Atomic file partial filename should be random (or use locking somehow)

[joonas-fi]

If two processes are racing to make the same final file:

rename /run/notifydedup/counter.json.part /run/notifydedup/counter.json: no such file or directory; also failed removing temp file: remove /run/notifydedup/counter.json.part: no such file or directory

Think timeline:

• Process 1 starts producing counter.json.part
• Process 2 starts producing counter.json.part, overriding first one
• Process 1 renames the part file to target (thinking it is its own part file), effectively replacing the target file with an incomplete file

Is it possible to use locking with the part file in a way that two competing processes are not racing to produce the target file?

[joonas-fi]

The above fix mitigates this, but leaving open to consider:

• Testing of this fix
• Should we use file locking on the partial file, so if a dangling file was left by a crashed process, we know that if we get a lock we know we can "regain ownership" of the partial file. NOTE: make sure file is truncated, so we don't build on top of garbage
  • Currently a dangling file prevents atomic file write from working at all