fix: add postgres.dump_db() function

pyproject.toml

@@ -44,6 +44,7 @@ tests = ["pytest-cov", "tox"]
 [project.urls]
 Homepage = "https://wuttaproject.org/"
 Repository = "https://forgejo.wuttaproject.org/wutta/wuttamess"
+Issues = "https://forgejo.wuttaproject.org/wutta/wuttamess/issues"
 Changelog = "https://forgejo.wuttaproject.org/wutta/wuttamess/src/branch/master/CHANGELOG.md"
 
 

src/wuttamess/postgres.py

@@ -152,3 +152,34 @@ def drop_db(c, name, checkfirst=True):
     """
     if not checkfirst or db_exists(c, name):
         c.sudo(f'dropdb {name}', user='postgres')
+
+
+def dump_db(c, name):
+    """
+    Dump a PostgreSQL database to file.
+
+    This uses the ``pg_dump`` and ``gzip`` commands to produce a
+    compressed SQL dump.  The filename returned is based on the
+    ``name`` provided, e.g. ``mydbname.sql.gz``.
+
+    :param c: Fabric connection.
+
+    :param name: Name of the database to dump.
+
+    :returns: Base name of the output file.  We only return the
+       filename and not the path, since the file is expected to exist
+       in the connected user's home folder.
+    """
+    sql_name = f'{name}.sql'
+    gz_name = f'{sql_name}.gz'
+    tmp_name = f'/tmp/{gz_name}'
+
+    # TODO: when pg_dump fails the command still succeeds!  (would this work?)
+    #cmd = f'set -e && pg_dump {name} | gzip -c > {tmp_name}'
+    cmd = f'pg_dump {name} | gzip -c > {tmp_name}'
+
+    c.sudo(cmd, user='postgres')
+    c.run(f"cp {tmp_name} {gz_name}")
+    c.run(f"rm {tmp_name}")
+
+    return gz_name

tests/test_postgres.py

@@ -122,3 +122,13 @@ class TestDropDb(TestCase):
             mod.drop_db(c, 'foo')
             db_exists.assert_called_once_with(c, 'foo')
             c.sudo.assert_not_called()
+
+
+class TestDumpDb(TestCase):
+
+    def test_basic(self):
+        c = MagicMock()
+        result = mod.dump_db(c, 'foo')
+        self.assertEqual(result, 'foo.sql.gz')
+        c.sudo.assert_called_once_with('pg_dump foo | gzip -c > /tmp/foo.sql.gz', user='postgres')
+        c.run.assert_called_with('rm /tmp/foo.sql.gz')