Fix Postgres data volume path in Breeze

dev/breeze/src/airflow_breeze/params/shell_params.py

@@ -595,6 +595,13 @@ def postgres_driver(self) -> str:
             return "psycopg2"
         return "psycopg"
 
+    @property
+    def postgres_data_volume_path(self) -> str:
+        """The ``VOLUME`` the postgres image declares; it moved from ``data/`` to its parent in 18."""
+        if self.backend == "postgres" and int(self.postgres_version) >= 18:
+            return "/var/lib/postgresql"
+        return "/var/lib/postgresql/data"
+
     @property
     def env_variables_for_docker_commands(self) -> dict[str, str]:
         """
@@ -718,6 +725,7 @@ def env_variables_for_docker_commands(self) -> dict[str, str]:
         _set_var(_env, "NUM_RUNS", self.num_runs)
         _set_var(_env, "ONLY_MIN_VERSION_UPDATE", self.only_min_version_update)
         _set_var(_env, "DISTRIBUTION_FORMAT", self.distribution_format)
+        _set_var(_env, "POSTGRES_DATA_VOLUME_PATH", self.postgres_data_volume_path)
         _set_var(_env, "POSTGRES_DRIVER", self.postgres_driver)
         _set_var(_env, "POSTGRES_HOST_PORT", None, POSTGRES_HOST_PORT)
         _set_var(_env, "POSTGRES_VERSION", self.postgres_version)

dev/breeze/tests/test_shell_params.py

@@ -266,6 +266,24 @@ def test_worktree_label_path_is_derived_from_checkout(tmp_path, monkeypatch, lin
             {"POSTGRES_DRIVER": "psycopg"},
             id="POSTGRES_DRIVER stays psycopg when installing from a GitHub branch",
         ),
+        pytest.param(
+            {},
+            {"backend": "postgres", "postgres_version": "17"},
+            {"POSTGRES_DATA_VOLUME_PATH": "/var/lib/postgresql/data"},
+            id="POSTGRES_DATA_VOLUME_PATH is the data directory up to Postgres 17",
+        ),
+        pytest.param(
+            {},
+            {"backend": "postgres", "postgres_version": "18"},
+            {"POSTGRES_DATA_VOLUME_PATH": "/var/lib/postgresql"},
+            id="POSTGRES_DATA_VOLUME_PATH is its parent from Postgres 18",
+        ),
+        pytest.param(
+            {},
+            {"backend": "none", "postgres_version": ""},
+            {"POSTGRES_DATA_VOLUME_PATH": "/var/lib/postgresql/data"},
+            id="POSTGRES_DATA_VOLUME_PATH ignores the empty version of non-postgres backends",
+        ),
     ],
 )
 def test_shell_params_to_env_var_conversion(
@@ -318,6 +336,16 @@ def test_pythonwarnings_is_forwarded_by_the_compose_base_file():
     assert "PYTHONWARNINGS" in base_compose_file["services"]["airflow"]["environment"]
 
 
+def test_postgres_data_volume_is_mounted_at_the_image_volume_path():
+    backend_compose_file = yaml.safe_load(
+        (SCRIPTS_CI_DOCKER_COMPOSE_PATH / "backend-postgres.yml").read_text()
+    )
+    assert (
+        "postgres-data-volume:${POSTGRES_DATA_VOLUME_PATH:-/var/lib/postgresql/data}"
+        in backend_compose_file["services"]["postgres"]["volumes"]
+    )
+
+
 @pytest.mark.parametrize(
     ("include_pycache_volume", "expected_prefix", "expected_dont_write"),
     [(True, PYCACHE_PREFIX_IN_CONTAINER, ""), (False, "", "true")],

scripts/ci/docker-compose/backend-postgres.yml

@@ -36,7 +36,7 @@ services:
       - POSTGRES_DB=airflow
       - POSTGRES_HOST_AUTH_METHOD=password
     volumes:
-      - postgres-data-volume:/var/lib/postgresql/{$POSTGRES_VERSION}/docker
+      - postgres-data-volume:${POSTGRES_DATA_VOLUME_PATH:-/var/lib/postgresql/data}
     healthcheck:
       test: ["CMD", "psql", "-h", "localhost", "-U", "postgres", "-c", "select 1", "airflow"]
       interval: 10s