Add mysql_compatible_index_length to migration helpers

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

db/migrate/20170222111732_create_gpg_keys.rb

 class CreateGpgKeys < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
   DOWNTIME = false
 
   def change
@@ -12,8 +14,8 @@ class CreateGpgKeys < ActiveRecord::Migration
 
       t.text :key
 
-      t.index :primary_keyid, unique: true, length: Gitlab::Database.mysql? ? 20 : nil
-      t.index :fingerprint, unique: true, length: Gitlab::Database.mysql? ? 20 : nil
+      t.index :primary_keyid, unique: true, length: mysql_compatible_index_length
+      t.index :fingerprint, unique: true, length: mysql_compatible_index_length
     end
   end
 end

db/migrate/20170613154149_create_gpg_signatures.rb

 class CreateGpgSignatures < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
   DOWNTIME = false
 
   def change
@@ -16,8 +18,8 @@ class CreateGpgSignatures < ActiveRecord::Migration
       t.text :gpg_key_user_name
       t.text :gpg_key_user_email
 
-      t.index :commit_sha, unique: true, length: Gitlab::Database.mysql? ? 20 : nil
-      t.index :gpg_key_primary_keyid, length: Gitlab::Database.mysql? ? 20 : nil
+      t.index :commit_sha, unique: true, length: mysql_compatible_index_length
+      t.index :gpg_key_primary_keyid, length: mysql_compatible_index_length
     end
   end
 end

db/migrate/20170827123848_add_index_on_merge_request_diff_commit_sha.rb

@@ -8,7 +8,7 @@ class AddIndexOnMergeRequestDiffCommitSha < ActiveRecord::Migration
   disable_ddl_transaction!
 
   def up
-    add_concurrent_index :merge_request_diff_commits, :sha, length: Gitlab::Database.mysql? ? 20 : nil
+    add_concurrent_index :merge_request_diff_commits, :sha, length: mysql_compatible_index_length
   end
 
   def down

db/migrate/20170927161718_create_gpg_key_subkeys.rb

 class CreateGpgKeySubkeys < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
   DOWNTIME = false
 
   def up
@@ -8,8 +10,8 @@ class CreateGpgKeySubkeys < ActiveRecord::Migration
       t.binary :keyid
       t.binary :fingerprint
 
-      t.index :keyid, unique: true, length: Gitlab::Database.mysql? ? 20 : nil
-      t.index :fingerprint, unique: true, length: Gitlab::Database.mysql? ? 20 : nil
+      t.index :keyid, unique: true, length: mysql_compatible_index_length
+      t.index :fingerprint, unique: true, length: mysql_compatible_index_length
     end
 
     add_reference :gpg_signatures, :gpg_key_subkey, index: true, foreign_key: { on_delete: :nullify }

lib/gitlab/database/migration_helpers.rb

@@ -1073,6 +1073,10 @@ into similar problems in the future (e.g. when new tables are created).
 
         connection.select_value(index_sql).to_i > 0
       end
+
+      def mysql_compatible_index_length
+        Gitlab::Database.mysql? ? 20 : nil
+      end
     end
   end
 end