nixos postgresql-backup: add compression option

This option allows basic configuration of the compression technique
used in the backup script. Specifically it adds `none` and `zstd` as
new alternatives, keeping `gzip` as the default.

nixos/tests/postgresql.nix

@@ -43,6 +43,7 @@ let
     testScript = let
       backupName = if backup-all then "all" else "postgres";
       backupService = if backup-all then "postgresqlBackup" else "postgresqlBackup-postgres";
+      backupFileBase = "/var/backup/postgresql/${backupName}";
     in ''
       def check_count(statement, lines):
           return 'test $(sudo -u postgres psql postgres -tAc "{}"|wc -l) -eq {}'.format(
@@ -72,9 +73,32 @@ let
       with subtest("Backup service works"):
           machine.succeed(
               "systemctl start ${backupService}.service",
-              "zcat /var/backup/postgresql/${backupName}.sql.gz | grep '<test>ok</test>'",
+              "zcat ${backupFileBase}.sql.gz | grep '<test>ok</test>'",
               "ls -hal /var/backup/postgresql/ >/dev/console",
-              "stat -c '%a' /var/backup/postgresql/${backupName}.sql.gz | grep 600",
+              "stat -c '%a' ${backupFileBase}.sql.gz | grep 600",
+          )
+      with subtest("Backup service removes prev files"):
+          machine.succeed(
+              # Create dummy prev files.
+              "touch ${backupFileBase}.prev.sql{,.gz,.zstd}",
+              "chown postgres:postgres ${backupFileBase}.prev.sql{,.gz,.zstd}",
+
+              # Run backup.
+              "systemctl start ${backupService}.service",
+              "ls -hal /var/backup/postgresql/ >/dev/console",
+
+              # Since nothing has changed in the database, the cur and prev files
+              # should match.
+              "zcat ${backupFileBase}.sql.gz | grep '<test>ok</test>'",
+              "cmp ${backupFileBase}.sql.gz ${backupFileBase}.prev.sql.gz",
+
+              # The prev files with unused suffix should be removed.
+              "[ ! -f '${backupFileBase}.prev.sql' ]",
+              "[ ! -f '${backupFileBase}.prev.sql.zstd' ]",
+
+              # Both cur and prev file should only be accessible by the postgres user.
+              "stat -c '%a' ${backupFileBase}.sql.gz | grep 600",
+              "stat -c '%a' '${backupFileBase}.prev.sql.gz' | grep 600",
           )
       with subtest("Backup service fails gracefully"):
           # Sabotage the backup process
@@ -84,8 +108,8 @@ let
           )
           machine.succeed(
               "ls -hal /var/backup/postgresql/ >/dev/console",
-              "zcat /var/backup/postgresql/${backupName}.prev.sql.gz | grep '<test>ok</test>'",
-              "stat /var/backup/postgresql/${backupName}.in-progress.sql.gz",
+              "zcat ${backupFileBase}.prev.sql.gz | grep '<test>ok</test>'",
+              "stat ${backupFileBase}.in-progress.sql.gz",
           )
           # In a previous version, the second run would overwrite prev.sql.gz,
           # so we test a second run as well.
@@ -93,8 +117,8 @@ let
               "systemctl start ${backupService}.service",
           )
           machine.succeed(
-              "stat /var/backup/postgresql/${backupName}.in-progress.sql.gz",
-              "zcat /var/backup/postgresql/${backupName}.prev.sql.gz | grep '<test>ok</test>'",
+              "stat ${backupFileBase}.in-progress.sql.gz",
+              "zcat ${backupFileBase}.prev.sql.gz | grep '<test>ok</test>'",
           )