fix: keep the ci test job off the network

tests/test_e2e.py

@@ -25,6 +25,7 @@ import tempfile
 import unittest
 import zipfile
 from pathlib import Path
+from unittest import mock
 
 sys.path.insert(0, os.path.join(os.path.dirname(__file__), "..", "scripts"))
 
@@ -5304,7 +5305,9 @@ struct BurnDriver BurnDrvneogeo = {
         original = largefiles.urllib.request.urlopen
         largefiles.urllib.request.urlopen = fake_urlopen
         try:
-            with tempfile.TemporaryDirectory() as tmpdir:
+            with tempfile.TemporaryDirectory() as tmpdir, mock.patch.dict(
+                os.environ, {"RETROBIOS_OFFLINE": ""}
+            ):
                 path = common.fetch_large_file(
                     "MAME 0.174 Arcade XML.dat", dest_dir=tmpdir
                 )

tests/test_large_file_cache.py

@@ -32,6 +32,20 @@ PAYLOAD_A = b"A" * (256 * 1024)
 PAYLOAD_B = b"B" * (256 * 1024)
 
 
+# These tests download through a mocked urlopen into their own directory;
+# the CI test job sets RETROBIOS_OFFLINE to keep every other test off the
+# network, so it is cleared here.
+_ONLINE = mock.patch.dict(os.environ, {"RETROBIOS_OFFLINE": ""})
+
+
+def setUpModule():
+    _ONLINE.start()
+
+
+def tearDownModule():
+    _ONLINE.stop()
+
+
 class _SlowResponse(io.BytesIO):
     """Serve a payload in chunks, yielding between them."""
 
@@ -220,6 +234,19 @@ class LargeFileCacheTest(unittest.TestCase):
         self.assertEqual(os.listdir(self.dir), ["asset.bin"])
 
 
+class TheOfflineSwitchHoldsEverywhere(unittest.TestCase):
+    """RETROBIOS_OFFLINE stops a download whatever the caller asked."""
+
+    def test_no_request_is_made(self):
+        with tempfile.TemporaryDirectory() as tmp, mock.patch.dict(
+            os.environ, {"RETROBIOS_OFFLINE": "1"}
+        ), mock.patch.object(largefiles.urllib.request, "urlopen") as urlopen:
+            self.assertIsNone(
+                largefiles.fetch_large_file("asset.bin", dest_dir=tmp, offline=False)
+            )
+            urlopen.assert_not_called()
+
+
 class HashCacheKeepsEveryDigest(unittest.TestCase):
     """A cache hit must serve the same five digests a fresh hash produces.