131 lines
4.4 KiB
Ruby
131 lines
4.4 KiB
Ruby
# frozen_string_literal: true
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module Mastodon::TimestampIds
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DEFAULT_REGEX = /timestamp_id\('(?<seq_prefix>\w+)'/
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class << self
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# Our ID will be composed of the following:
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# 6 bytes (48 bits) of millisecond-level timestamp
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# 2 bytes (16 bits) of sequence data
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#
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# The 'sequence data' is intended to be unique within a
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# given millisecond, yet obscure the 'serial number' of
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# this row.
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#
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# To do this, we hash the following data:
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# * Table name (if provided, skipped if not)
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# * Secret salt (should not be guessable)
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# * Timestamp (again, millisecond-level granularity)
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#
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# We then take the first two bytes of that value, and add
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# the lowest two bytes of the table ID sequence number
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# (`table_name`_id_seq). This means that even if we insert
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# two rows at the same millisecond, they will have
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# distinct 'sequence data' portions.
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#
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# If this happens, and an attacker can see both such IDs,
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# they can determine which of the two entries was inserted
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# first, but not the total number of entries in the table
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# (even mod 2**16).
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#
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# The table name is included in the hash to ensure that
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# different tables derive separate sequence bases so rows
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# inserted in the same millisecond in different tables do
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# not reveal the table ID sequence number for one another.
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#
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# The secret salt is included in the hash to ensure that
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# external users cannot derive the sequence base given the
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# timestamp and table name, which would allow them to
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# compute the table ID sequence number.
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def define_timestamp_id
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return if already_defined?
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connection.execute(<<~SQL)
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CREATE OR REPLACE FUNCTION timestamp_id(table_name text)
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RETURNS bigint AS
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$$
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DECLARE
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time_part bigint;
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sequence_base bigint;
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tail bigint;
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BEGIN
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time_part := (
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-- Get the time in milliseconds
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((date_part('epoch', now()) * 1000))::bigint
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-- And shift it over two bytes
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<< 16);
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sequence_base := (
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'x' ||
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-- Take the first two bytes (four hex characters)
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substr(
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-- Of the MD5 hash of the data we documented
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md5(table_name ||
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'#{SecureRandom.hex(16)}' ||
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time_part::text
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),
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1, 4
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)
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-- And turn it into a bigint
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)::bit(16)::bigint;
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-- Finally, add our sequence number to our base, and chop
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-- it to the last two bytes
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tail := (
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(sequence_base + nextval(table_name || '_id_seq'))
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& 65535);
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-- Return the time part and the sequence part. OR appears
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-- faster here than addition, but they're equivalent:
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-- time_part has no trailing two bytes, and tail is only
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-- the last two bytes.
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RETURN time_part | tail;
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END
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$$ LANGUAGE plpgsql VOLATILE;
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SQL
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end
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def ensure_id_sequences_exist
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# Find tables using timestamp IDs.
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connection.tables.each do |table|
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# We're only concerned with "id" columns.
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next unless (id_col = connection.columns(table).find { |col| col.name == 'id' })
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# And only those that are using timestamp_id.
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next unless (data = DEFAULT_REGEX.match(id_col.default_function))
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seq_name = data[:seq_prefix] + '_id_seq'
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# If we were on Postgres 9.5+, we could do CREATE SEQUENCE IF
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# NOT EXISTS, but we can't depend on that. Instead, catch the
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# possible exception and ignore it.
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# Note that seq_name isn't a column name, but it's a
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# relation, like a column, and follows the same quoting rules
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# in Postgres.
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connection.execute(<<~SQL)
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DO $$
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BEGIN
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CREATE SEQUENCE #{connection.quote_column_name(seq_name)};
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EXCEPTION WHEN duplicate_table THEN
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-- Do nothing, we have the sequence already.
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END
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$$ LANGUAGE plpgsql;
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SQL
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end
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end
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private
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def already_defined?
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connection.execute(<<~SQL).values.first.first
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SELECT EXISTS(
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SELECT * FROM pg_proc WHERE proname = 'timestamp_id'
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);
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SQL
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end
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def connection
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ActiveRecord::Base.connection
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end
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end
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end
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