

[Antoche]

On Monday, 25 November 2013 at 11:48:06 UTC, Shammah Chancellor 
wrote:
> On 2013-11-25 06:03:27 +0000, Antoche said:
>
>> The following code compiles but doesn't work as expected:
>> 
>>      import std.stdio;
>>      import std.concurrency;
>> 
>>      class A
>>      {
>>          this() immutable {}
>>      }
>> 
>>      void main()
>>      {
>>          auto tid = spawn( &fooBar, thisTid );
>>          while(true)
>>          {
>>              receive(
>>                  (Variant any) {
>>                      writeln( "Received a variant" );
>>                      writeln( "Received ", any );
>>                  }
>>              );
>>          }
>>      }
>> 
>>      void fooBar( Tid masterTid )
>>      {
>>          scope(failure) writeln( "fooBar failed" );
>>          scope(success) writeln( "fooBar succeeded" );
>>          scope(exit) writeln( "fooBar exiting" );
>>          try
>>          {
>>              immutable A b = new immutable A();
>>              masterTid.send( 42 ); // This works
>>              masterTid.send( b );  // This doesn't
>>          }
>>          catch( Exception e )
>>          {
>>              writeln( "Exception received" );
>>          }
>>      }
>> 
>> 
>> I see this in the console:
>> 
>>      fooBar exiting
>>      fooBar failed
>>      Received a variant
>>      Received 42
>> 
>> (then it just hangs)
>> 
>> I'm especially puzzled by:
>>   * Sending an int as a message works but not an immutable 
>> object.
>> Wasn't this  (safely sharing objects across threads) one of the
>> basic use cases for the immutable type qualifier?
>>   * scope(failure) failed but my exception handler didn't catch
>> anything. How is this possible? What could cause that?
>> Assertions/abort?
>> 
>> There seemed to be a 3-year-old ticket on this issue
>> (http://d.puremagic.com/issues/show_bug.cgi?id=5538) with very
>> little activity, which is a bit surprising given how much
>> emphasis is given to this feature (the D homepage mentions "D
>> offers an innovative approach to concurrency, featuring true
>> immutable data, message passing, no sharing by default, and
>> controlled mutable sharing across threads", and TDPL devotes a
>> whole chapter on it). This thread
>> (http://forum.dlang.org/thread/kgk8hc$12fa$1@digitalmars.com)
>> also says std.concurrency is "very buggy".
>> 
>> If I can't use std.concurrency, is there any other safe
>> alternative for multithreaded programming with D?
>> 
>> Thanks,
>> A.
>
>
> There is a bug with the internals of send/receive.   It has to 
> be able to copy the reference to a another place while sending 
> --  and it seems it can't do that because it's immutable.     I 
> don't think many people are using it.    I personally did not 
> realize the bug was from 2011.  I think DMD has been fixed 
> enough to make a patch to phobos now.  I'm going to ping this 
> bug report.
>
> However,  even CONSTRUCTING immutable objects right now is very 
> difficult.   It's arguably "correct" but, seems impossible to 
> actually be able to make something of use without many many 
> idups.  It's kind of in the same boat as shared.
>
> What I am currently doing, is casting to shared, and then back.
>  I tried making my classes shared, and also immutable as you 
> have.   Nothing else seems to work:
>
>> class A
>>      {
>>          this() {}
>>      }
>> 
>>      void main()
>>      {
>>          auto tid = spawn( &fooBar, thisTid );
>>          while(true)
>>          {
>>              receive(
> 		   (shared A _m)
> 	     	   {
> 			auto m = cast(A)_m;
> 			//Do stuff.
> 		   },
>>                  (Variant any) {
>>                      writeln( "Received a variant" );
>>                      writeln( "Received ", any );
>>                  }
>>              );
>>          }
>>      }
>> 
>>      void fooBar( Tid masterTid )
>>      {
>>          scope(failure) writeln( "fooBar failed" );
>>          scope(success) writeln( "fooBar succeeded" );
>>          scope(exit) writeln( "fooBar exiting" );
>>          try
>>          {
>>              immutable A b = new A();
>>              masterTid.send( 42 ); // This works
>>              masterTid.send( cast(shared) b );  // Should work
>>          }
>>          catch( Exception e )
>>          {
>>              writeln( "Exception received" );
>>          }
>>      }
>
> -Shammah

Thanks for the suggestion. My problem with casting is that it 
results in undefined behaviour, so I have no guarantee whatsoever 
what the program is actually going to do. I could simply not use 
immutable at all, and use shared instead, but then I might as 
well stick to C++ and volatile. The whole point of using D was 
trying to write safer code with equivalent performance.

I see std.parallelism seems to offer some features that might 
compensate for the broken std.concurrency. Is it working better 
or is it as broken as std.concurrency? Is there any other way to 
write safe multithreaded programs?

Finally, can anyone explained why the scope(failure) is reached? 
Looking at the stacktrace in gdb at this point gives me no 
indication of what's happening.