Add a new task semaphore system

playground/task_semaphore/wscript

@@ -0,0 +1,44 @@
+#! /usr/bin/env python
+
+"""
+Task semaphore demo. Compare the runtimes:
+
+	waf configure build --fast  # 0m08
+	waf configure build         # 1m15
+"""
+
+import random, time
+from waflib import Task, TaskGen, Utils
+
+def options(opt):
+	opt.add_option('--fast', action='store_true', default=False, help='Disable the semaphore to compare the runtime', dest='fast')
+
+def configure(conf):
+	pass
+
+def build(bld):
+	# max 20 jobs globally
+	bld.jobs = 20
+
+	bld(features='make_100')
+
+	class Foo(Task.Task):
+		always_run = True
+
+		if not bld.options.fast:
+			semaphore = Task.TaskSemaphore(2) # 2 jobs maximum
+
+		def uid(self):
+			# unique id for each object
+			return Utils.h_list(self.num)
+
+		def run(self):
+			time.sleep(random.randint(1000, 2000) / 1000.)
+			print("Task %r" % self.num)
+
+	@TaskGen.feature('make_100')
+	def make_100_bound_tasks(self):
+		for x in range(100):
+			tsk = self.create_task('Foo')
+			tsk.num = x
+

waflib/Runner.py

@@ -329,6 +329,14 @@ class Parallel(object):
 					try_unfreeze(x)
 			del self.revdeps[tsk]
 
+		if hasattr(tsk, 'semaphore'):
+			sem = tsk.semaphore
+			sem.release(tsk)
+			while sem.waiting and not sem.is_locked():
+				# take a frozen task, make it ready to run
+				x = sem.waiting.pop()
+				self._add_task(x)
+
 	def get_out(self):
 		"""
 		Waits for a Task that task consumers add to :py:attr:`waflib.Runner.Parallel.out` after execution.
@@ -352,8 +360,29 @@ class Parallel(object):
 		:param tsk: task instance
 		:type tsk: :py:attr:`waflib.Task.Task`
 		"""
+		# TODO change in waf 2.1
 		self.ready.put(tsk)
 
+	def _add_task(self, tsk):
+		if hasattr(tsk, 'semaphore'):
+			sem = tsk.semaphore
+			try:
+				sem.acquire(tsk)
+			except IndexError:
+				sem.waiting.add(tsk)
+				return
+
+		self.count += 1
+		self.processed += 1
+		if self.numjobs == 1:
+			tsk.log_display(tsk.generator.bld)
+			try:
+				self.process_task(tsk)
+			finally:
+				self.out.put(tsk)
+		else:
+			self.add_task(tsk)
+
 	def process_task(self, tsk):
 		"""
 		Processes a task and attempts to stop the build in case of errors
@@ -453,17 +482,7 @@ class Parallel(object):
 
 			st = self.task_status(tsk)
 			if st == Task.RUN_ME:
-				self.count += 1
-				self.processed += 1
-
-				if self.numjobs == 1:
-					tsk.log_display(tsk.generator.bld)
-					try:
-						self.process_task(tsk)
-					finally:
-						self.out.put(tsk)
-				else:
-					self.add_task(tsk)
+				self._add_task(tsk)
 			elif st == Task.ASK_LATER:
 				self.postpone(tsk)
 			elif st == Task.SKIP_ME:

waflib/Task.py

@@ -133,10 +133,12 @@ evil = store_task_type('evil', (object,), {})
 
 class Task(evil):
 	"""
-	This class deals with the filesystem (:py:class:`waflib.Node.Node`). The method :py:class:`waflib.Task.Task.runnable_status`
-	uses a hash value (from :py:class:`waflib.Task.Task.signature`) which is persistent from build to build. When the value changes,
-	the task has to be executed. The method :py:class:`waflib.Task.Task.post_run` will assign the task signature to the output
-	nodes (if present).
+	Task objects represents actions to perform such as commands to execute by calling the `run` method.
+
+	Detecting when to execute a task occurs in the method :py:meth:`waflib.Task.Task.runnable_status`.
+
+	Detecting which tasks to execute is performed through a hash value returned by
+	:py:meth:`waflib.Task.Task.signature`. The task signature is persistent from build to build.
 	"""
 	vars = []
 	"""ConfigSet variables that should trigger a rebuild (class attribute used for :py:meth:`waflib.Task.Task.sig_vars`)"""
@@ -1339,3 +1341,54 @@ def deep_inputs(cls):
 TaskBase = Task
 "Provided for compatibility reasons, TaskBase should not be used"
 
+class TaskSemaphore(object):
+	"""
+	Task semaphores provide a simple and efficient way of throttling the amount of
+	a particular task to run concurrently. The throttling value is capped
+	by the amount of maximum jobs, so for example, a `TaskSemaphore(10)`
+	has no effect in a `-j2` build.
+
+	Task semaphores are typically specified on the task class level::
+
+		class compile(waflib.Task.Task):
+			semaphore = waflib.Task.TaskSemaphore(2)
+			run_str = 'touch ${TGT}'
+
+	Task semaphores are meant to be used by the build scheduler in the main
+	thread, so there are no guarantees of thread safety.
+	"""
+	def __init__(self, num):
+		"""
+		:param num: maximum value of concurrent tasks
+		:type num: int
+		"""
+		self.num = num
+		self.locking = set()
+		self.waiting = set()
+
+	def is_locked(self):
+		"""Returns True if this semaphore cannot be acquired by more tasks"""
+		return len(self.locking) >= self.num
+
+	def acquire(self, tsk):
+		"""
+		Mark the semaphore as used by the given task (not re-entrant).
+
+		:param tsk: task object
+		:type tsk: :py:class:`waflib.Task.Task`
+		:raises: :py:class:`IndexError` in case the resource is already acquired
+		"""
+		if self.is_locked():
+			raise IndexError('Cannot lock more %r' % self.locking)
+		self.locking.add(tsk)
+
+	def release(self, tsk):
+		"""
+		Mark the semaphore as unused by the given task.
+
+		:param tsk: task object
+		:type tsk: :py:class:`waflib.Task.Task`
+		:raises: :py:class:`KeyError` in case the resource is not acquired by the task
+		"""
+		self.locking.remove(tsk)
+