Merge upstream 2.0ish #165

6 files changed, 1212 insertions, 6 deletions

diff --git a/lib/mastodon/migration_helpers.rb b/lib/mastodon/migration_helpers.rb
new file mode 100644
index 000000000..ed716501e
--- /dev/null
+++ b/lib/mastodon/migration_helpers.rb
@@ -0,0 +1,988 @@
+# frozen_string_literal: true
+
+# This file is copied almost entirely from GitLab, which has done a large
+# amount of work to ensure that migrations can happen with minimal downtime.
+# Many thanks to those engineers.
+
+# Changes have been made to remove dependencies on other GitLab files and to
+# shorten temporary column names.
+
+# Documentation on using these functions (and why one might do so):
+# https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/what_requires_downtime.md
+
+# The file itself:
+# https://gitlab.com/gitlab-org/gitlab-ce/blob/master/lib/gitlab/database/migration_helpers.rb
+
+# It is licensed as follows:
+
+# Copyright (c) 2011-2017 GitLab B.V.
+
+# Permission is hereby granted, free of charge, to any person obtaining a copy
+# of this software and associated documentation files (the "Software"), to deal
+# in the Software without restriction, including without limitation the rights
+# to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+# copies of the Software, and to permit persons to whom the Software is
+# furnished to do so, subject to the following conditions:
+
+# The above copyright notice and this permission notice shall be included in
+# all copies or substantial portions of the Software.
+
+# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+# IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+# FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+# AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+# LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+# OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+# THE SOFTWARE.
+
+# This is bad form, but there are enough differences that it's impractical to do
+# otherwise:
+# rubocop:disable all
+
+module Mastodon
+  module MigrationHelpers
+    # Stub for Database.postgresql? from GitLab
+    def self.postgresql?
+      ActiveRecord::Base.configurations[Rails.env]['adapter'].casecmp('postgresql').zero?
+    end
+
+    # Stub for Database.mysql? from GitLab
+    def self.mysql?
+      ActiveRecord::Base.configurations[Rails.env]['adapter'].casecmp('mysql2').zero?
+    end
+
+    # Model that can be used for querying permissions of a SQL user.
+    class Grant < ActiveRecord::Base
+      self.table_name =
+        if Mastodon::MigrationHelpers.postgresql?
+          'information_schema.role_table_grants'
+        else
+          'mysql.user'
+        end
+
+      def self.scope_to_current_user
+        if Mastodon::MigrationHelpers.postgresql?
+          where('grantee = user')
+        else
+          where("CONCAT(User, '@', Host) = current_user()")
+        end
+      end
+
+      # Returns true if the current user can create and execute triggers on the
+      # given table.
+      def self.create_and_execute_trigger?(table)
+        priv =
+          if Mastodon::MigrationHelpers.postgresql?
+            where(privilege_type: 'TRIGGER', table_name: table)
+          else
+            where(Trigger_priv: 'Y')
+          end
+
+        priv.scope_to_current_user.any?
+      end
+    end
+
+    BACKGROUND_MIGRATION_BATCH_SIZE = 1000 # Number of rows to process per job
+    BACKGROUND_MIGRATION_JOB_BUFFER_SIZE = 1000 # Number of jobs to bulk queue at a time
+    
+    # Gets an estimated number of rows for a table
+    def estimate_rows_in_table(table_name)
+      exec_query('SELECT reltuples FROM pg_class WHERE relname = ' +
+        "'#{table_name}'").to_a.first['reltuples']
+    end
+
+    # Adds `created_at` and `updated_at` columns with timezone information.
+    #
+    # This method is an improved version of Rails' built-in method `add_timestamps`.
+    #
+    # Available options are:
+    # default - The default value for the column.
+    # null - When set to `true` the column will allow NULL values.
+    #        The default is to not allow NULL values.
+    def add_timestamps_with_timezone(table_name, options = {})
+      options[:null] = false if options[:null].nil?
+
+      [:created_at, :updated_at].each do |column_name|
+        if options[:default] && transaction_open?
+          raise '`add_timestamps_with_timezone` with default value cannot be run inside a transaction. ' \
+            'You can disable transactions by calling `disable_ddl_transaction!` ' \
+            'in the body of your migration class'
+        end
+
+        # If default value is presented, use `add_column_with_default` method instead.
+        if options[:default]
+          add_column_with_default(
+            table_name,
+            column_name,
+            :datetime_with_timezone,
+            default: options[:default],
+            allow_null: options[:null]
+          )
+        else
+          add_column(table_name, column_name, :datetime_with_timezone, options)
+        end
+      end
+    end
+
+    # Creates a new index, concurrently when supported
+    #
+    # On PostgreSQL this method creates an index concurrently, on MySQL this
+    # creates a regular index.
+    #
+    # Example:
+    #
+    #     add_concurrent_index :users, :some_column
+    #
+    # See Rails' `add_index` for more info on the available arguments.
+    def add_concurrent_index(table_name, column_name, options = {})
+      if transaction_open?
+        raise 'add_concurrent_index can not be run inside a transaction, ' \
+          'you can disable transactions by calling disable_ddl_transaction! ' \
+          'in the body of your migration class'
+      end
+
+      if MigrationHelpers.postgresql?
+        options = options.merge({ algorithm: :concurrently })
+        disable_statement_timeout
+      end
+
+      add_index(table_name, column_name, options)
+    end
+
+    # Removes an existed index, concurrently when supported
+    #
+    # On PostgreSQL this method removes an index concurrently.
+    #
+    # Example:
+    #
+    #     remove_concurrent_index :users, :some_column
+    #
+    # See Rails' `remove_index` for more info on the available arguments.
+    def remove_concurrent_index(table_name, column_name, options = {})
+      if transaction_open?
+        raise 'remove_concurrent_index can not be run inside a transaction, ' \
+          'you can disable transactions by calling disable_ddl_transaction! ' \
+          'in the body of your migration class'
+      end
+
+      if supports_drop_index_concurrently?
+        options = options.merge({ algorithm: :concurrently })
+        disable_statement_timeout
+      end
+
+      remove_index(table_name, options.merge({ column: column_name }))
+    end
+
+    # Removes an existing index, concurrently when supported
+    #
+    # On PostgreSQL this method removes an index concurrently.
+    #
+    # Example:
+    #
+    #     remove_concurrent_index :users, "index_X_by_Y"
+    #
+    # See Rails' `remove_index` for more info on the available arguments.
+    def remove_concurrent_index_by_name(table_name, index_name, options = {})
+      if transaction_open?
+        raise 'remove_concurrent_index_by_name can not be run inside a transaction, ' \
+          'you can disable transactions by calling disable_ddl_transaction! ' \
+          'in the body of your migration class'
+      end
+
+      if supports_drop_index_concurrently?
+        options = options.merge({ algorithm: :concurrently })
+        disable_statement_timeout
+      end
+
+      remove_index(table_name, options.merge({ name: index_name }))
+    end
+
+    # Only available on Postgresql >= 9.2
+    def supports_drop_index_concurrently?
+      return false unless MigrationHelpers.postgresql?
+
+      version = select_one("SELECT current_setting('server_version_num') AS v")['v'].to_i
+
+      version >= 90200
+    end
+
+    # Adds a foreign key with only minimal locking on the tables involved.
+    #
+    # This method only requires minimal locking when using PostgreSQL. When
+    # using MySQL this method will use Rails' default `add_foreign_key`.
+    #
+    # source - The source table containing the foreign key.
+    # target - The target table the key points to.
+    # column - The name of the column to create the foreign key on.
+    # on_delete - The action to perform when associated data is removed,
+    #             defaults to "CASCADE".
+    def add_concurrent_foreign_key(source, target, column:, on_delete: :cascade, target_col: 'id')
+      # Transactions would result in ALTER TABLE locks being held for the
+      # duration of the transaction, defeating the purpose of this method.
+      if transaction_open?
+        raise 'add_concurrent_foreign_key can not be run inside a transaction'
+      end
+
+      # While MySQL does allow disabling of foreign keys it has no equivalent
+      # of PostgreSQL's "VALIDATE CONSTRAINT". As a result we'll just fall
+      # back to the normal foreign key procedure.
+      if MigrationHelpers.mysql?
+        return add_foreign_key(source, target,
+                               column: column,
+                               on_delete: on_delete)
+      else
+        on_delete = 'SET NULL' if on_delete == :nullify
+      end
+
+      disable_statement_timeout
+
+      key_name = concurrent_foreign_key_name(source, column, target_col)
+
+      # Using NOT VALID allows us to create a key without immediately
+      # validating it. This means we keep the ALTER TABLE lock only for a
+      # short period of time. The key _is_ enforced for any newly created
+      # data.
+      execute <<-EOF.strip_heredoc
+      ALTER TABLE #{source}
+      ADD CONSTRAINT #{key_name}
+      FOREIGN KEY (#{column})
+      REFERENCES #{target} (#{target_col})
+      #{on_delete ? "ON DELETE #{on_delete.upcase}" : ''}
+      NOT VALID;
+      EOF
+
+      # Validate the existing constraint. This can potentially take a very
+      # long time to complete, but fortunately does not lock the source table
+      # while running.
+      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, target_col)
+      "fk_#{Digest::SHA256.hexdigest("#{table}_#{column}_#{target_col}_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)
+    def disable_statement_timeout
+      execute('SET statement_timeout TO 0') if MigrationHelpers.postgresql?
+    end
+
+    # Updates the value of a column in batches.
+    #
+    # This method updates the table in batches of 5% of the total row count.
+    # This method will continue updating rows until no rows remain.
+    #
+    # When given a block this method will yield two values to the block:
+    #
+    # 1. An instance of `Arel::Table` for the table that is being updated.
+    # 2. The query to run as an Arel object.
+    #
+    # By supplying a block one can add extra conditions to the queries being
+    # executed. Note that the same block is used for _all_ queries.
+    #
+    # Example:
+    #
+    #     update_column_in_batches(:projects, :foo, 10) do |table, query|
+    #       query.where(table[:some_column].eq('hello'))
+    #     end
+    #
+    # This would result in this method updating only rows where
+    # `projects.some_column` equals "hello".
+    #
+    # table - The name of the table.
+    # column - The name of the column to update.
+    # value - The value for the column.
+    #
+    # Rubocop's Metrics/AbcSize metric is disabled for this method as Rubocop
+    # determines this method to be too complex while there's no way to make it
+    # less "complex" without introducing extra methods (which actually will
+    # make things _more_ complex).
+    #
+    # rubocop: disable Metrics/AbcSize
+    def update_column_in_batches(table_name, column, value)
+      if transaction_open?
+        raise 'update_column_in_batches can not be run inside a transaction, ' \
+          'you can disable transactions by calling disable_ddl_transaction! ' \
+          'in the body of your migration class'
+      end
+
+      table = Arel::Table.new(table_name)
+      
+      total = estimate_rows_in_table(table_name).to_i
+      if total == 0
+        count_arel = table.project(Arel.star.count.as('count'))
+        count_arel = yield table, count_arel if block_given?
+        
+        total = exec_query(count_arel.to_sql).to_hash.first['count'].to_i
+        
+        return if total == 0
+      end
+
+      # Update in batches of 5% until we run out of any rows to update.
+      batch_size = ((total / 100.0) * 5.0).ceil
+      max_size = 1000
+
+      # The upper limit is 1000 to ensure we don't lock too many rows. For
+      # example, for "merge_requests" even 1% of the table is around 35 000
+      # rows for GitLab.com.
+      batch_size = max_size if batch_size > max_size
+
+      start_arel = table.project(table[:id]).order(table[:id].asc).take(1)
+      start_arel = yield table, start_arel if block_given?
+      start_id = exec_query(start_arel.to_sql).to_hash.first['id'].to_i
+      
+      say "Migrating #{table_name}.#{column} (~#{total.to_i} rows)"
+
+      started_time = Time.now
+      last_time = Time.now
+      migrated = 0
+      loop do
+        stop_row = nil
+        
+        suppress_messages do
+          stop_arel = table.project(table[:id])
+            .where(table[:id].gteq(start_id))
+            .order(table[:id].asc)
+            .take(1)
+            .skip(batch_size)
+
+          stop_arel = yield table, stop_arel if block_given?
+          stop_row = exec_query(stop_arel.to_sql).to_hash.first
+
+          update_arel = Arel::UpdateManager.new
+            .table(table)
+            .set([[table[column], value]])
+            .where(table[:id].gteq(start_id))
+
+          if stop_row
+            stop_id = stop_row['id'].to_i
+            start_id = stop_id
+            update_arel = update_arel.where(table[:id].lt(stop_id))
+          end
+
+          update_arel = yield table, update_arel if block_given?
+
+          execute(update_arel.to_sql)
+        end
+        
+        migrated += batch_size
+        if Time.now - last_time > 1
+          status = "Migrated #{migrated} rows"
+          
+          percentage = 100.0 * migrated / total
+          status += " (~#{sprintf('%.2f', percentage)}%, "
+          
+          remaining_time = (100.0 - percentage) * (Time.now - started_time) / percentage
+          
+          status += "#{(remaining_time / 60).to_i}:"
+          status += sprintf('%02d', remaining_time.to_i % 60)
+          status += ' remaining, '
+          
+          # Tell users not to interrupt if we're almost done.
+          if remaining_time > 10
+            status += 'safe to interrupt'
+          else
+            status += 'DO NOT interrupt'
+          end
+          
+          status += ')'
+          
+          say status, true
+          last_time = Time.now
+        end
+
+        # There are no more rows left to update.
+        break unless stop_row
+      end
+    end
+
+    # Adds a column with a default value without locking an entire table.
+    #
+    # This method runs the following steps:
+    #
+    # 1. Add the column with a default value of NULL.
+    # 2. Change the default value of the column to the specified value.
+    # 3. Update all existing rows in batches.
+    # 4. Set a `NOT NULL` constraint on the column if desired (the default).
+    #
+    # These steps ensure a column can be added to a large and commonly used
+    # table without locking the entire table for the duration of the table
+    # modification.
+    #
+    # table - The name of the table to update.
+    # column - The name of the column to add.
+    # type - The column type (e.g. `:integer`).
+    # default - The default value for the column.
+    # limit - Sets a column limit. For example, for :integer, the default is
+    #         4-bytes. Set `limit: 8` to allow 8-byte integers.
+    # allow_null - When set to `true` the column will allow NULL values, the
+    #              default is to not allow NULL values.
+    #
+    # This method can also take a block which is passed directly to the
+    # `update_column_in_batches` method.
+    def add_column_with_default(table, column, type, default:, limit: nil, allow_null: false, &block)
+      if transaction_open?
+        raise 'add_column_with_default can not be run inside a transaction, ' \
+          'you can disable transactions by calling disable_ddl_transaction! ' \
+          'in the body of your migration class'
+      end
+
+      disable_statement_timeout
+
+      transaction do
+        if limit
+          add_column(table, column, type, default: nil, limit: limit)
+        else
+          add_column(table, column, type, default: nil)
+        end
+
+        # Changing the default before the update ensures any newly inserted
+        # rows already use the proper default value.
+        change_column_default(table, column, default)
+      end
+
+      begin
+        update_column_in_batches(table, column, default, &block)
+
+        change_column_null(table, column, false) unless allow_null
+      # We want to rescue _all_ exceptions here, even those that don't inherit
+      # from StandardError.
+      rescue Exception => error # rubocop: disable all
+        remove_column(table, column)
+
+        raise error
+      end
+    end
+
+    # Renames a column without requiring downtime.
+    #
+    # Concurrent renames work by using database triggers to ensure both the
+    # old and new column are in sync. However, this method will _not_ remove
+    # the triggers or the old column automatically; this needs to be done
+    # manually in a post-deployment migration. This can be done using the
+    # method `cleanup_concurrent_column_rename`.
+    #
+    # table - The name of the database table containing the column.
+    # old - The old column name.
+    # new - The new column name.
+    # type - The type of the new column. If no type is given the old column's
+    #        type is used.
+    def rename_column_concurrently(table, old, new, type: nil)
+      if transaction_open?
+        raise 'rename_column_concurrently can not be run inside a transaction'
+      end
+
+      check_trigger_permissions!(table)
+      trigger_name = rename_trigger_name(table, old, new)
+      
+      # If we were in the middle of update_column_in_batches, we should remove
+      # the old column and start over, as we have no idea where we were.
+      if column_for(table, new)
+        if MigrationHelpers.postgresql?
+          remove_rename_triggers_for_postgresql(table, trigger_name)
+        else
+          remove_rename_triggers_for_mysql(trigger_name)
+        end
+        
+        remove_column(table, new)
+      end
+
+      old_col = column_for(table, old)
+      new_type = type || old_col.type
+
+      col_opts = {
+        precision: old_col.precision,
+        scale: old_col.scale,
+      }
+
+      # We may be trying to reset the limit on an integer column type, so let
+      # Rails handle that.
+      unless [:bigint, :integer].include?(new_type)
+        col_opts[:limit] = old_col.limit
+      end
+
+      add_column(table, new, new_type, col_opts)
+
+      # We set the default value _after_ adding the column so we don't end up
+      # updating any existing data with the default value. This isn't
+      # necessary since we copy over old values further down.
+      change_column_default(table, new, old_col.default) if old_col.default
+
+      quoted_table = quote_table_name(table)
+      quoted_old = quote_column_name(old)
+      quoted_new = quote_column_name(new)
+
+      if MigrationHelpers.postgresql?
+        install_rename_triggers_for_postgresql(trigger_name, quoted_table,
+                                               quoted_old, quoted_new)
+      else
+        install_rename_triggers_for_mysql(trigger_name, quoted_table,
+                                          quoted_old, quoted_new)
+      end
+
+      update_column_in_batches(table, new, Arel::Table.new(table)[old])
+
+      change_column_null(table, new, false) unless old_col.null
+
+      copy_indexes(table, old, new)
+      copy_foreign_keys(table, old, new)
+    end
+
+    # Changes the type of a column concurrently.
+    #
+    # table - The table containing the column.
+    # column - The name of the column to change.
+    # new_type - The new column type.
+    def change_column_type_concurrently(table, column, new_type)
+      temp_column = rename_column_name(column)
+
+      rename_column_concurrently(table, column, temp_column, type: new_type)
+      
+      # Primary keys don't necessarily have an associated index.
+      if ActiveRecord::Base.get_primary_key(table) == column.to_s
+        old_pk_index_name = "index_#{table}_on_#{column}"
+        new_pk_index_name = "index_#{table}_on_#{column}_cm"
+        
+        unless indexes_for(table, column).find{|i| i.name == old_pk_index_name}
+          add_concurrent_index(table, [temp_column], {
+            unique: true,
+            name: new_pk_index_name
+          })
+        end
+      end
+    end
+
+    # Performs cleanup of a concurrent type change.
+    #
+    # table - The table containing the column.
+    # column - The name of the column to change.
+    # new_type - The new column type.
+    def cleanup_concurrent_column_type_change(table, column)
+      temp_column = rename_column_name(column)
+
+      # Wait for the indices to be built
+      indexes_for(table, column).each do |index|
+        expected_name = index.name + '_cm'
+        
+        puts "Waiting for index #{expected_name}"
+        sleep 1 until indexes_for(table, temp_column).find {|i| i.name == expected_name }
+      end
+      
+      was_primary = (ActiveRecord::Base.get_primary_key(table) == column.to_s)
+      old_default_fn = column_for(table, column).default_function
+      
+      old_fks = []
+      if was_primary
+        # Get any foreign keys pointing at this column we need to recreate, and
+        # remove the old ones.
+        # Based on code from:
+        # http://errorbank.blogspot.com/2011/03/list-all-foreign-keys-references-for.html
+        old_fks_res = execute <<-EOF.strip_heredoc
+          select m.relname as src_table,
+            (select a.attname
+              from pg_attribute a
+              where a.attrelid = m.oid
+                and a.attnum = o.conkey[1]
+                and a.attisdropped = false) as src_col,
+            o.conname as name,
+            o.confdeltype as on_delete
+          from pg_constraint o
+          left join pg_class f on f.oid = o.confrelid 
+          left join pg_class c on c.oid = o.conrelid
+          left join pg_class m on m.oid = o.conrelid
+          where o.contype = 'f'
+            and o.conrelid in (
+              select oid from pg_class c where c.relkind = 'r')
+            and f.relname = '#{table}';
+          EOF
+        old_fks = old_fks_res.to_a
+        old_fks.each do |old_fk|
+          add_concurrent_foreign_key(
+            old_fk['src_table'],
+            table,
+            column: old_fk['src_col'],
+            target_col: temp_column,
+            on_delete: extract_foreign_key_action(old_fk['on_delete'])
+          )
+          
+          remove_foreign_key(old_fk['src_table'], name: old_fk['name'])
+        end
+      end
+
+      # If there was a sequence owned by the old column, make it owned by the
+      # new column, as it will otherwise be deleted when we get rid of the
+      # old column.
+      if (seq_match = /^nextval\('([^']*)'(::text|::regclass)?\)/.match(old_default_fn))
+        seq_name = seq_match[1]
+        execute("ALTER SEQUENCE #{seq_name} OWNED BY #{table}.#{temp_column}")
+      end
+
+      transaction do
+        # This has to be performed in a transaction as otherwise we might have
+        # inconsistent data.
+        
+        cleanup_concurrent_column_rename(table, column, temp_column)
+        rename_column(table, temp_column, column)
+        
+        # If there was an old default function, we didn't copy it. Do that now
+        # in the transaction, so we don't miss anything.
+        change_column_default(table, column, -> { old_default_fn }) if old_default_fn
+      end
+      
+      # Rename any indices back to what they should be.
+      indexes_for(table, column).each do |index|
+        next unless index.name.end_with?('_cm')
+
+        real_index_name = index.name.sub(/_cm$/, '')
+        rename_index(table, index.name, real_index_name)
+      end
+      
+      # Rename any foreign keys back to names based on the real column.
+      foreign_keys_for(table, column).each do |fk|
+        old_fk_name = concurrent_foreign_key_name(fk.from_table, temp_column, 'id')
+        new_fk_name = concurrent_foreign_key_name(fk.from_table, column, 'id')
+        execute("ALTER TABLE #{fk.from_table} RENAME CONSTRAINT " +
+          "#{old_fk_name} TO #{new_fk_name}")
+      end
+      
+      # Rename any foreign keys from other tables to names based on the real
+      # column.
+      old_fks.each do |old_fk|
+        old_fk_name = concurrent_foreign_key_name(old_fk['src_table'],
+          old_fk['src_col'], temp_column)
+        new_fk_name = concurrent_foreign_key_name(old_fk['src_table'],
+          old_fk['src_col'], column)
+        execute("ALTER TABLE #{old_fk['src_table']} RENAME CONSTRAINT " +
+          "#{old_fk_name} TO #{new_fk_name}")
+      end
+      
+      # If the old column was a primary key, mark the new one as a primary key.
+      if was_primary
+        execute("ALTER TABLE #{table} ADD PRIMARY KEY USING INDEX " +
+          "index_#{table}_on_#{column}")
+      end
+    end
+
+    # Cleans up a concurrent column name.
+    #
+    # This method takes care of removing previously installed triggers as well
+    # as removing the old column.
+    #
+    # table - The name of the database table.
+    # old - The name of the old column.
+    # new - The name of the new column.
+    def cleanup_concurrent_column_rename(table, old, new)
+      trigger_name = rename_trigger_name(table, old, new)
+
+      check_trigger_permissions!(table)
+
+      if MigrationHelpers.postgresql?
+        remove_rename_triggers_for_postgresql(table, trigger_name)
+      else
+        remove_rename_triggers_for_mysql(trigger_name)
+      end
+
+      remove_column(table, old)
+    end
+
+    # Performs a concurrent column rename when using PostgreSQL.
+    def install_rename_triggers_for_postgresql(trigger, table, old, new)
+      execute <<-EOF.strip_heredoc
+      CREATE OR REPLACE FUNCTION #{trigger}()
+      RETURNS trigger AS
+      $BODY$
+      BEGIN
+        NEW.#{new} := NEW.#{old};
+        RETURN NEW;
+      END;
+      $BODY$
+      LANGUAGE 'plpgsql'
+      VOLATILE
+      EOF
+
+      execute <<-EOF.strip_heredoc
+      CREATE TRIGGER #{trigger}
+      BEFORE INSERT OR UPDATE
+      ON #{table}
+      FOR EACH ROW
+      EXECUTE PROCEDURE #{trigger}()
+      EOF
+    end
+
+    # Installs the triggers necessary to perform a concurrent column rename on
+    # MySQL.
+    def install_rename_triggers_for_mysql(trigger, table, old, new)
+      execute <<-EOF.strip_heredoc
+      CREATE TRIGGER #{trigger}_insert
+      BEFORE INSERT
+      ON #{table}
+      FOR EACH ROW
+      SET NEW.#{new} = NEW.#{old}
+      EOF
+
+      execute <<-EOF.strip_heredoc
+      CREATE TRIGGER #{trigger}_update
+      BEFORE UPDATE
+      ON #{table}
+      FOR EACH ROW
+      SET NEW.#{new} = NEW.#{old}
+      EOF
+    end
+
+    # Removes the triggers used for renaming a PostgreSQL column concurrently.
+    def remove_rename_triggers_for_postgresql(table, trigger)
+      execute("DROP TRIGGER IF EXISTS #{trigger} ON #{table}")
+      execute("DROP FUNCTION IF EXISTS #{trigger}()")
+    end
+
+    # Removes the triggers used for renaming a MySQL column concurrently.
+    def remove_rename_triggers_for_mysql(trigger)
+      execute("DROP TRIGGER IF EXISTS #{trigger}_insert")
+      execute("DROP TRIGGER IF EXISTS #{trigger}_update")
+    end
+
+    # Returns the (base) name to use for triggers when renaming columns.
+    def rename_trigger_name(table, old, new)
+      'trigger_' + Digest::SHA256.hexdigest("#{table}_#{old}_#{new}").first(12)
+    end
+
+    # Returns the name to use for temporary rename columns.
+    def rename_column_name(base)
+      base.to_s + '_cm'
+    end
+
+    # Returns an Array containing the indexes for the given column
+    def indexes_for(table, column)
+      column = column.to_s
+
+      indexes(table).select { |index| index.columns.include?(column) }
+    end
+
+    # Returns an Array containing the foreign keys for the given column.
+    def foreign_keys_for(table, column)
+      column = column.to_s
+
+      foreign_keys(table).select { |fk| fk.column == column }
+    end
+
+    # Copies all indexes for the old column to a new column.
+    #
+    # table - The table containing the columns and indexes.
+    # old - The old column.
+    # new - The new column.
+    def copy_indexes(table, old, new)
+      old = old.to_s
+      new = new.to_s
+
+      indexes_for(table, old).each do |index|
+        new_columns = index.columns.map do |column|
+          column == old ? new : column
+        end
+
+        # This is necessary as we can't properly rename indexes such as
+        # "ci_taggings_idx".
+        name = index.name + '_cm'
+        
+        # If the order contained the old column, map it to the new one.
+        order = index.orders
+        if order.key?(old)
+          order[new] = order.delete(old)
+        end
+
+        options = {
+          unique: index.unique,
+          name: name,
+          length: index.lengths,
+          order: order
+        }
+
+        # These options are not supported by MySQL, so we only add them if
+        # they were previously set.
+        options[:using] = index.using if index.using
+        options[:where] = index.where if index.where
+
+        add_concurrent_index(table, new_columns, options)
+      end
+    end
+
+    # Copies all foreign keys for the old column to the new column.
+    #
+    # table - The table containing the columns and indexes.
+    # old - The old column.
+    # new - The new column.
+    def copy_foreign_keys(table, old, new)
+      foreign_keys_for(table, old).each do |fk|
+        add_concurrent_foreign_key(fk.from_table,
+                                   fk.to_table,
+                                   column: new,
+                                   on_delete: fk.on_delete)
+      end
+    end
+
+    # Returns the column for the given table and column name.
+    def column_for(table, name)
+      name = name.to_s
+
+      columns(table).find { |column| column.name == name }
+    end
+
+    # This will replace the first occurance of a string in a column with
+    # the replacement
+    # On postgresql we can use `regexp_replace` for that.
+    # On mysql we find the location of the pattern, and overwrite it
+    # with the replacement
+    def replace_sql(column, pattern, replacement)
+      quoted_pattern = Arel::Nodes::Quoted.new(pattern.to_s)
+      quoted_replacement = Arel::Nodes::Quoted.new(replacement.to_s)
+
+      if MigrationHelpers.mysql?
+        locate = Arel::Nodes::NamedFunction
+          .new('locate', [quoted_pattern, column])
+        insert_in_place = Arel::Nodes::NamedFunction
+          .new('insert', [column, locate, pattern.size, quoted_replacement])
+
+        Arel::Nodes::SqlLiteral.new(insert_in_place.to_sql)
+      else
+        replace = Arel::Nodes::NamedFunction
+          .new("regexp_replace", [column, quoted_pattern, quoted_replacement])
+        Arel::Nodes::SqlLiteral.new(replace.to_sql)
+      end
+    end
+
+    def remove_foreign_key_without_error(*args)
+      remove_foreign_key(*args)
+    rescue ArgumentError
+    end
+
+    def sidekiq_queue_migrate(queue_from, to:)
+      while sidekiq_queue_length(queue_from) > 0
+        Sidekiq.redis do |conn|
+          conn.rpoplpush "queue:#{queue_from}", "queue:#{to}"
+        end
+      end
+    end
+
+    def sidekiq_queue_length(queue_name)
+      Sidekiq.redis do |conn|
+        conn.llen("queue:#{queue_name}")
+      end
+    end
+
+    def check_trigger_permissions!(table)
+      unless Grant.create_and_execute_trigger?(table)
+        dbname = ActiveRecord::Base.configurations[Rails.env]['database']
+        user = ActiveRecord::Base.configurations[Rails.env]['username'] || ENV['USER']
+
+        raise <<-EOF
+Your database user is not allowed to create, drop, or execute triggers on the
+table #{table}.
+
+If you are using PostgreSQL you can solve this by logging in to the GitLab
+database (#{dbname}) using a super user and running:
+
+    ALTER #{user} WITH SUPERUSER
+
+For MySQL you instead need to run:
+
+    GRANT ALL PRIVILEGES ON *.* TO #{user}@'%'
+
+Both queries will grant the user super user permissions, ensuring you don't run
+into similar problems in the future (e.g. when new tables are created).
+        EOF
+      end
+    end
+
+    # Bulk queues background migration jobs for an entire table, batched by ID range.
+    # "Bulk" meaning many jobs will be pushed at a time for efficiency.
+    # If you need a delay interval per job, then use `queue_background_migration_jobs_by_range_at_intervals`.
+    #
+    # model_class - The table being iterated over
+    # job_class_name - The background migration job class as a string
+    # batch_size - The maximum number of rows per job
+    #
+    # Example:
+    #
+    #     class Route < ActiveRecord::Base
+    #       include EachBatch
+    #       self.table_name = 'routes'
+    #     end
+    #
+    #     bulk_queue_background_migration_jobs_by_range(Route, 'ProcessRoutes')
+    #
+    # Where the model_class includes EachBatch, and the background migration exists:
+    #
+    #     class Gitlab::BackgroundMigration::ProcessRoutes
+    #       def perform(start_id, end_id)
+    #         # do something
+    #       end
+    #     end
+    def bulk_queue_background_migration_jobs_by_range(model_class, job_class_name, batch_size: BACKGROUND_MIGRATION_BATCH_SIZE)
+      raise "#{model_class} does not have an ID to use for batch ranges" unless model_class.column_names.include?('id')
+
+      jobs = []
+
+      model_class.each_batch(of: batch_size) do |relation|
+        start_id, end_id = relation.pluck('MIN(id), MAX(id)').first
+
+        if jobs.length >= BACKGROUND_MIGRATION_JOB_BUFFER_SIZE
+          # Note: This code path generally only helps with many millions of rows
+          # We push multiple jobs at a time to reduce the time spent in
+          # Sidekiq/Redis operations. We're using this buffer based approach so we
+          # don't need to run additional queries for every range.
+          BackgroundMigrationWorker.perform_bulk(jobs)
+          jobs.clear
+        end
+
+        jobs << [job_class_name, [start_id, end_id]]
+      end
+
+      BackgroundMigrationWorker.perform_bulk(jobs) unless jobs.empty?
+    end
+
+    # Queues background migration jobs for an entire table, batched by ID range.
+    # Each job is scheduled with a `delay_interval` in between.
+    # If you use a small interval, then some jobs may run at the same time.
+    #
+    # model_class - The table being iterated over
+    # job_class_name - The background migration job class as a string
+    # delay_interval - The duration between each job's scheduled time (must respond to `to_f`)
+    # batch_size - The maximum number of rows per job
+    #
+    # Example:
+    #
+    #     class Route < ActiveRecord::Base
+    #       include EachBatch
+    #       self.table_name = 'routes'
+    #     end
+    #
+    #     queue_background_migration_jobs_by_range_at_intervals(Route, 'ProcessRoutes', 1.minute)
+    #
+    # Where the model_class includes EachBatch, and the background migration exists:
+    #
+    #     class Gitlab::BackgroundMigration::ProcessRoutes
+    #       def perform(start_id, end_id)
+    #         # do something
+    #       end
+    #     end
+    def queue_background_migration_jobs_by_range_at_intervals(model_class, job_class_name, delay_interval, batch_size: BACKGROUND_MIGRATION_BATCH_SIZE)
+      raise "#{model_class} does not have an ID to use for batch ranges" unless model_class.column_names.include?('id')
+
+      model_class.each_batch(of: batch_size) do |relation, index|
+        start_id, end_id = relation.pluck('MIN(id), MAX(id)').first
+
+        # `BackgroundMigrationWorker.bulk_perform_in` schedules all jobs for
+        # the same time, which is not helpful in most cases where we wish to
+        # spread the work over time.
+        BackgroundMigrationWorker.perform_in(delay_interval * index, job_class_name, [start_id, end_id])
+      end
+    end
+  end
+end
+
+# rubocop:enable all
diff --git a/lib/mastodon/snowflake.rb b/lib/mastodon/snowflake.rb
new file mode 100644
index 000000000..219e323d4
--- /dev/null
+++ b/lib/mastodon/snowflake.rb
@@ -0,0 +1,162 @@
+# frozen_string_literal: true
+
+module Mastodon::Snowflake
+  DEFAULT_REGEX = /timestamp_id\('(?<seq_prefix>\w+)'/
+
+  class Callbacks
+    def self.around_create(record)
+      now = Time.now.utc
+
+      if record.created_at.nil? || record.created_at >= now || record.created_at == record.updated_at
+        yield
+      else
+        record.id = Mastodon::Snowflake.id_at(record.created_at)
+        tries     = 0
+
+        begin
+          yield
+        rescue ActiveRecord::RecordNotUnique
+          raise if tries > 100
+
+          tries     += 1
+          record.id += rand(100)
+
+          retry
+        end
+      end
+    end
+  end
+
+  class << self
+    # Our ID will be composed of the following:
+    # 6 bytes (48 bits) of millisecond-level timestamp
+    # 2 bytes (16 bits) of sequence data
+    #
+    # The 'sequence data' is intended to be unique within a
+    # given millisecond, yet obscure the 'serial number' of
+    # this row.
+    #
+    # To do this, we hash the following data:
+    # * Table name (if provided, skipped if not)
+    # * Secret salt (should not be guessable)
+    # * Timestamp (again, millisecond-level granularity)
+    #
+    # We then take the first two bytes of that value, and add
+    # the lowest two bytes of the table ID sequence number
+    # (`table_name`_id_seq). This means that even if we insert
+    # two rows at the same millisecond, they will have
+    # distinct 'sequence data' portions.
+    #
+    # If this happens, and an attacker can see both such IDs,
+    # they can determine which of the two entries was inserted
+    # first, but not the total number of entries in the table
+    # (even mod 2**16).
+    #
+    # The table name is included in the hash to ensure that
+    # different tables derive separate sequence bases so rows
+    # inserted in the same millisecond in different tables do
+    # not reveal the table ID sequence number for one another.
+    #
+    # The secret salt is included in the hash to ensure that
+    # external users cannot derive the sequence base given the
+    # timestamp and table name, which would allow them to
+    # compute the table ID sequence number.
+    def define_timestamp_id
+      return if already_defined?
+
+      connection.execute(<<~SQL)
+        CREATE OR REPLACE FUNCTION timestamp_id(table_name text)
+        RETURNS bigint AS
+        $$
+          DECLARE
+            time_part bigint;
+            sequence_base bigint;
+            tail bigint;
+          BEGIN
+            time_part := (
+              -- Get the time in milliseconds
+              ((date_part('epoch', now()) * 1000))::bigint
+              -- And shift it over two bytes
+              << 16);
+
+            sequence_base := (
+              'x' ||
+              -- Take the first two bytes (four hex characters)
+              substr(
+                -- Of the MD5 hash of the data we documented
+                md5(table_name ||
+                  '#{SecureRandom.hex(16)}' ||
+                  time_part::text
+                ),
+                1, 4
+              )
+            -- And turn it into a bigint
+            )::bit(16)::bigint;
+
+            -- Finally, add our sequence number to our base, and chop
+            -- it to the last two bytes
+            tail := (
+              (sequence_base + nextval(table_name || '_id_seq'))
+              & 65535);
+
+            -- Return the time part and the sequence part. OR appears
+            -- faster here than addition, but they're equivalent:
+            -- time_part has no trailing two bytes, and tail is only
+            -- the last two bytes.
+            RETURN time_part | tail;
+          END
+        $$ LANGUAGE plpgsql VOLATILE;
+      SQL
+    end
+
+    def ensure_id_sequences_exist
+      # Find tables using timestamp IDs.
+      connection.tables.each do |table|
+        # We're only concerned with "id" columns.
+        next unless (id_col = connection.columns(table).find { |col| col.name == 'id' })
+
+        # And only those that are using timestamp_id.
+        next unless (data = DEFAULT_REGEX.match(id_col.default_function))
+
+        seq_name = data[:seq_prefix] + '_id_seq'
+
+        # If we were on Postgres 9.5+, we could do CREATE SEQUENCE IF
+        # NOT EXISTS, but we can't depend on that. Instead, catch the
+        # possible exception and ignore it.
+        # Note that seq_name isn't a column name, but it's a
+        # relation, like a column, and follows the same quoting rules
+        # in Postgres.
+        connection.execute(<<~SQL)
+          DO $$
+            BEGIN
+              CREATE SEQUENCE #{connection.quote_column_name(seq_name)};
+            EXCEPTION WHEN duplicate_table THEN
+              -- Do nothing, we have the sequence already.
+            END
+          $$ LANGUAGE plpgsql;
+        SQL
+      end
+    end
+
+    def id_at(timestamp)
+      id  = timestamp.to_i * 1000 + rand(1000)
+      id  = id << 16
+      id += rand(2**16)
+      id
+    end
+
+    private
+
+    def already_defined?
+      connection.execute(<<~SQL).values.first.first
+        SELECT EXISTS(
+          SELECT * FROM pg_proc WHERE proname = 'timestamp_id'
+        );
+      SQL
+    end
+
+    def connection
+      ActiveRecord::Base.connection
+    end
+  end
+end
diff --git a/lib/mastodon/version.rb b/lib/mastodon/version.rb
index 8b692c29d..0f2fc5ac6 100644
--- a/lib/mastodon/version.rb
+++ b/lib/mastodon/version.rb
@@ -5,15 +5,15 @@ module Mastodon
     module_function
 
     def major
-      1
+      2
     end
 
     def minor
-      6
+      0
     end
 
     def patch
-      1
+      0
     end
 
     def pre
@@ -21,7 +21,7 @@ module Mastodon
     end
 
     def flags
-      ''
+      'rc2'
     end
 
     def to_a
diff --git a/lib/tasks/assets.rake b/lib/tasks/assets.rake
index 44896afc7..f60c1b9f2 100644
--- a/lib/tasks/assets.rake
+++ b/lib/tasks/assets.rake
@@ -10,7 +10,7 @@ end
 namespace :assets do
   desc 'Generate static pages'
   task :generate_static_pages do
-    render_static_page 'errors/500', layout: 'error', dest: Rails.root.join('public', '500.html')
+    render_static_page 'errors/500', layout: 'error', dest: Rails.root.join('public', 'assets', '500.html')
   end
 end
 
diff --git a/lib/tasks/db.rake b/lib/tasks/db.rake
index 7a055bf25..32039c31d 100644
--- a/lib/tasks/db.rake
+++ b/lib/tasks/db.rake
@@ -1,5 +1,36 @@
 # frozen_string_literal: true
 
+require_relative '../mastodon/snowflake'
+
+def each_schema_load_environment
+  # If we're in development, also run this for the test environment.
+  # This is a somewhat hacky way to do this, so here's why:
+  # 1. We have to define this before we load the schema, or we won't
+  #    have a timestamp_id function when we get to it in the schema.
+  # 2. db:setup calls db:schema:load_if_ruby, which calls
+  #    db:schema:load, which we define above as having a prerequisite
+  #    of this task.
+  # 3. db:schema:load ends up running
+  #    ActiveRecord::Tasks::DatabaseTasks.load_schema_current, which
+  #    calls a private method `each_current_configuration`, which
+  #    explicitly also does the loading for the `test` environment
+  #    if the current environment is `development`, so we end up
+  #    needing to do the same, and we can't even use the same method
+  #    to do it.
+
+  if Rails.env == 'development'
+    test_conf = ActiveRecord::Base.configurations['test']
+
+    if test_conf['database']&.present?
+      ActiveRecord::Base.establish_connection(:test)
+      yield
+      ActiveRecord::Base.establish_connection(Rails.env.to_sym)
+    end
+  end
+
+  yield
+end
+
 namespace :db do
   namespace :migrate do
     desc 'Setup the db or migrate depending on state of db'
@@ -16,4 +47,29 @@ namespace :db do
       end
     end
   end
+
+  # Before we load the schema, define the timestamp_id function.
+  # Idiomatically, we might do this in a migration, but then it
+  # wouldn't end up in schema.rb, so we'd need to figure out a way to
+  # get it in before doing db:setup as well. This is simpler, and
+  # ensures it's always in place.
+  Rake::Task['db:schema:load'].enhance ['db:define_timestamp_id']
+
+  # After we load the schema, make sure we have sequences for each
+  # table using timestamp IDs.
+  Rake::Task['db:schema:load'].enhance do
+    Rake::Task['db:ensure_id_sequences_exist'].invoke
+  end
+
+  task :define_timestamp_id do
+    each_schema_load_environment do
+      Mastodon::Snowflake.define_timestamp_id
+    end
+  end
+
+  task :ensure_id_sequences_exist do
+    each_schema_load_environment do
+      Mastodon::Snowflake.ensure_id_sequences_exist
+    end
+  end
 end
diff --git a/lib/tasks/emojis.rake b/lib/tasks/emojis.rake
index cd5e30e96..625a6e55d 100644
--- a/lib/tasks/emojis.rake
+++ b/lib/tasks/emojis.rake
@@ -17,7 +17,7 @@ namespace :emojis do
   task :generate do
     source = 'http://www.unicode.org/Public/emoji/5.0/emoji-test.txt'
     codes  = []
-    dest   = Rails.root.join('app', 'javascript', 'mastodon', 'emoji_map.json')
+    dest   = Rails.root.join('app', 'javascript', 'mastodon', 'features', 'emoji', 'emoji_map.json')
 
     puts "Downloading emojos from source... (#{source})"