[cards] bug fix with error card renders (#1893)

- Error cards didn't contain a reload token replacement because of which the UI keeps trying to poll for new cards.
- This is not the best situation because the UI will keep refreshing the card constantly until the UI catches hold of the card.
- The UI changes will obviously follow for situations where a data update might be published but a card doesn't end up getting saved because of some data store issue; This PR will fix the bug and ensures that if a error card gets created instead of an expected card then the UI still gets the right reload token.

metaflow/plugins/cards/card_cli.py

@@ -388,6 +388,22 @@ def wrapper(*args, **kwargs):
     return wrapper
 
 
+def _extract_reload_token(data, task, mf_card):
+    if "render_seq" not in data:
+        return "never"
+
+    if data["render_seq"] == "final":
+        # final data update should always trigger a card reload to show
+        # the final card, hence a different token for the final update
+        return "final"
+    elif mf_card.RELOAD_POLICY == mf_card.RELOAD_POLICY_ALWAYS:
+        return "render-seq-%s" % data["render_seq"]
+    elif mf_card.RELOAD_POLICY == mf_card.RELOAD_POLICY_NEVER:
+        return "never"
+    elif mf_card.RELOAD_POLICY == mf_card.RELOAD_POLICY_ONCHANGE:
+        return mf_card.reload_content_token(task, data)
+
+
 def update_card(mf_card, mode, task, data, timeout_value=None):
     """
     This method will be responsible for creating a card/data-update based on the `mode`.
@@ -432,31 +448,21 @@ def update_card(mf_card, mode, task, data, timeout_value=None):
             - `timeout_stack_trace` : stack trace of the function if it timed out.
     """
 
-    def _reload_token():
-        if "render_seq" not in data:
-            return "never"
-
-        if data["render_seq"] == "final":
-            # final data update should always trigger a card reload to show
-            # the final card, hence a different token for the final update
-            return "final"
-        elif mf_card.RELOAD_POLICY == mf_card.RELOAD_POLICY_ALWAYS:
-            return "render-seq-%s" % data["render_seq"]
-        elif mf_card.RELOAD_POLICY == mf_card.RELOAD_POLICY_NEVER:
-            return "never"
-        elif mf_card.RELOAD_POLICY == mf_card.RELOAD_POLICY_ONCHANGE:
-            return mf_card.reload_content_token(task, data)
-
     def _add_token_html(html):
         if html is None:
             return None
-        return html.replace(mf_card.RELOAD_POLICY_TOKEN, _reload_token())
+        return html.replace(
+            mf_card.RELOAD_POLICY_TOKEN,
+            _extract_reload_token(data=data, task=task, mf_card=mf_card),
+        )
 
     def _add_token_json(json_msg):
         if json_msg is None:
             return None
         return {
-            "reload_token": _reload_token(),
+            "reload_token": _extract_reload_token(
+                data=data, task=task, mf_card=mf_card
+            ),
             "data": json_msg,
             "created_on": time.time(),
         }
@@ -740,8 +746,16 @@ def create(
     #   3. `refresh` is implemented but it raises an exception. (We do nothing. Don't store anything.)
     #   4. `refresh` is implemented but it times out. (We do nothing. Don't store anything.)
 
+    def _render_error_card(stack_trace):
+        _card = error_card()
+        token = _extract_reload_token(data, task, _card)
+        return _card.render(
+            task,
+            stack_trace=stack_trace,
+        ).replace(mf_card.RELOAD_POLICY_TOKEN, token)
+
     if error_stack_trace is not None and mode != "refresh":
-        rendered_content = error_card().render(task, stack_trace=error_stack_trace)
+        rendered_content = _render_error_card(error_stack_trace)
     elif (
         rendered_info.is_implemented
         and rendered_info.timed_out
@@ -753,18 +767,15 @@ def create(
             "To increase the timeout duration for card rendering, please set the `timeout` parameter in the @card decorator. "
             "\nStack Trace : \n%s"
         ) % (timeout, rendered_info.timeout_stack_trace)
-        rendered_content = error_card().render(
-            task,
-            stack_trace=timeout_stack_trace,
-        )
+        rendered_content = _render_error_card(timeout_stack_trace)
     elif (
         rendered_info.is_implemented
         and rendered_info.data is None
         and render_error_card
         and mode != "refresh"
     ):
-        rendered_content = error_card().render(
-            task, stack_trace="No information rendered from card of type %s" % type
+        rendered_content = _render_error_card(
+            "No information rendered from card of type %s" % type
         )
     elif (
         not rendered_info.is_implemented
@@ -775,7 +786,7 @@ def create(
             "Card of type %s is a runtime time card with no `render_runtime` implemented. "
             "Please implement `render_runtime` method to allow rendering this card at runtime."
         ) % type
-        rendered_content = error_card().render(task, stack_trace=message)
+        rendered_content = _render_error_card(message)
 
     # todo : should we save native type for error card or error type ?
     if type is not None and re.match(CARD_ID_PATTERN, type) is not None:

metaflow/plugins/cards/card_modules/basic.py

@@ -542,11 +542,24 @@ class ErrorCard(MetaflowCard):
 
     type = "error"
 
+    RELOAD_POLICY = MetaflowCard.RELOAD_POLICY_ONCHANGE
+
     def __init__(self, options={}, components=[], graph=None):
         self._only_repr = True
         self._graph = None if graph is None else transform_flow_graph(graph)
         self._components = components
 
+    def reload_content_token(self, task, data):
+        """
+        The reload token will change when the component array has changed in the Metaflow card.
+        The change in the component array is signified by the change in the component_update_ts.
+        """
+        if task.finished:
+            return "final"
+        # `component_update_ts` will never be None. It is set to a default value when the `ComponentStore` is instantiated
+        # And it is updated when components added / removed / changed from the `ComponentStore`.
+        return "runtime-%s" % (str(data["component_update_ts"]))
+
     def render(self, task, stack_trace=None):
         RENDER_TEMPLATE = read_file(RENDER_TEMPLATE_PATH)
         JS_DATA = read_file(JS_PATH)