Wednesday, August 16

Using the PostgreSQL TAP framework in extensions

Support for using the TAP protocol to run extended regression tests was added to PostgreSQL back in 9.4 with the adoption of Perl’s prove tool and Test::More to test initdb, pg_basebackup, etc.

Since then the TAP-based tests have been greatly expanded, particularly with the advent of the src/test/recovery tests and the PostgresNode module in PostgreSQL 9.6. PostgreSQL now comes with a built-in test harness for easily starting up postgres instances, creating and restoring backups for replication, setting up streaming, and lots more.

You can now use this to test your extensions.

pg_regress and its limitations

Extensions have long supported pg_regress based tests. Just drop the test scripts in sql/. Put the expected results in expected/. List the test names (sans directory and file extension) in your Makefile‘s REGRESS target. Run make check.

This is great as far as it goes, but it falls short when:

  • You need to test with multiple different server configurations that require a server restart;
  • You need to test replication, failover, or run tests on a read-replica;
  • You need to run client programs like pg_recvlogical, pg_basebackup, pg_dump etc in your tests;
  • You need to test behaviour that isn’t always exactly the same each time you run it;
  • You can’t avoid outputting things like dates, transaction IDs, or other data that varies for each test run;
  • Different postgres versions require tests with quite different inputs and/or outputs.

Even when it’s posible to use pg_regress, a TAP based test may be more convenient. It can be too much hassle to use conditionals in the makefile and multiple pg_regress. Using alternate output files is confusing and error-prone, and adds maintenance burden.

TAP helps you write integration tests. It’s great. Use it.

How to use TAP and Test::More in extensions

Make sure your PostgreSQL install was configured --with-tap-tests when it was compiled. This is not (yet) the default.

Add the following to your Makefile, remembering to indent with tabs:

prove_installcheck: install
        rm -rf $(CURDIR)/tmp_check/log
        cd $(srcdir) && TESTDIR='$(CURDIR)' PATH="$(bindir):$$PATH" PGPORT='6$(DEF_PGPORT)' PG_REGRESS='$(top_builddir)/src/test/regress/pg_regress' $(PROVE) $(PG_PROVE_FLAGS) $(PROVE_FLAGS) $(if $(PROVE_TESTS),$(PROVE_TESTS),t/*.pl)
 
installcheck: prove_installcheck

Now create a directory with the name t/ in the same directory as your Makefile.

There, add Perl scripts to exercise your module’s functionality. Here’s a trivial example taken from src/test/recovery/t/001_stream_rep.pl in the PostgreSQL source tree and trimmed:

#!/usr/bin/env perl
use strict;
use warnings;
use Cwd;
use Config;
use TestLib;
use Test::More tests => 1;
# Initialize master node
my $node_master = get_new_node('master');
$node_master->init(allows_streaming => 1);
$node_master->start;
my $backup_name = 'my_backup';

# Take backup
$node_master->backup($backup_name);

# Create streaming standby linking to master
my $node_standby_1 = get_new_node('standby_1');
$node_standby_1->init_from_backup($node_master, $backup_name,
        has_streaming => 1);
$node_standby_1->append_conf('postgresql.conf', qq[
hot_standby_feedback = on
]);
$node_standby_1->start;

is(
  $node_standby_1->safe_psql('postgres', 'SELECT pg_is_in_recovery()'),
  't',
  'replica is in recovery'
);

Run it with

make -s installcheck_prove

It will print a results summary:

$ make -s prove_installcheck
t/test.pl .. 
1..1
ok 1 - replica is in recovery
ok
All tests successful.

Details – server logs, a more detailed test log, etc – will be output in tmp_check/.

If you got

TAP tests not enabled

instead, recompile PostgreSQL --with-tap-tests, install, and try again.

The test isn’t that exciting, but it shows the sorts of things you can do that simply aren’t possible with pg_regress. You can find much more interesting examples in PostgreSQL’s recovery test suite and elsewhere in the source tree.

Extension authors can also write modules to use to share functionality across tests, which is well worth doing for bigger test suites. Ahaha

The TAP framework has made developing for PostgreSQL massively easier and it’s now aiding development of complex extensions like BDR and pglogical too. I strongly recommend getting a handle on it.

The main downside right now is that PGXS doesn’t know how to make a proper temp-install within an extension’s sub-directory. You have to install the extension to test it. This is no different to PGXS‘s support for make installcheck, though.

But, perl?

PostgreSQL’s TAP tests use Perl. But because it’s the “test anything protocol”, you don’t have to use Perl to write your extension’s tests. You’re free to use any tools and languages you want … but if you don’t use Perl, you won’t get the goodies the test framework bundles for Perl like PostgresNode, so Perl is usually the path of least resistance.

In-core tests are required to run on Perl 5.8 using only core modules + the IPC::Run module. No such restrictions apply to out-of-core modules, so you can at lest make your Perl code nicer to write and test.

Honestly, once you get used to it writing tests in Perl isn’t so bad. Make friends with Devel::Trace, use the test log files, the diag, note and explain Test::More commands, and make sure to use Carp. You’ll forget you’re writing Perl in no time.

If you really can’t stand the idea, you can always write some Perl glue to set up your required server environment then run your test scripts with the tools and languages of your choice. So long as they can produce TAP-compatible output, all will be well.

Future improvements

I’d quite like to backport the new PostgresNode.pm stuff to PostgreSQL 9.4, where the TAP support first appeared. This would make extension testing a lot easier.

PGXS could use true support for running the prove tests like it has for pg_regress with REGRESS=.

Thanks!

Huge credit for making this possible goes to Peter Eisentraut, Michael Paquier and Álvaro Herrera plus numerous reviewers and contributors.

Thanks to Tom Lane for applying the patches to install PostgresNode.pm etc, so we can start using this in extensions.

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