tests: Add tests for python application factories

Add the following tests cases:

1. When "factory" key is used inside the "targets" option.
2. When "factory" key is used at the root level of python application
   config.
3. When factory returns invalid callable or When factory is invalid
   callable

Link: <#1336>
[ Commit subject & message formatting tweaks - Andrew ]
Signed-off-by: Andrew Clayton <a.clayton@nginx.com>

test/python/factory/wsgi.py

@@ -0,0 +1,23 @@
+def wsgi_a(env, start_response):
+    start_response("200", [("Content-Length", "1")])
+    return [b"1"]
+
+
+def wsgi_b(env, start_response):
+    start_response("200", [("Content-Length", "1")])
+    return [b"2"]
+
+
+def wsgi_a_factory():
+    return wsgi_a
+
+
+def wsgi_b_factory():
+    return wsgi_b
+
+
+wsgi_invalid_callable = None
+
+
+def wsgi_factory_returning_invalid_callable():
+    return wsgi_invalid_callable

test/test_python_factory.py

@@ -0,0 +1,140 @@
+from unit.applications.lang.python import ApplicationPython
+from unit.option import option
+
+prerequisites = {"modules": {"python": "all"}}
+
+client = ApplicationPython()
+
+
+def test_python_factory_targets():
+    python_dir = f"{option.test_dir}/python"
+
+    assert "success" in client.conf(
+        {
+            "listeners": {
+                "*:8080": {"pass": "applications/targets/1"},
+                "*:8081": {"pass": "applications/targets/2"},
+                "*:8082": {"pass": "applications/targets/factory-1"},
+                "*:8083": {"pass": "applications/targets/factory-2"},
+            },
+            "applications": {
+                "targets": {
+                    "type": client.get_application_type(),
+                    "working_directory": f"{python_dir}/factory/",
+                    "path": f"{python_dir}/factory/",
+                    "targets": {
+                        "1": {
+                            "module": "wsgi",
+                            "callable": "wsgi_a",
+                            "factory": False,
+                        },
+                        "2": {
+                            "module": "wsgi",
+                            "callable": "wsgi_b",
+                            "factory": False,
+                        },
+                        "factory-1": {
+                            "module": "wsgi",
+                            "callable": "wsgi_a_factory",
+                            "factory": True,
+                        },
+                        "factory-2": {
+                            "module": "wsgi",
+                            "callable": "wsgi_b_factory",
+                            "factory": True,
+                        },
+                    },
+                }
+            },
+        }
+    )
+
+    resp = client.get(port=8080)
+    assert resp["status"] == 200
+    assert resp["body"] == "1"
+
+    resp = client.get(port=8081)
+    assert resp["status"] == 200
+    assert resp["body"] == "2"
+
+    resp = client.get(port=8082)
+    assert resp["status"] == 200
+    assert resp["body"] == "1"
+
+    resp = client.get(port=8083)
+    assert resp["status"] == 200
+    assert resp["body"] == "2"
+
+
+def test_python_factory_without_targets():
+    python_dir = f"{option.test_dir}/python"
+
+    assert "success" in client.conf(
+        {
+            "listeners": {
+                "*:8080": {"pass": "applications/python-app-factory"},
+                "*:8081": {"pass": "applications/python-app"},
+            },
+            "applications": {
+                "python-app-factory": {
+                    "type": client.get_application_type(),
+                    "working_directory": f"{python_dir}/factory/",
+                    "path": f"{python_dir}/factory/",
+                    "module": "wsgi",
+                    "callable": "wsgi_a_factory",
+                    "factory": True,
+                },
+                "python-app": {
+                    "type": client.get_application_type(),
+                    "working_directory": f"{python_dir}/factory/",
+                    "path": f"{python_dir}/factory/",
+                    "module": "wsgi",
+                    "callable": "wsgi_b",
+                    "factory": False,
+                },
+            },
+        }
+    )
+
+    resp = client.get(port=8080)
+    assert resp["status"] == 200
+    assert resp["body"] == "1"
+
+    resp = client.get(port=8081)
+    assert resp["status"] == 200
+    assert resp["body"] == "2"
+
+
+def test_python_factory_invalid_callable_value(skip_alert):
+    skip_alert(
+        r"failed to apply new conf",
+        r"did not return callable object",
+        r"can not be called to fetch callable",
+    )
+    python_dir = f"{option.test_dir}/python"
+
+    invalid_callable_values = [
+        "wsgi_factory_returning_invalid_callable",
+        "wsgi_invalid_callable",
+    ]
+
+    for callable_value in invalid_callable_values:
+        assert "error" in client.conf(
+            {
+                "listeners": {"*:8080": {"pass": "applications/targets/1"}},
+                "applications": {
+                    "targets": {
+                        "type": client.get_application_type(),
+                        "working_directory": f"{python_dir}/factory/",
+                        "path": f"{python_dir}/factory/",
+                        "targets": {
+                            "1": {
+                                "module": "wsgi",
+                                "callable": callable_value,
+                                "factory": True,
+                            },
+                        },
+                    }
+                },
+            }
+        )