Fix uploads.path length for long filenames

This will prevent our other migration for adding old files to the uploads table from breaking.

db/migrate/20171103000000_set_uploads_path_size_for_mysql.rb

+# See http://doc.gitlab.com/ce/development/migration_style_guide.html
+# for more information on how to write migrations for GitLab.
+
+class SetUploadsPathSizeForMysql < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  # Set this constant to true if this migration requires downtime.
+  DOWNTIME = false
+
+  def up
+    # We need at least 297 at the moment. For more detail on that number, see:
+    # https://gitlab.com/gitlab-org/gitlab-ce/issues/40168#what-is-the-expected-correct-behavior
+    #
+    # Rails + PostgreSQL `string` is equivalent to a `text` field, but
+    # Rails + MySQL `string` is `varchar(255)` by default. Also, note that we
+    # have an upper limit because with a unique index, MySQL has a max key
+    # length of 3072 bytes which seems to correspond to `varchar(1024)`.
+    change_column :uploads, :path, :string, limit: 511
+  end
+
+  def down
+    # It was unspecified, which is varchar(255) by default in Rails for MySQL.
+    change_column :uploads, :path, :string
+  end
+end

db/schema.rb

@@ -1721,7 +1721,7 @@ ActiveRecord::Schema.define(version: 20171124150326) do
   create_table "uploads", force: :cascade do |t|
     t.integer "size", limit: 8, null: false
-    t.string "path", null: false
+    t.string "path", limit: 511, null: false
     t.string "checksum", limit: 64
     t.integer "model_id"
     t.string "model_type"