python: Support application factories

Adds support for the app factory pattern to the Python language module.
A factory is a callable that returns a WSGI or ASGI application object.

Unit does not support passing arguments to factories.

Setting the `factory` option to `true` instructs Unit to treat the
configured `callable` as a factory.

For example:

    "my-app": {
        "type": "python",
        "path": "/srv/www/",
        "module": "hello",
        "callable": "create_app",
        "factory": true
    }

This is similar to other WSGI / ASGI servers. E.g.,

    $ uvicorn --factory hello:create_app
    $ gunicorn 'hello:create_app()'

The factory setting defaults to false.

Closes: https://github.com/nginx/unit/issues/1106
Link: <https://github.com/nginx/unit/pull/1336#issuecomment-2179381605>
[ Commit message - Dan / Minor code tweaks - Andrew ]
Signed-off-by: Andrew Clayton <a.clayton@nginx.com>

src/nxt_conf_validation.c

@@ -841,6 +841,11 @@ static nxt_conf_vldt_object_t  nxt_conf_vldt_python_members[] = {
         .type       = NXT_CONF_VLDT_STRING,
         .validator  = nxt_conf_vldt_targets_exclusive,
         .u.string   = "callable",
+    }, {
+        .name       = nxt_string("factory"),
+        .type       = NXT_CONF_VLDT_BOOLEAN,
+        .validator  = nxt_conf_vldt_targets_exclusive,
+        .u.string   = "factory",
     }, {
         .name       = nxt_string("prefix"),
         .type       = NXT_CONF_VLDT_STRING,
@@ -865,6 +870,9 @@ static nxt_conf_vldt_object_t  nxt_conf_vldt_python_target_members[] = {
     }, {
         .name       = nxt_string("callable"),
         .type       = NXT_CONF_VLDT_STRING,
+    }, {
+        .name       = nxt_string("factory"),
+        .type       = NXT_CONF_VLDT_BOOLEAN,
     }, {
         .name       = nxt_string("prefix"),
         .type       = NXT_CONF_VLDT_STRING,
@@ -883,6 +891,9 @@ static nxt_conf_vldt_object_t  nxt_conf_vldt_python_notargets_members[] = {
     }, {
         .name       = nxt_string("callable"),
         .type       = NXT_CONF_VLDT_STRING,
+    }, {
+        .name       = nxt_string("factory"),
+        .type       = NXT_CONF_VLDT_BOOLEAN,
     }, {
         .name       = nxt_string("prefix"),
         .type       = NXT_CONF_VLDT_STRING,

src/python/nxt_python.c

@@ -403,11 +403,13 @@ nxt_python_set_target(nxt_task_t *task, nxt_python_target_t *target,
     char              *callable, *module_name;
     PyObject          *module, *obj;
     nxt_str_t         str;
+    nxt_bool_t        is_factory = 0;
     nxt_conf_value_t  *value;
 
     static nxt_str_t  module_str = nxt_string("module");
     static nxt_str_t  callable_str = nxt_string("callable");
     static nxt_str_t  prefix_str = nxt_string("prefix");
+    static nxt_str_t  factory_flag_str = nxt_string("factory");
 
     module = obj = NULL;
 
@@ -449,7 +451,30 @@ nxt_python_set_target(nxt_task_t *task, nxt_python_target_t *target,
         goto fail;
     }
 
-    if (nxt_slow_path(PyCallable_Check(obj) == 0)) {
+    value = nxt_conf_get_object_member(conf, &factory_flag_str, NULL);
+    if (value != NULL) {
+        is_factory = nxt_conf_get_boolean(value);
+    }
+
+    if (is_factory) {
+        if (nxt_slow_path(PyCallable_Check(obj) == 0)) {
+            nxt_alert(task,
+                      "factory \"%s\" in module \"%s\" "
+                      "can not be called to fetch callable",
+                      callable, module_name);
+            goto fail;
+        }
+
+        obj = PyObject_CallObject(obj, NULL);
+        if (nxt_slow_path(PyCallable_Check(obj) == 0)) {
+            nxt_alert(task,
+                      "factory \"%s\" in module \"%s\" "
+                      "did not return callable object",
+                      callable, module_name);
+            goto fail;
+        }
+
+    } else if (nxt_slow_path(PyCallable_Check(obj) == 0)) {
         nxt_alert(task, "\"%s\" in module \"%s\" is not a callable object",
                   callable, module_name);
         goto fail;