Condition Mutexes

[Graham St Jack]

On Wed, 21 Oct 2009 00:56:13 +0000, dsimcha wrote:

> I'm messing around w/ core.sync.  Does anyone know what I'm doing wrong
> in this program?  It just hangs. If I could figure out ()##$) condition
> mutexes (right now, I'm using busy spinning), I might have a decent
> implementation of parallelForeach over ranges.
> 
> import core.sync.mutex, core.sync.condition, core.thread, std.stdio;
> 
> __gshared Condition condition;
> 
> void waitThenPrint() {
>     condition.wait();
>     writeln("FOO");
> }
> 
> void main() {
>     condition = new Condition( new Mutex() ); auto T = new
>     Thread(&waitThenPrint);
>     T.start();
>     condition.notify();  // Never wakes up and prints FOO.
> }

There are a few problems. The most serious is that you have to lock the 
mutex before calling condition.wait(). The underlying operating-system 
stuff atomically 

This means that the mutex needs to be an attribute of the class, and 
waitThenPrint() should be more like this:

void waitThenPrint() {
  synchronized(myMutex) {
    condition.wait();
  }
  writeln("FOO");
}

While it isn't strictly necessary in this case, you should also:

Put the condition.notify() call into a synchronized(myMutex) block.

When some state variables are involved in the condition, you should do 
something like this:

void waitThenPrint() {
  synchronized(myMutex) {
    while (state_not_right()) {
      condition.wait();
    }
  }
  writeln("FOO");
}

and

synchronized(myMutex) {
  set_state_to_right();
  condition.notify();
}