Only download mermaid.js if MERMAID_USE_CDN is False

This is a follow-up to
#20

#21 added a CDN feature,
which means the added script tag will point to the CDN version of
mermaid.js. But it still tries to download a copy to the package's
static directory in AppConfig.ready.

Because this isn't necessary in CDN mode and causes us a PermissionError
during deployment, I'm proposing we only download in non-CDN mode.

Author: maiksprenger

src/django_mermaid/apps.py

@@ -7,18 +7,24 @@
 from .templatetags import DEFAULT_VERSION
 from .templatetags import MERMAID_CDN
 
+def download_if_necessary(version):
+    """Download mermaid.js from CDN if not already present"""
+    cdn = "https://cdnjs.cloudflare.com/ajax/libs/mermaid/%s/mermaid.min.js" % version
+    current_dir = pathlib.Path(__file__).parent
+    static_dir = current_dir / "static"
+    mermaid_dir = static_dir / "mermaid" / version
+    mermaid_js = mermaid_dir / "mermaid.js"
+    if not mermaid_js.exists() or \
+            mermaid_js.stat().st_size == 0:
+        mermaid_dir.mkdir(parents=True, exist_ok=True)
+        urlretrieve(MERMAID_CDN % version, str(mermaid_js))
+
 
 class MermaidConfig(AppConfig):
     name = "django_mermaid"
 
     def ready(self):
-        """Download mermaid.js from CDN if not already present"""
-        version = getattr(settings, "MERMAID_VERSION", DEFAULT_VERSION)
-        current_dir = pathlib.Path(__file__).parent
-        static_dir = current_dir / "static"
-        mermaid_dir = static_dir / "mermaid" / version
-        mermaid_js = mermaid_dir / "mermaid.js"
-        if not mermaid_js.exists() or \
-               mermaid_js.stat().st_size == 0:
-            mermaid_dir.mkdir(parents=True, exist_ok=True)
-            urlretrieve(MERMAID_CDN % version, str(mermaid_js))
+        if not getattr(settings, "MERMAID_USE_CDN", False):
+            version = getattr(settings, "MERMAID_VERSION", DEFAULT_VERSION)
+            download_if_necessary(version)
+

tests/test_package.py

@@ -0,0 +1,21 @@
+from django.test import override_settings
+from unittest.mock import patch
+from django.apps import apps
+
+@override_settings(MERMAID_USE_CDN=False)
+@patch('django_mermaid.apps.download_if_necessary')
+def test_download_called_when_use_cdn_false(mock_download):
+    # Re-run the AppConfig.ready() method manually
+    app_config = apps.get_app_config('django_mermaid')
+    app_config.ready()
+
+    mock_download.assert_called_once()
+
+
+@override_settings(MERMAID_USE_CDN=True)
+@patch('django_mermaid.apps.download_if_necessary')
+def test_download_not_called_when_use_cdn_true(mock_download):
+    app_config = apps.get_app_config('django_mermaid')
+    app_config.ready()
+
+    mock_download.assert_not_called()