Source

SCons / src / scons / Job.py

"""scons.Job

This module defines the Serial and Parallel classes that execute tasks to
complete a build.

"""

__revision__ = "Job.py __REVISION__ __DATE__ __DEVELOPER__"

class Serial:
    """This class is used to execute tasks in series, and is more efficient
    than Parallel, but is only appropriate for non-parallel builds. Only
    one instance of this class should be in existence at a time.

    This class is not thread safe.
    """

    def __init__(self, taskmaster):
        """Create a new serial job given a taskmaster. 

        The taskmaster's next_task() method should return the next task that
        needs to be executed, or None if there are no more tasks. The
        taskmaster's executed() method will be called for each task when it is
        finished being executed. The taskmaster's is_blocked() method will not
        be called.
        """
        
        self.taskmaster = taskmaster

    def start(self):
        
        """Start the job. This will begin pulling tasks from the taskmaster
        and executing them, and return when there are no more tasks.  """
        
        while 1:
            task = self.taskmaster.next_task()

            if task is None:
                break
 
            task.execute()
            self.taskmaster.executed(task)

    def stop(self):
        """Serial jobs are always finished when start() returns, so there
        is nothing to do here"""
        
        pass

    def wait(self):
        """Serial jobs are always finished when start() returns, so there
        is nothing to do here"""
        pass


# The will hold a condition variable once the first parallel task
# is created.
cv = None

class Parallel:
    """This class is used to execute tasks in parallel, and is less
    efficient than Serial, but is appropriate for parallel builds. Create
    an instance of this class for each job or thread you want.

    This class is thread safe.
    """


    def __init__(self, taskmaster):

        """Create a new parallel job given a taskmaster. Multiple jobs will
        be using the taskmaster in parallel, but all method calls to taskmaster
        methods are serialized by the jobs themselves.

        The taskmaster's next_task() method should return the next task
        that needs to be executed, or None if there are no more tasks. The
        taskmaster's executed() method will be called for each task when it
        is finished being executed. The taskmaster's is_blocked() method
        should return true iff there are more tasks, but they can't be
        executed until one or more other tasks have been
        executed. next_task() will be called iff is_blocked() returned
        false.

        Note: calls to taskmaster are serialized, but calls to execute() on
        distinct tasks are not serialized, because that is the whole point
        of parallel jobs: they can execute multiple tasks
        simultaneously. """

        global cv
        
        # import threading here so that everything in the Job module
        # but the Parallel class will work if the interpreter doesn't
        # support threads
        import threading
        
        self.taskmaster = taskmaster
        self.thread = threading.Thread(None, self.__run)
        self.stop_running = 0

        if cv is None:
            cv = threading.Condition()

    def start(self):
        """Start the job. This will spawn a thread that will begin pulling
        tasks from the task master and executing them. This method returns
        immediately and doesn't wait for the jobs to be executed.

        To stop the job, call stop().
        To wait for the job to finish, call wait().
        """
        self.thread.start()

    def stop(self):
        """Stop the job. This will cause the job to finish after the
        currently executing task is done. A job that has been stopped can
        not be restarted.

        To wait for the job to finish, call wait().
        """
        self.stop_running = 1

    def wait(self):
        """Wait for the job to finish. A job is finished when either there
        are no more tasks or the job has been stopped and it is no longer
        executing a task.

        This method should only be called after start() has been called.

        To stop the job, call stop().
        """
        self.thread.join()

    def __run(self):
        """private method that actually executes the tasks"""

        cv.acquire()

        try:

            while 1:
                while self.taskmaster.is_blocked():
                    cv.wait(None)

                task = self.taskmaster.next_task()

                if task == None or self.stop_running:
                    break

                cv.release()
                task.execute()
                cv.acquire()

                self.taskmaster.executed(task)

                if not self.taskmaster.is_blocked():
                    cv.notifyAll()

        finally:
            cv.release()