_pipeline.py: Fix the planner to retain the original order when depth sorting

The algorithm adds elements to a dictionary and then sorts the dictionary
by the discovered depths while recursing - using an OrderedDict is enough
to ensure a stable order.

This fixes `bst show --deps plan ...` reporting different results on
each invocation.

This fixes an aspect of #824

1 files changed, 2 insertions, 1 deletions

diff --git a/buildstream/_pipeline.py b/buildstream/_pipeline.py
index 1dd4ec815..3dc3019eb 100644
--- a/buildstream/_pipeline.py
+++ b/buildstream/_pipeline.py
@@ -22,6 +22,7 @@
 import os
 import itertools
 from operator import itemgetter
+from collections import OrderedDict
 
 from ._exceptions import PipelineError
 from ._message import Message, MessageType
@@ -479,7 +480,7 @@ class Pipeline():
 #
 class _Planner():
     def __init__(self):
-        self.depth_map = {}
+        self.depth_map = OrderedDict()
         self.visiting_elements = set()
 
     # Here we want to traverse the same element more than once when