This ensures that if a functional test runs QEMU with a writable
disk pointing to a cached asset, an error will be reported, rather
than silently modifying the cache file.

As an example, tweaking test_sbsaref.py to set snapshot=off,
results in a clear error:

  Command: ./build/qemu-system-aarch64 ...snip... -drive file=/var/home/berrange/.cache/qemu/download/44cdbae275ef1bb6dab1d5fbb59473d4f741e1c8ea8a80fd9e906b531d6ad461,format=raw,snapshot=off -cpu max,pauth=off
  Output: qemu-system-aarch64: Could not open '/var/home/berrange/.cache/qemu/download/44cdbae275ef1bb6dab1d5fbb59473d4f741e1c8ea8a80fd9e906b531d6ad461': Permission denied

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
---
 tests/functional/qemu_test/asset.py | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/tests/functional/qemu_test/asset.py b/tests/functional/qemu_test/asset.py
index e47bfac035..f126cd5863 100644
--- a/tests/functional/qemu_test/asset.py
+++ b/tests/functional/qemu_test/asset.py
@@ -8,6 +8,7 @@
 import hashlib
 import logging
 import os
+import stat
 import subprocess
 import sys
 import unittest
@@ -143,6 +144,8 @@ def fetch(self):
             raise Exception("Hash of %s does not match %s" %
                             (self.url, self.hash))
         tmp_cache_file.replace(self.cache_file)
+        # Remove write perms to stop tests accidentally modifying them
+        os.chmod(self.cache_file, stat.S_IRUSR | stat.S_IRGRP)
 
         self.log.info("Cached %s at %s" % (self.url, self.cache_file))
         return str(self.cache_file)
-- 
2.46.0