Add optional ordering of QA patch files

To explicitly specify the order of patches executed, the QA provides an
"order" file. It can contain names of patches that will be executed
first, and in the order listed, before all the other patches that still
follow an alphabetical order.
Signed-off-by: Hrvoje Ribicic <riba@google.com>
Reviewed-by: Michele Tartara <mtartara@google.com>

qa/qa_config.py

@@ -45,6 +45,7 @@ _ENABLED_DISK_TEMPLATES_KEY = "enabled-disk-templates"
 _QA_BASE_PATH = os.path.dirname(__file__)
 _QA_DEFAULT_PATCH = "qa-patch.json"
 _QA_PATCH_DIR = "patch"
+_QA_PATCH_ORDER_FILE = "order"
 
 #: QA configuration (L{_QaConfig})
 _config = None
@@ -296,8 +297,11 @@ class _QaConfig(object):
   def ApplyPatches(data, patch_module, patches):
     """Applies any patches present, and returns the modified QA configuration.
 
-    First, patches from the patch directory are applied, ordered alphabetically
-    by name.
+    First, patches from the patch directory are applied. They are ordered
+    alphabetically, unless there is an ``order`` file present - any patches
+    listed within are applied in that order, and any remaining ones in
+    alphabetical order again. Finally, the default patch residing in the
+    top-level QA directory is applied.
 
     @type data: dict (deserialized json)
     @param data: The QA configuration
@@ -309,10 +313,32 @@ class _QaConfig(object):
     @return: The modified configuration data.
 
     """
+    ordered_patches = []
+    order_path = os.path.join(_QA_BASE_PATH, _QA_PATCH_DIR,
+                              _QA_PATCH_ORDER_FILE)
+    if os.path.exists(order_path):
+      order_file = open(order_path, 'r')
+      ordered_patches = order_file.read().splitlines()
+      # Removes empty lines
+      ordered_patches = filter(None, ordered_patches)
+
+    # Add the patch dir
+    ordered_patches = map(lambda x: os.path.join(_QA_PATCH_DIR, x),
+                          ordered_patches)
+
+    # First the ordered patches
+    for patch in ordered_patches:
+      if patch not in patches:
+        raise qa_error.Error("Patch %s specified in the ordering file does not "
+                             "exist" % patch)
+      data = patch_module.apply_patch(data, patches[patch])
+
+    # Then the other non-default ones
     for patch in sorted(patches):
-      if patch != _QA_DEFAULT_PATCH:
+      if patch != _QA_DEFAULT_PATCH and patch not in ordered_patches:
         data = patch_module.apply_patch(data, patches[patch])
 
+    # Finally the default one
     if _QA_DEFAULT_PATCH in patches:
       data = patch_module.apply_patch(data, patches[_QA_DEFAULT_PATCH])