Hash concurrent foreign key names similar to Rails

This was initially not implemented simply because I forgot about the
size limit of constraint names in PostgreSQL (63 bytes). Using the old
technique we can't add foreign keys for certain tables. For example,
adding a foreign key on
protected_branch_merge_access_levels.protected_branch_id would lead to
the following key name:

    fk_protected_branch_merge_access_levels_protected_branches_protected_branch_id

This key is 78 bytes long, thus violating the PostgreSQL size
requirements.

The hashing strategy is copied from Rails' foreign_key_name() method,
which unfortunately is private and subject to change without notice.

lib/gitlab/database/migration_helpers.rb

@@ -54,7 +54,7 @@ module Gitlab
 
         disable_statement_timeout
 
-        key_name = "fk_#{source}_#{target}_#{column}"
+        key_name = concurrent_foreign_key_name(source, column)
 
         # Using NOT VALID allows us to create a key without immediately
         # validating it. This means we keep the ALTER TABLE lock only for a
@@ -74,6 +74,15 @@ module Gitlab
         execute("ALTER TABLE #{source} VALIDATE CONSTRAINT #{key_name};")
       end
 
+      # Returns the name for a concurrent foreign key.
+      #
+      # PostgreSQL constraint names have a limit of 63 bytes. The logic used
+      # here is based on Rails' foreign_key_name() method, which unfortunately
+      # is private so we can't rely on it directly.
+      def concurrent_foreign_key_name(table, column)
+        "fk_#{Digest::SHA256.hexdigest("#{table}_#{column}_fk").first(10)}"
+      end
+
       # Long-running migrations may take more than the timeout allowed by
       # the database. Disable the session's statement timeout to ensure
       # migrations don't get killed prematurely. (PostgreSQL only)

spec/lib/gitlab/database/migration_helpers_spec.rb

@@ -101,6 +101,16 @@ describe Gitlab::Database::MigrationHelpers, lib: true do
     end
   end
 
+  describe '#concurrent_foreign_key_name' do
+    it 'returns the name for a foreign key' do
+      name = model.concurrent_foreign_key_name(:this_is_a_very_long_table_name,
+                                               :with_a_very_long_column_name)
+
+      expect(name).to be_an_instance_of(String)
+      expect(name.length).to eq(13)
+    end
+  end
+
   describe '#disable_statement_timeout' do
     context 'using PostgreSQL' do
       it 'disables statement timeouts' do