=begin pod

=TITLE class Semaphore

=SUBTITLE Control access to shared resources by multiple processes

  class Semaphore { }

Protect your shared code, data or device access using semaphores. An example is
a printer manager managing a pool of printers without the need of storing print
jobs when all printers are occupied. The next job is just blocked until a
printer becomes available.

  class print-manager {
    has Array $!printers;
    has Semaphore $!print-control;

    method BUILD( Int:D :$nbr-printers ) {
      for ^$nbr-printers -> $pc {
        $!printers[$pc] = { :name{"printer-$pc"} };
      }

      $!print-control .= new($nbr-printers);
    }

    method find-available-printer-and-print-it($job) { say "Is printed!"; }

    method print( $print-job ) {
      $!print-control.acquire;

      self.find-available-printer-and-print-it($print-job);

      $!print-control.release;
    }
  }

Another example is a protection around code updating sensitive data. In such a
case the semaphore is typically initialized to 1.

It is important to have a release on every exit of your program! While this is
obvious, it is easy to fall in traps such as throwing an exception caused by some
event. When the program dies there is no problem. When the exception is caught
your program might eventually come back to the acquire method and will hang
indefinitely.

=head1 Methods

=head2 method new

  method new( int $permits )

Initialize the semaphore with the number of permitted accesses. E.g. when set to
2, program threads can pass the acquire method twice until it blocks on the
third time acquire is called.

=head2 method acquire

  method acquire()

Acquire access. When other threads have called the method before and the the
number of permits are used up, the process blocks until threads passed before
releases the semaphore.

=head2 method try_acquire

  method try_acquire(--> Bool)

Same as acquire but will not block. Instead it returns C<True> if access is
permitted or C<False> otherwise.

=head2 method release

  method release()

Release the semaphore raising the number of permissions. Any blocked thread will
get access after that.

=end pod
