atomic_helper.write_file missing os.fsync() leaves 0-byte files on sudden reboot/power-loss, causing fatal JSONDecodeError in stages.py

[snehadhar-source]

cloud-init/cloudinit/atomic_helper.py

Line 78 in 494f7de

os.rename(tf.name, filename)

When a system crashes or is forcefully rebooted (e.g. SIGKILL, hard reset) right after cloud-init writes instance cache files, network-config.json can become a 0-byte file on reboot.
On the next boot, cloud-init crashes on an unhandled JSONDecodeError and fails to initialize networking.

Root Cause

1. Missing fsync before rename: cloudinit/atomic_helper.py writes to a tempfile and calls os.rename() without tf.flush() or os.fsync(). On filesystems like ext4 (with delayed allocation), rename commits directory metadata to the journal while dirty data remains in volatile RAM. A sudden crash before background writeback leaves an empty file (Size: 0, Blocks: 0).
2. Missing error handling: cloudinit/stages.py:448 calls util.load_json() without try...except, raising unhandled json.decoder.JSONDecodeError on empty/corrupted cache.

Suggested Fix

1. cloudinit/atomic_helper.py: Flush and sync before rename:

   tf.write(content)
   tf.flush()
   try:
   os.fsync(tf.fileno())
   except OSError:
   pass
   os.chmod(tf.name, mode)
   os.rename(tf.name, filename)

2. cloudinit/stages.py: Handle corrupted/empty JSON gracefully

if os.path.isfile(net_cfg_fname):
try:
content = util.load_text_file(net_cfg_fname)
if content:
return util.load_json(content)
except (json.decoder.JSONDecodeError, ValueError) as exc:
LOG.warning("Failed to parse cached network config at %s: %s", net_cfg_fname, exc)