Change priority and destination lists to sets

By using sets instead of lists for the valid priority and destination,
the checks for item uniqueness in these datatypes is now handled
inherently, with little detriment.

The only drawback to this is that the system that chooses the default
priority for each tool now simply chooses the first element from
iter(), rather than the one in the middle of the list. This isn't a
huge deal, as that was no more than a guess at what a mid-level
priority would be, and would even cause errors when the chosen
priority wasn't in the tool's default destination section.

Using sets insetad still result in the same issue, so this will need
to be fixed soon, as it prevents certain tool destination configs
from working properly.

lib/galaxy/jobs/dynamic_tool_destination.py

@@ -24,13 +24,13 @@
 list of all valid priorities, inferred from the global
 default_desinations section of the config
 """
-priority_list = []
+priority_list = set()
 
 """
 list of all valid destinations, retrieved from the
 job configuration file
 """
-valid_destinations = []
+valid_destinations = set()
 
 
 class MalformedYMLException(Exception):
@@ -785,6 +785,7 @@ def infinite_defaultdict():
 
         if 'default_destination' in obj:
             if isinstance(obj['default_destination'], str):
+                priority_list.add(obj['default_destination'])
                 if obj['default_destination'] in valid_destinations:
                     new_config["default_destination"] = obj['default_destination']
                 else:
@@ -803,8 +804,7 @@ def infinite_defaultdict():
 
                         if isinstance(obj['default_destination']['priority'][priority],
                                       str):
-                            if priority not in priority_list:
-                                priority_list.append(priority)
+                            priority_list.add(priority)
 
                             if obj['default_destination']['priority'][priority] in valid_destinations:
                                 new_config['default_destination']['priority'][priority] = obj[
@@ -1300,11 +1300,12 @@ def map_tool_to_destination(
     # For each different rule for the tool that's running
     fail_message = None
 
-    # set default priority to whatever the middle one is in the list
-    # The priorities won't necessarily be in order, but it can't be
-    # worse than choosing a random value (probably)
+    # assign default priority to the first element that comes out of the
+    # set. Definitely not the best way to pick fallback default
+    # destinations for tools, but there's not many other options without
+    # making default destinations a mandatory field for all tools
     if len(priority_list) > 0:
-        default_priority = priority_list[len(priority_list) / 2]
+        default_priority = next(iter(priority_list))
         priority = default_priority
     else:
         fail_message = ("No priorities declared in config file!" +
@@ -1455,7 +1456,7 @@ def map_tool_to_destination(
 
     if destination == "fail":
         if fail_message:
-            raise JobMappingException(fail_message)
+            raise JobMappingException(fail_message)  ###TODO: Should an exception occur when there are no default destinations?
         else:
             raise JobMappingException(matched_rule["fail_message"])
 
@@ -1484,7 +1485,7 @@ def get_valid_destinations_from_config(config_location='/config/job_conf.xml'):
     @return: A list of all of the destination IDs declared in the job
                 configuration file.
     """
-    valid_destinations = []
+    valid_destinations = set()
 
     # os.path.realpath gets the path of DynamicToolDestination.py
     # and then os.path.join is used to go back four directories
@@ -1496,7 +1497,7 @@ def get_valid_destinations_from_config(config_location='/config/job_conf.xml'):
 
     for destination in job_conf.getroot().iter("destination"):
         if isinstance(destination.get("id"), str):
-            valid_destinations.append(destination.get("id"))
+            valid_destinations.add(destination.get("id"))
 
         else:
             error = "Destination ID '" + str(destination)