feat: wait for meilisearch index creation to succeed

In `search.meilisearch.create_indexes`, we were not waiting for the
index creation tasks to complete. This was causing a potential race
condition, where the `create_indexes` function would fail because it
took a few seconds for the index creation to succeed.

See the relevant conversation here:
openedx/edx-platform#35743 (comment)

edxsearch/__init__.py

@@ -1,3 +1,3 @@
 """ Container module for testing / demoing search """
 
-__version__ = '4.1.0'
+__version__ = '4.1.1'

search/meilisearch.py

@@ -115,9 +115,22 @@ class MeilisearchEngine(SearchEngine):
     compliant with edx-search's ElasticSearchEngine.
     """
 
-    def __init__(self, index=None):
+    def __init__(self, index=None) -> None:
         super().__init__(index=index)
-        self.meilisearch_index = get_meilisearch_index(self.index_name)
+        self._meilisearch_index: t.Optional[meilisearch.index.Index] = None
+
+    @property
+    def meilisearch_index(self) -> meilisearch.index.Index:
+        """
+        Lazy load meilisearch index.
+        """
+        if self._meilisearch_index is None:
+            meilisearch_index_name = get_meilisearch_index_name(self.index_name)
+            meilisearch_client = get_meilisearch_client()
+            self._meilisearch_index = meilisearch_client.get_index(
+                meilisearch_index_name
+            )
+        return self._meilisearch_index
 
     @property
     def meilisearch_index_name(self):
@@ -211,7 +224,7 @@ def print_failed_meilisearch_tasks(count: int = 10):
         print(result)
 
 
-def create_indexes(index_filterables: dict[str, list[str]] = None):
+def create_indexes(index_filterables: t.Optional[dict[str, list[str]]] = None):
     """
     This is an initialization function that creates indexes and makes sure that they
     support the right facetting.
@@ -225,38 +238,68 @@ def create_indexes(index_filterables: dict[str, list[str]] = None):
     client = get_meilisearch_client()
     for index_name, filterables in index_filterables.items():
         meilisearch_index_name = get_meilisearch_index_name(index_name)
-        try:
-            index = client.get_index(meilisearch_index_name)
-        except meilisearch.errors.MeilisearchApiError as e:
-            if e.code != "index_not_found":
-                raise
-            client.create_index(
-                meilisearch_index_name, {"primaryKey": PRIMARY_KEY_FIELD_NAME}
-            )
-            # Get the index again
-            index = client.get_index(meilisearch_index_name)
+        index = get_or_create_meilisearch_index(client, meilisearch_index_name)
+        update_index_filterables(client, index, filterables)
+
+
+def get_or_create_meilisearch_index(
+    client: meilisearch.Client, index_name: str
+) -> meilisearch.index.Index:
+    """
+    Get an index. If it does not exist, create it.
 
-        # Update filterables if there are some new elements
-        if filterables:
-            existing_filterables = set(index.get_filterable_attributes())
-            if not set(filterables).issubset(existing_filterables):
-                all_filterables = list(existing_filterables.union(filterables))
-                index.update_filterable_attributes(all_filterables)
+    This will fail with a RuntimeError if we fail to create the index. It will fail with
+    a MeilisearchApiError in other failure cases.
+    """
+    try:
+        return client.get_index(index_name)
+    except meilisearch.errors.MeilisearchApiError as e:
+        if e.code != "index_not_found":
+            raise
+        task_info = client.create_index(
+            index_name, {"primaryKey": PRIMARY_KEY_FIELD_NAME}
+        )
+        wait_for_task_to_succeed(client, task_info)
+        # Get the index again
+        return client.get_index(index_name)
 
 
-def get_meilisearch_index(index_name: str):
+def update_index_filterables(
+    client: meilisearch.Client, index: meilisearch.index.Index, filterables: list[str]
+) -> None:
     """
-    Return a meilisearch index.
+    Make sure that the filterable fields of an index include the given list of fields.
 
-    Note that the index may not exist, and it will be created on first insertion.
-    ideally, the initialisation function `create_indexes` should be run first.
+    If existing fields are present, they are preserved.
     """
-    meilisearch_client = get_meilisearch_client()
-    meilisearch_index_name = get_meilisearch_index_name(index_name)
-    return meilisearch_client.index(meilisearch_index_name)
+    if not filterables:
+        return
+    existing_filterables = set(index.get_filterable_attributes())
+    if set(filterables).issubset(existing_filterables):
+        # all filterables fields are already present
+        return
+    all_filterables = list(existing_filterables.union(filterables))
+    task_info = index.update_filterable_attributes(all_filterables)
+    wait_for_task_to_succeed(client, task_info)
+
+
+def wait_for_task_to_succeed(
+    client: meilisearch.Client,
+    task_info: meilisearch.task.TaskInfo,
+    timeout_in_ms: int = 5000,
+) -> None:
+    """
+    Wait for a Meilisearch task to succeed. If it does not, raise RuntimeError.
+    """
+    task = client.wait_for_task(task_info.task_uid, timeout_in_ms=timeout_in_ms)
+    if task.status != "succeeded":
+        raise RuntimeError(f"Failed meilisearch task: {task}")
 
 
 def get_meilisearch_client():
+    """
+    Return a Meilisearch client with the right settings.
+    """
     return meilisearch.Client(MEILISEARCH_URL, api_key=MEILISEARCH_API_KEY)
 
 
@@ -332,7 +375,7 @@ def get_search_params(
     Return a dictionary of parameters that should be passed to the Meilisearch client
     `.search()` method.
     """
-    params = {"showRankingScore": True}
+    params: dict[str, t.Any] = {"showRankingScore": True}
 
     # Aggregation
     if aggregation_terms: