Add migrations to limit Design filename

This change adds two post-migrations that enforce a character limit
for design_management_designs.filename of 255.

The first post-migration adds the limit to the filename column.

The second post-migration modifies any design filenames larger than 255
characters to the string "gitlab-modified-#{id}".

This will ensure the filename remains valid (by having a valid image
extension .jpg and being unique).

Any records modified by this data migration will appear as a broken
image when viewed within the GitLab app, as filenames are used to look
up the design file from the design git repository.

However, this is an intentional consequence given the 255 character
limit was selected because that is the largest filename that most
filesystems support, and also because GitLab.com has 6 records with
filenames larger than this but they are all abuse.

https://gitlab.com/gitlab-org/gitlab/-/issues/209025

changelogs/unreleased/209025-design-filename-limit-migrations.yml

+---
+title: Add database migrations to design_management_designs.filename to enforce
+  a 255 character limit, and modify any filenames that exceed that limit
+merge_request: 33565
+author:
+type: changed

db/post_migrate/20200602013900_add_limit_to_designs_filename.rb

+# frozen_string_literal: true
+
+class AddLimitToDesignsFilename < ActiveRecord::Migration[6.0]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  def up
+    add_text_limit(:design_management_designs, :filename, 255, validate: false)
+  end
+
+  def down
+    remove_text_limit(:design_management_designs, :filename)
+  end
+end

db/post_migrate/20200602013901_cap_designs_filename_length_to_new_limit.rb

+# frozen_string_literal: true
+
+class CapDesignsFilenameLengthToNewLimit < ActiveRecord::Migration[6.0]
+  DOWNTIME = false
+
+  CHAR_LENGTH = 255
+  MODIFIED_NAME = 'gitlab-modified-'
+  MODIFIED_EXTENSION = '.jpg'
+
+  def up
+    arel_table = Arel::Table.new(:design_management_designs)
+
+    # Design filenames larger than the limit will be renamed to "gitlab-modified-{id}.jpg",
+    # which will be valid and unique. The design file itself will appear broken, as it is
+    # understood that no designs with filenames that exceed this limit will be legitimate.
+    # See https://gitlab.com/gitlab-org/gitlab/-/merge_requests/33565/diffs#note_355789080
+    new_value_clause = Arel::Nodes::NamedFunction.new(
+      'CONCAT',
+      [
+        Arel::Nodes.build_quoted(MODIFIED_NAME),
+        arel_table[:id],
+        Arel::Nodes.build_quoted(MODIFIED_EXTENSION)
+      ]
+    )
+    where_clause = Arel::Nodes::NamedFunction.new(
+      'CHAR_LENGTH',
+      [arel_table[:filename]]
+    ).gt(CHAR_LENGTH)
+
+    update_arel = Arel::UpdateManager.new.table(arel_table)
+                                         .set([[arel_table[:filename], new_value_clause]])
+                                         .where(where_clause)
+
+    ActiveRecord::Base.connection.execute(update_arel.to_sql)
+  end
+
+  def down
+    # no-op : the original filename is lost forever
+  end
+end

db/structure.sql

@@ -8121,6 +8121,9 @@ ALTER TABLE ONLY public.chat_names
 ALTER TABLE ONLY public.chat_teams
     ADD CONSTRAINT chat_teams_pkey PRIMARY KEY (id);
 
+ALTER TABLE public.design_management_designs
+    ADD CONSTRAINT check_07155e2715 CHECK ((char_length((filename)::text) <= 255)) NOT VALID;
+
 ALTER TABLE public.ci_job_artifacts
     ADD CONSTRAINT check_27f0f6dbab CHECK ((file_store IS NOT NULL)) NOT VALID;
 
@@ -13832,6 +13835,8 @@ COPY "schema_migrations" (version) FROM STDIN;
 20200528125905
 20200528171933
 20200601210148
+20200602013900
+20200602013901
 20200603073101
 20200604143628
 \.

spec/migrations/cap_designs_filename_length_to_new_limit_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+require Rails.root.join('db', 'post_migrate', '20200602013901_cap_designs_filename_length_to_new_limit')
+
+describe CapDesignsFilenameLengthToNewLimit, :migration, schema: 20200528125905 do
+  let(:namespaces) { table(:namespaces) }
+  let(:projects) { table(:projects) }
+  let(:issues) { table(:issues) }
+  let(:designs) { table(:design_management_designs) }
+
+  let(:filename_below_limit) { generate_filename(254) }
+  let(:filename_at_limit) { generate_filename(255) }
+  let(:filename_above_limit) { generate_filename(256) }
+
+  let!(:namespace) { namespaces.create!(name: 'foo', path: 'foo') }
+  let!(:project) { projects.create!(name: 'gitlab', path: 'gitlab-org/gitlab', namespace_id: namespace.id) }
+  let!(:issue) { issues.create!(description: 'issue', project_id: project.id) }
+
+  def generate_filename(length, extension: '.png')
+    name = 'a' * (length - extension.length)
+
+    "#{name}#{extension}"
+  end
+
+  def create_design(filename)
+    designs.create!(
+      issue_id: issue.id,
+      project_id: project.id,
+      filename: filename
+    )
+  end
+
+  it 'correctly sets filenames that are above the limit' do
+    [
+      filename_below_limit,
+      filename_at_limit,
+      filename_above_limit
+    ].each(&method(:create_design))
+
+    migrate!
+
+    expect(designs.find(1).filename).to eq(filename_below_limit)
+    expect(designs.find(2).filename).to eq(filename_at_limit)
+    expect(designs.find(3).filename).to eq([described_class::MODIFIED_NAME, 3, described_class::MODIFIED_EXTENSION].join)
+  end
+
+  it 'runs after filename limit has been set' do
+    # This spec file uses the `schema:` keyword to run these tests
+    # against a schema version before the one that sets the limit,
+    # as otherwise we can't create the design data with filenames greater
+    # than the limit.
+    #
+    # For this test, we migrate any skipped versions up to this migration.
+    migration_context.migrate(20200602013901)
+
+    create_design(filename_at_limit)
+    expect { create_design(filename_above_limit) }.to raise_error(ActiveRecord::StatementInvalid)
+  end
+end