Bug 1320136 - Mark redundant RunSQL migration operations as elidable

Django's `squashmigrations` command cannot optimise past `RunSQL` or
`RunPython` migration operations, since it cannot guarantee that the
order of migration operations isn't important. However as of Django 1.10
these commands can be marked as `elidable`, if they are not required in
the final migration (eg if they only manipulate legacy data and have
already run in all necessary environments), which solves this problem.

treeherder/model/migrations/0001_initial.py

@@ -367,13 +367,19 @@ class Migration(migrations.Migration):
         migrations.RunSQL(
             sql='CREATE FULLTEXT INDEX `idx_crash_signature` on bugscache (`crash_signature`);',
             reverse_sql='ALTER TABLE bugscache DROP INDEX idx_crash_signature',
+            # Since this cancels out the RunSQL command marked as elidable in 0043_bugscache_cleanup.py.
+            elidable=True,
         ),
         migrations.RunSQL(
             sql='CREATE FULLTEXT INDEX `idx_keywords` on bugscache (`keywords`);',
             reverse_sql='ALTER TABLE bugscache DROP INDEX idx_keywords',
+            # Since this cancels out the RunSQL command marked as elidable in 0043_bugscache_cleanup.py.
+            elidable=True,
         ),
         migrations.RunSQL(
             sql='CREATE FULLTEXT INDEX `idx_all_full_text` on bugscache (`summary`, `crash_signature`, `keywords`);',
             reverse_sql='ALTER TABLE bugscache DROP INDEX idx_all_full_text',
+            # Since this cancels out the RunSQL command marked as elidable in 0043_bugscache_cleanup.py.
+            elidable=True,
         ),
     ]

treeherder/model/migrations/0018_merge_duplicate_classifications.py

@@ -44,5 +44,8 @@ class Migration(migrations.Migration):
     ]
 
     operations = [
-        migrations.RunPython(combine_classified_failures, noop)
+        # Marked as elidable since this migration is only required to fix existing
+        # bad data (which has already occurred on stage/prod) and so can be omitted
+        # when this migration is squashed in the future.
+        migrations.RunPython(combine_classified_failures, noop, elidable=True)
     ]

treeherder/model/migrations/0035_job_detail_field_lengths_decrease.py

@@ -15,7 +15,11 @@ class Migration(migrations.Migration):
         migrations.RunSQL(
             ("UPDATE job_detail SET "
              "  title = SUBSTRING(title, 1, 70), "
-             "  value = SUBSTRING(value, 1, 125)")
+             "  value = SUBSTRING(value, 1, 125)"),
+            # Marked as elidable since this migration is only required to fix existing
+            # bad data (which has already occurred on stage/prod) and so can be omitted
+            # when this migration is squashed in the future.
+            elidable=True,
         ),
         migrations.AlterField(
             model_name='jobdetail',

treeherder/model/migrations/0043_bugscache_cleanup.py

@@ -31,13 +31,19 @@ class Migration(migrations.Migration):
         migrations.RunSQL(
             sql='ALTER TABLE bugscache DROP INDEX idx_crash_signature',
             reverse_sql='CREATE FULLTEXT INDEX `idx_crash_signature` on bugscache (`crash_signature`);',
+            # Since this cancels out the RunSQL command marked as elidable in 0001_initial.py.
+            elidable=True,
         ),
         migrations.RunSQL(
             sql='ALTER TABLE bugscache DROP INDEX idx_keywords',
             reverse_sql='CREATE FULLTEXT INDEX `idx_keywords` on bugscache (`keywords`);',
+            # Since this cancels out the RunSQL command marked as elidable in 0001_initial.py.
+            elidable=True,
         ),
         migrations.RunSQL(
             sql='ALTER TABLE bugscache DROP INDEX idx_all_full_text',
             reverse_sql='CREATE FULLTEXT INDEX `idx_all_full_text` on bugscache (`summary`, `crash_signature`, `keywords`);',
+            # Since this cancels out the RunSQL command marked as elidable in 0001_initial.py.
+            elidable=True,
         ),
     ]