"example #2 seems just fine, assuming rename() is atomic and a power failure would just leave the old contents in the file."
Unfortunately, the 'bug' is that this does not happen. Instead, the truncate hits disk before the new data is written, so that a power loss leaves the file empty rather than merely out-of-date. This behaviour is different on ext4 than on ext3, hence it has only recently become an issue.
To my untrained eye, regardless of what the POSIX standard 'allows', this is a bug that should be fixed. On the bright side, it looks like there are patches ready to be applied that will change the behaviour to match ext3, which is as you describe. If this happens, #2 would remain a relatively safe choice.
Unfortunately, the 'bug' is that this does not happen. Instead, the truncate hits disk before the new data is written, so that a power loss leaves the file empty rather than merely out-of-date. This behaviour is different on ext4 than on ext3, hence it has only recently become an issue.
To my untrained eye, regardless of what the POSIX standard 'allows', this is a bug that should be fixed. On the bright side, it looks like there are patches ready to be applied that will change the behaviour to match ext3, which is as you describe. If this happens, #2 would remain a relatively safe choice.