I was seeing this when trying to run flatpak's tests in ostree's CI:
https://github.com/ostreedev/ostree/pull/824
The race here is that the python process can still be writing to the output
while sed is reading it, and hence we'll find a difference on the next line.
Fix this by making a tmp copy of the file, which then both sed and cmp will
read consistently.
I'm not *entirely* sure this will fix the problem as I couldn't easily reproduce
the race locally, but I believe it at least fixes *a* race.
48b5dd5e7b
