fix: claim a partial only under its own name

Opening and locking a partial download are two steps. Between them another run could finish, rename the same file into place and release its lock. The second run then locked the installed BIOS, read its full size as a finished partial and truncated it to download again. After the lock the partial's name must still point at the locked file, or the run downloads into a private copy.

tests/test_install_experience.py

@@ -145,6 +145,31 @@ class TwoRunsOneFolder(unittest.TestCase):
             self.assertIsNotNone(install._claim_partial(part))
 
 
+class APartialRenamedBeforeTheLock(unittest.TestCase):
+    """Open and lock are two steps. The run holding the partial could finish
+    in between, rename it into place and release it: the second run then
+    locked the installed BIOS, took it for a finished partial and truncated it."""
+
+    @unittest.skipIf(os.name == "nt", "flock semantics")
+    def test_the_installed_file_is_not_claimed(self):
+        import fcntl
+
+        with tempfile.TemporaryDirectory() as tmp:
+            part = Path(tmp) / ".big.bin.part"
+            final = Path(tmp) / "big.bin"
+            part.write_bytes(PAYLOAD)
+            real_flock = fcntl.flock
+
+            def finish_first(fd, op):
+                os.replace(part, final)
+                return real_flock(fd, op)
+
+            with mock.patch.object(fcntl, "flock", finish_first):
+                claimed = install._claim_partial(part)
+            self.assertIsNone(claimed)
+            self.assertEqual(final.read_bytes(), PAYLOAD)
+
+
 class InstalledFileModes(unittest.TestCase):
     """A file came out 0600, the mode of the temporary it was written to: run
     with sudo for a root-owned BIOS folder, the emulator's own user could not