[LNT] Python 3 support: sort benchmark regressing

Summary:
Test server/ui/change_processing.py makes an assumption about the order
in which benchmark are listed when a regression occurs but these are put
in a set before being iterated over for printing. Since set have no
guarantee of order, the test is fragile and fails on Python 3 due to
different implementation of Set. This commit sort the benchmark when
printing them so that the output is consistent. It also adapts the
testcase accordingly.

Reviewers: cmatthews, hubert.reinterpretcast, kristof.beyls, leandron, PrzemekWirkus

Reviewed By: PrzemekWirkus

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D68803

Author: Thomas Preud'homme

lnt/server/db/regression.py

@@ -71,7 +71,7 @@ def rebuild_title(session, ts, regression):
             fc = ri.field_change
             benchmarks.add(shortname(fc.test.name))
         FMT = "Regression of {} benchmarks: {}"
-        title = FMT.format(new_size, ', '.join(benchmarks))
+        title = FMT.format(new_size, ', '.join(sorted(benchmarks)))
         # Crop long titles.
         title = (title[:120] + '...') if len(title) > 120 else title
         regression.title = title

tests/server/ui/change_processing.py

@@ -212,7 +212,8 @@ def test_change_grouping_criteria(self):
         session.commit()
 
         r2 = rebuild_title(session, ts_db, self.regression)
-        expected_title = "Regression of 6 benchmarks: foo, bar"
+        # The list of benchmark regressing is guaranteed to be sorted.
+        expected_title = "Regression of 6 benchmarks: bar, foo"
         self.assertEqual(r2.title, expected_title)
 
     def test_regression_evolution(self):