Hardening Applications with Property Tests

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Hardening a Library with Property Tests

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Parker Selbert @sorentwo Soren / Dscout

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Adopting Elixir and a library named Kiq

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Nobody likes ﬁnding bugs in production.

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Kiq Oban Initially, an experiment to model queues as streams.

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Starting over meant building up new example tests, with many possible regressions.

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Property-Based Testing • PropEr • StreamData  • Proper Book

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Properties Aspects of code that hold for a set of randomized test data Functions that dynamically generate inﬁnite streams of data Generators

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EX1 Reliable Execution

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What is absolute?

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What is absolute? All inserted jobs are attempted at least once.

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property "inserted jobs are executed at least once" do check all jobs <- list_of(job()) do for %{id: id, args: %{"ref" => ref}} <- Oban.insert_all(jobs) do assert_receive {:ok, ^ref} assert %{attempt: 1, attempted_at: %DateTime{}} = Repo.get(Job, job.id) end end end

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defp job do gen all queue <- member_of(~w(alpha beta gamma delta)), ref <- integer() do Job.new(%{ref: ref}, queue: queue, worker: Worker) end end

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1) property inserted jobs are executed at least once (Oban.ExecutingTest) Failed with generated values (after 13 successful runs): * Clause: jobs <- list_of(job()) Generated: [#Ecto.Changeset] Assertion with == failed code: assert job.attempt == 1 left: 0 right: 1

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✂ Make Oban Work…

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. Finished in 1.4 seconds 1 property, 0 tests, 0 failures

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Additional Absolutes •Record errors on failure •Schedule failed jobs for retry •Discard jobs without more attempts

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case job.state do "completed" -> assert job.completed_at "discarded" -> refute job.completed_at assert job.discarded_at "retryable" -> refute job.completed_at assert DateTime.compare(job.scheduled_at, DateTime.utc_now()) == :gt assert length(job.errors) > 0 assert [%{"attempt" => 1, "at" => _, "error" => _} | _] = job.errors "scheduled" -> refute job.completed_at assert DateTime.compare(job.scheduled_at, DateTime.utc_now()) == :gt assert job.max_attempts > 1 end

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defp job do gen all queue <- member_of(~w(alpha beta gamma delta)), action <- member_of(~w(OK DISCARD ERROR EXIT FAIL SNOOZE)), max_attempts <- integer(1..20), ref <- integer() do args = %{ref: ref, action: action} opts = [queue: queue, max_attempts: max_attempts, worker: Worker] Job.new(args, opts) end end

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1 generator  1 semi-stateful property test ~2000 lines of library code

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EX2 Unique Jobs

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The Blog Recipe •Partial Index •Insert Helper •Manual Implementation

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What is absolute?

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What is absolute? Duplicate jobs are never inserted, regardless of concurrency.

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property "preventing multiple inserts” do check all args <- arg_map(), runs <- integer(2..5) do fun = fn -> unique_insert!(args) end ids = 1..runs |> Enum.map(fn _ -> Task.async(fun) end) |> Enum.map(&Task.await/1) |> Enum.map(fn %Job{id: id} -> id end) |> Enum.reject(&is_nil/1) |> Enum.uniq() assert 1 == length(ids) end end

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def arg_map, do: map_of(arg_key(), arg_val()) def arg_key, do: one_of([integer(), string(:ascii)]) def arg_val, do: one_of([integer(), float(), string(:ascii), list_of(integer())])

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defmodule UniqueWorker do use Oban.Worker, unique: [period: 30] @impl Worker def perform(_job), do: :ok end defp unique_insert!(args, opts \\ []) do args |> UniqueWorker.new(opts) |> Oban.insert!() end

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1) property preventing multiple inserts (Oban.UniquenessTest) Failed with generated values (after 0 successful runs): * Clause: args <- arg_map() Generated: %{"" => 0} * Clause: runs <- integer(2..5) Generated: 2 Assertion with == failed code: assert 1 == length(ids) left: 1 right: 4

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✂ Prevent concurrent writes…

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. Finished in 0.7 seconds 1 property, 0 tests, 0 failures

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3 generators  1 property test 2+ concurrent inserts 0 reported† bugs † if you don’t count that preﬁx thing…

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EX3 Safe Migrations

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Progressive Migrations, v1..v8 •Tables •Columns •Functions •Triggers •Indexes

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What is absolute?

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What is absolute? Migrating up or down between any two versions is successful.

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@base_migration 100_000  @current_version 8   property "migrating up and down between arbitrary versions" do check all up <- integer(1..@current_version), down <- integer(1..(@current_version - 1)) do Application.put_env(:oban, :up_version, up) Application.put_env(:oban, :down_version, down) assert :ok = Ecto.Migrator.up(Repo, @base_migration, StepMigration) assert :ok = Ecto.Migrator.down(Repo, @base_migration, StepMigration) clear_migrated() end end

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defmodule StepMigration do use Ecto.Migration def up do up_version = Application.get_env(:oban, :up_version) Oban.Migrations.up(version: up_version) end def down do down_version = Application.get_env(:oban, :up_version)   Oban.Migrations.down(version: down_version) end end

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1) property migrating up and down between arbitrary versions (Oban.MigratingTest) Failed with generated values (after 1 successful run): * Clause: up <- integer(2..current_version()) Generated: 8 * Clause: down <- integer(1..current_version() - 1) Generated: 1 got exception: ** (Postgrex.Error) ERROR 42P07 (duplicate_table) relation already exists

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✂ Ensure up/down safety…

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. Finished in 5.7 seconds 1 property, 0 tests, 0 failures

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iex> length(for x <- 1..8, y <- 1..7, do: {x, y}) 56 ⏲ How many permutations are there?

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@base_migration 100_000  @current_version 8   test "migrating up and down between arbitrary versions" do for up <- 1..@current_version, down <- 1..(@current_version - 1) do Application.put_env(:oban, :up_version, up) Application.put_env(:oban, :down_version, down) assert :ok = Ecto.Migrator.up(Repo, @base_migration, StepMigration) assert :ok = Ecto.Migrator.down(Repo, @base_migration, StepMigration) clear_migrated() end end

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. Finished in 1.7 seconds 1 test, 0 failures, 0 excluded

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1 test for up 1 test for up+down  120 permutations

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EX4 Cron Parsing

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Barrier to Adoption •We depend on periodic jobs… •Periodic jobs depends on unique jobs… •Unique jobs shipped…

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* 0-5,*/3 7-14 */2 MON,TUE “At every minute past every hour from 0 through 5 and every 3rd hour on every day-of-month from 7 through 14 and on Monday and Tuesday in every 2nd month”

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property "literal values and aliases are parsed" do check all minutes <- expression(), hours <- integer(0..23), days <- integer(1..31), months <- months(), weekdays <- weekdays(), spaces <- spaces() do spacing = :erlang.iolist_to_binary(spaces) [minutes, hours, days, months, weekdays] |> Enum.join(spacing) |> Cron.parse!() end end

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property “valid cron expressions are parsed" do check all minutes <- expression(), hours <- integer(0..23), days <- integer(1..31), months <- months(), weekdays <- weekdays(), spaces <- spaces() do spacing = :erlang.iolist_to_binary(spaces) [minutes, hours, days, months, weekdays] |> Enum.join(spacing) |> Cron.parse!() end end

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defp months do one_of([integer(1..12), constant("JAN"), ...]) end defp weekdays do one_of([integer(0..6), constant("MON"), ...]) end defp spaces do list_of(one_of([constant(" "), constant("\t")]), min_length: 1) end

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defp expression do one_of([ constant("*"), map(integer(1..59), &"*/#{&1}"), map(integer(1..58), &"#{&1}-#{&1 + 1}"), map(integer(1..57), &"#{&1}-#{&1 + 2}/2"), list_of(integer(0..59), length: 1..10) ]) end

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property "literal values and aliases are parsed" do check all minutes <- integer(0..59), hours <- integer(0..23), days <- integer(1..31), months <- months(), weekdays <- weekdays(), spaces <- spaces() do spacing = :erlang.iolist_to_binary(spaces) [minutes, hours, days, months, weekdays] |> Enum.join(spacing) |> Cron.parse!() end end

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1) property valid expressions and aliases are parsed (Oban.CronTest) ** (ExUnitProperties.Error) failed with generated values (after 1 successful run): * Clause: minutes <- integer(0..59) Generated: 44 * Clause: hours <- integer(0..23) Generated: 14 * Clause: days <- integer(1..31) Generated: 5 * Clause: months <- months() Generated: "JUL" * Clause: weekdays <- weekdays() Generated: 0 * Clause: spaces <- spaces() Generated: [" ", " "] got exception: ** (ArgumentError) expected string "*" or ASCII character in the range '0' to '9'...

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✂ Write and Reﬁne Cron Parser…

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. Finished in 0.1 seconds 1 property, 0 tests, 0 failures

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test "parsing expressions that are out of bounds fails" do invalid_expressions = [ "60 * * * *", "* 24 * * *", "* * 32 * *", "* * * 13 *", "* * * * 7", "*/0 * * * *", "ONE * * * *", "* * * jan *", "* * * * sun" ] for expression <- invalid_expressions, do: assert_unparsable(expression) end defp assert_unparsable(expression) do assert_raise ArgumentError, fn -> Cron.parse!(expression) end end

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1 property for valid expressions 1 test for invalid expressions  1 reported bug

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Look for absolute truths. Once you understand them the properties and generators fall out naturally.

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Write fewer, more expressive tests. Not just for your libraries, but your applications too.

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