Add retries to taxonomy reading. (#205)

When we try to load a taxonomy multiple times concurrently, we run into
`FileExistsError`s as the various threads try to cache files at the same
location.

src/ferc_xbrl_extractor/arelle_interface.py

@@ -1,6 +1,7 @@
 """Abstract away interface to Arelle XBRL Library."""
 
 import io
+import time
 from pathlib import Path
 from typing import Literal
 
@@ -12,12 +13,22 @@
 from pydantic import BaseModel
 
 
-def _taxonomy_view(taxonomy_source: str | FileSource.FileSource):
+def _taxonomy_view(taxonomy_source: str | FileSource.FileSource, max_retries: int = 7):
     """Actually use Arelle to get a taxonomy and its relationships."""
     cntlr = Cntlr.Cntlr()
     cntlr.startLogging(logFileName="logToPrint")
     model_manager = ModelManager.initialize(cntlr)
-    taxonomy = ModelXbrl.load(model_manager, taxonomy_source)
+    for try_count in range(max_retries):
+        try:
+            cntlr.logger.debug(f"Try #{try_count}: {taxonomy_source=}")
+            taxonomy = ModelXbrl.load(model_manager, taxonomy_source)
+            continue
+        except FileExistsError as e:
+            if (try_count + 1) == max_retries:
+                raise e
+            backoff = 2 ** (try_count + 1)
+            cntlr.logger.warning(f"Failed try #{try_count}, retrying in {backoff}s")
+            time.sleep(backoff)
 
     view = ViewRelationshipSet(taxonomy, "taxonomy.json", "roles", None, None, None)
     view.view(XbrlConst.parentChild, None, None, None)

tests/integration/arelle_interface_test.py

@@ -0,0 +1,21 @@
+import concurrent.futures
+from unittest.mock import patch
+
+from arelle import Cntlr
+
+from ferc_xbrl_extractor.arelle_interface import load_taxonomy
+
+
+def test_concurrent_taxonomy_load(tmp_path):
+    cntlr = Cntlr.Cntlr()
+    cntlr.webCache.cacheDir = str(tmp_path)
+    cntlr.webCache.clear()
+    path = "https://eCollection.ferc.gov/taxonomy/form60/2022-01-01/form/form60/form-60_2022-01-01.xsd"
+    with patch("ferc_xbrl_extractor.arelle_interface.Cntlr.Cntlr", lambda: cntlr):
+        with concurrent.futures.ThreadPoolExecutor(max_workers=2) as executor:
+            futures = [executor.submit(load_taxonomy, path) for _ in range(2)]
+        done, _not_done = concurrent.futures.wait(
+            futures, timeout=10, return_when=concurrent.futures.ALL_COMPLETED
+        )
+    errored = {fut for fut in done if fut.exception()}
+    assert len(errored) == 0