package: ensure patches are applied in-order

This helps to ensure that patches are applied consistently and will also
be used as the source for the patch part of full package hashes.

lib/spack/spack/package.py

@@ -37,6 +37,7 @@
 import copy
 import functools
 import inspect
+import itertools
 import os
 import re
 import shutil
@@ -1107,7 +1108,7 @@ def do_patch(self):
 
         # Apply all the patches for specs that match this one
         patched = False
-        for patch in patches:
+        for patch in self.patches_to_apply():
             try:
                 with working_dir(self.stage.source_path):
                     patch.apply(self.stage)
@@ -1151,6 +1152,15 @@ def do_patch(self):
         else:
             touch(no_patches_file)
 
+    def patches_to_apply(self):
+        """If the patch set does not change between two invocations of spack,
+           then all patches in the set will be applied in the same order"""
+        patchesToApply = itertools.chain.from_iterable(
+            patch_list
+            for spec, patch_list in self.patches.items()
+            if self.spec.satisfies(spec))
+        return sorted(patchesToApply, key=lambda p: p.path_or_url)
+
     @property
     def namespace(self):
         namespace, dot, module = self.__module__.rpartition('.')