From 2908ca8f2b417f7b0824a934770619d0f8cf8909 Mon Sep 17 00:00:00 2001
From: James Ennis <james.ennis@codethink.co.uk>
Date: Tue, 28 May 2019 17:31:07 +0100
Subject: queue.py: Use heapq for the ready queue

This patch includes setting a _depth to each element
once the pipeline has been sorted. This is necessary
as we need to store elements in the heapq sorted by
their depth.
---
 src/buildstream/_pipeline.py               |  5 +++++
 src/buildstream/_scheduler/queues/queue.py |  7 ++++---
 src/buildstream/element.py                 | 12 ++++++++++++
 3 files changed, 21 insertions(+), 3 deletions(-)

diff --git a/src/buildstream/_pipeline.py b/src/buildstream/_pipeline.py
index c176b82f6..d44813348 100644
--- a/src/buildstream/_pipeline.py
+++ b/src/buildstream/_pipeline.py
@@ -513,4 +513,9 @@ class _Planner():
             self.plan_element(root, 0)
 
         depth_sorted = sorted(self.depth_map.items(), key=itemgetter(1), reverse=True)
+
+        # Set the depth of each element
+        for index, item in enumerate(depth_sorted):
+            item[0]._set_depth(index)
+
         return [item[0] for item in depth_sorted if plan_cached or not item[0]._cached_success()]
diff --git a/src/buildstream/_scheduler/queues/queue.py b/src/buildstream/_scheduler/queues/queue.py
index 951b28c2a..9a07f633c 100644
--- a/src/buildstream/_scheduler/queues/queue.py
+++ b/src/buildstream/_scheduler/queues/queue.py
@@ -22,6 +22,7 @@
 import os
 from collections import deque
 from enum import Enum
+import heapq
 import traceback
 
 # Local imports
@@ -73,7 +74,7 @@ class Queue():
         #
         self._scheduler = scheduler
         self._resources = scheduler.resources  # Shared resource pool
-        self._ready_queue = deque()            # Ready elements
+        self._ready_queue = []                 # Ready elements
         self._done_queue = deque()             # Processed / Skipped elements
         self._max_retries = 0
 
@@ -210,7 +211,7 @@ class Queue():
             if not reserved:
                 break
 
-            element = self._ready_queue.popleft()
+            _, element = heapq.heappop(self._ready_queue)
             ready.append(element)
 
         return [
@@ -355,7 +356,7 @@ class Queue():
             self._done_queue.append(element)            # Elements to proceed to the next queue
         elif status == QueueStatus.READY:
             # Push elements which are ready to be processed immediately into the queue
-            self._ready_queue.append(element)
+            heapq.heappush(self._ready_queue, (element._depth, element))
         else:
             # Register a queue specific callback for pending elements
             self.register_pending_element(element)
diff --git a/src/buildstream/element.py b/src/buildstream/element.py
index 355773f76..84c8f20ff 100644
--- a/src/buildstream/element.py
+++ b/src/buildstream/element.py
@@ -244,6 +244,8 @@ class Element(Plugin):
         self.__can_query_cache_callback = None        # Callback to PullQueue/FetchQueue
         self.__buildable_callback = None              # Callback to BuildQueue
 
+        self._depth = None                            # Depth of Element in its current dependency graph
+
         # Ensure we have loaded this class's defaults
         self.__init_defaults(project, plugin_conf, meta.kind, meta.is_junction)
 
@@ -2310,6 +2312,16 @@ class Element(Plugin):
     def _set_buildable_callback(self, callback):
         self.__buildable_callback = callback
 
+    # _set_depth()
+    #
+    # Set the depth of the Element.
+    #
+    # The depth represents the position of the Element within the current
+    # session's dependency graph. A depth of zero represents the bottommost element.
+    #
+    def _set_depth(self, depth):
+        self._depth = depth
+
     #############################################################
     #                   Private Local Methods                   #
     #############################################################
-- 
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