fix: judge a cached large file from one read

_fetch_asset hashed the cache copy, then hashed the path again for the
release check. A copy another run promoted in between answered the second
question, and the caller was told the cache held nothing when it held
the file asked for. Both checks now read one snapshot, taken while the
inode stays the same.

tests/test_large_file_cache.py

@@ -691,7 +691,10 @@ class ASameSizeReuploadIsSeen(unittest.TestCase):
     def replaced(self, digests: dict, size: int | None) -> bool:
         with mock.patch.object(largefiles, "_served_digests", lambda: digests), \
                 mock.patch.object(largefiles, "_served_size", lambda name: size):
-            return largefiles._replaced_on_release("dsi_nand.bin", str(self.cached), False)
+            return largefiles._replaced_on_release(
+                "dsi_nand.bin", hashlib.sha256(self.cached.read_bytes()).hexdigest(),
+                self.cached.stat().st_size, False,
+            )
 
     def test_the_digest_decides(self):
         same = hashlib.sha256(b"OLD NAND").hexdigest()
@@ -703,6 +706,30 @@ class ASameSizeReuploadIsSeen(unittest.TestCase):
         self.assertFalse(self.replaced({}, 8))
         self.assertTrue(self.replaced({}, 9))
 
+    def test_a_copy_promoted_during_the_check_is_the_one_judged(self):
+        """Another run promotes the served revision while this one hashes the
+        stale copy: the release check reread the path, found the served
+        bytes, and the caller was told the cache held no such file."""
+        new = b"NEW NAND"
+        real = largefiles.compute_hashes
+
+        def promote_while_hashing(path, *args, **kwargs):
+            result = real(path, *args, **kwargs)
+            if Path(path).read_bytes() != new:
+                promoted = self.dir / "promoted.tmp"
+                promoted.write_bytes(new)
+                os.replace(promoted, self.cached)
+            return result
+
+        digests = {"dsi_nand.bin": hashlib.sha256(new).hexdigest()}
+        with mock.patch.object(largefiles, "compute_hashes", promote_while_hashing), \
+                mock.patch.object(largefiles, "_served_digests", lambda: digests):
+            found = largefiles._fetch_asset(
+                "dsi_nand.bin", str(self.dir), hashlib.sha1(new).hexdigest(), "",
+                offline=False,
+            )
+        self.assertEqual(found, str(self.cached))
+
     def test_the_release_check_compares_digests_at_equal_size(self):
         expected = {"bios/Nintendo/DSi/dsi_nand.bin": 8}
         assets = {"dsi_nand.bin": 8}