--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/piecrust/baking/scheduler.py	Sun Nov 30 21:46:42 2014 -0800
@@ -0,0 +1,97 @@
+import logging
+import threading
+
+
+logger = logging.getLogger(__name__)
+
+
+class BakeScheduler(object):
+    _EMPTY = object()
+    _WAIT = object()
+
+    def __init__(self, record, jobs=None):
+        self.record = record
+        self.jobs = list(jobs) if jobs is not None else []
+        self._active_jobs = []
+        self._lock = threading.Lock()
+        self._added_event = threading.Event()
+        self._done_event = threading.Event()
+
+    def addJob(self, job):
+        logger.debug("Queuing job '%s:%s'." % (
+            job.factory.source.name, job.factory.rel_path))
+        with self._lock:
+            self.jobs.append(job)
+        self._added_event.set()
+
+    def onJobFinished(self, job):
+        logger.debug("Removing job '%s:%s'." % (
+            job.factory.source.name, job.factory.rel_path))
+        with self._lock:
+            self._active_jobs.remove(job)
+        self._done_event.set()
+
+    def getNextJob(self, wait_timeout=None, empty_timeout=None):
+        self._added_event.clear()
+        self._done_event.clear()
+        job = self._doGetNextJob()
+        while job in (self._EMPTY, self._WAIT):
+            if job == self._EMPTY:
+                if empty_timeout is None:
+                    return None
+                logger.debug("Waiting for a new job to be added...")
+                res = self._added_event.wait(empty_timeout)
+            elif job == self._WAIT:
+                if wait_timeout is None:
+                    return None
+                logger.debug("Waiting for a job to be finished...")
+                res = self._done_event.wait(wait_timeout)
+            if not res:
+                logger.debug("Timed-out. No job found.")
+                return None
+            job = self._doGetNextJob()
+        return job
+
+    def _doGetNextJob(self):
+        with self._lock:
+            if len(self.jobs) == 0:
+                return self._EMPTY
+
+            job = self.jobs.pop(0)
+            first_job = job
+            while True:
+                ready, wait_on_src = self._isJobReady(job)
+                if ready:
+                    break
+
+                logger.debug("Job '%s:%s' isn't ready yet: waiting on pages "
+                             "from source '%s' to finish baking." %
+                             (job.factory.source.name,
+                                 job.factory.rel_path, wait_on_src))
+                self.jobs.append(job)
+                job = self.jobs.pop(0)
+                if job == first_job:
+                    # None of the jobs are ready... we need to wait.
+                    self.jobs.append(job)
+                    return self._WAIT
+
+            logger.debug("Job '%s:%s' is ready to go, moving to active "
+                    "queue." % (job.factory.source.name, job.factory.rel_path))
+            self._active_jobs.append(job)
+            return job
+
+    def _isJobReady(self, job):
+        e = self.record.getPreviousEntry(job.factory.source.name,
+                job.factory.rel_path)
+        if not e:
+            return (True, None)
+        for sn, rp in e.used_source_names:
+            if sn == job.factory.source.name:
+                continue
+            if any(filter(lambda j: j.factory.source.name == sn, self.jobs)):
+                return (False, sn)
+            if any(filter(lambda j: j.factory.source.name == sn,
+                    self._active_jobs)):
+                return (False, sn)
+        return (True, None)
+