Fix template error when iterating None value and fix params documenta…

…tion (#31078)

* Check if form value is iterable

* Update doc params list to array

* Add test render, prevent value mutaion in js

airflow/www/static/js/trigger.js

@@ -46,7 +46,7 @@ function updateJSONconf() {
             values[values.length] = lines[j].trim();
           }
         }
-        params[keyName] = values;
+        params[keyName] = values.length === 0 ? params[keyName] : values;
       } else if (elements[i].value.length === 0) {
         params[keyName] = null;
       } else if (

airflow/www/templates/airflow/trigger.html

@@ -80,9 +80,11 @@
     {% elif form_details.schema and "array" in form_details.schema.type %}
       <textarea class="form-control" name="element_{{ form_key }}" id="element_{{ form_key }}" valuetype="array" rows="6"
         {%- if not "null" in form_details.schema.type %} required=""{% endif -%}>
-        {%- for txt in form_details.value -%}
-          {{ txt }}{{ "\n" }}
-        {%- endfor -%}
+        {%- if form_details.value is sequence %}
+          {%- for txt in form_details.value -%}
+            {{ txt }}{{ "\n" }}
+          {%- endfor -%}
+        {% endif -%}
       </textarea>
     {% elif form_details.schema and "object" in form_details.schema.type %}
       <textarea class="form-control" name="element_{{ form_key }}" id="element_{{ form_key }}" valuetype="object" rows="6"

docs/apache-airflow/core-concepts/params.rst

@@ -189,7 +189,7 @@ The following features are supported in the Trigger UI Form:
       You can add the parameters ``minimum`` and ``maximum`` to restrict number range accepted.
     - ``boolean``: Generates a toggle button to be used as ``True`` or ``False``.
     - ``date``, ``datetime`` and ``time``: Generate date and/or time picker
-    - ``list``: Generates a HTML multi line text field, every line edited will be made into a string array as value
+    - ``array``: Generates a HTML multi line text field, every line edited will be made into a string array as value
     - ``object``: Generates a JSON entry field
     - Note: Per default if you specify a type, a field will be made required with input - because of JSON validation.
       If you want to have a field value being added optional only, you must allow JSON schema validation allowing null values via:

tests/www/views/test_views_trigger_dag.py

@@ -260,3 +260,29 @@ def test_viewer_cant_trigger_dag(app):
         resp = client.get(url, follow_redirects=True)
         response_data = resp.data.decode()
         assert "Access is Denied" in response_data
+
+
+def test_trigger_dag_params_array_value_none_render(admin_client, dag_maker, session, app, monkeypatch):
+    """
+    Test that textarea in Trigger DAG UI is pre-populated
+    with param value None and type ["null", "array"] set in DAG.
+    """
+    expected_conf = {"dag_param": None}
+    expected_dag_conf = json.dumps(expected_conf, indent=4).replace('"', """)
+    DAG_ID = "params_dag"
+    param = Param(
+        None,
+        type=["null", "array"],
+        minItems=0,
+    )
+    with monkeypatch.context() as m:
+        with dag_maker(dag_id=DAG_ID, serialized=True, session=session, params={"dag_param": param}):
+            EmptyOperator(task_id="task1")
+
+        m.setattr(app, "dag_bag", dag_maker.dagbag)
+        resp = admin_client.get(f"trigger?dag_id={DAG_ID}")
+
+    check_content_in_response(
+        f'<textarea style="display: none;" id="json_start" name="json_start">{expected_dag_conf}</textarea>',
+        resp,
+    )