Fix migration for removing MR diff indexes

For whatever reason on some PostgreSQL installations there would be a
separate UNIQUE constraint on the merge_request_id column. Rails'
index_exists?() returns false for this constraint (even when using the
full name), yet the indexes() method returns it.

This commit changes the migration so that the constraint is dropped as
well. MySQL installations don't appear to be affected.

Fixes gitlab-org/gitlab-ce#22136

db/migrate/20160725104020_merge_request_diff_remove_uniq.rb

@@ -8,14 +8,28 @@ class MergeRequestDiffRemoveUniq < ActiveRecord::Migration
   DOWNTIME = false
 
   def up
+    constraint_name = 'merge_request_diffs_merge_request_id_key'
+
+    transaction do
       if index_exists?(:merge_request_diffs, :merge_request_id)
-      remove_index :merge_request_diffs, :merge_request_id
+        remove_index(:merge_request_diffs, :merge_request_id)
+      end
+
+      # In some bizarre cases PostgreSQL might have a separate unique constraint
+      # that we'll need to drop.
+      if constraint_exists?(constraint_name) && Gitlab::Database.postgresql?
+        execute("ALTER TABLE merge_request_diffs DROP CONSTRAINT IF EXISTS #{constraint_name};")
+      end
     end
   end
 
   def down
     unless index_exists?(:merge_request_diffs, :merge_request_id)
-      add_concurrent_index :merge_request_diffs, :merge_request_id, unique: true
+      add_concurrent_index(:merge_request_diffs, :merge_request_id, unique: true)
+    end
   end
+
+  def constraint_exists?(name)
+    indexes(:merge_request_diffs).map(&:name).include?(name)
   end
 end