Fix race condition scheduling $tempdir deletion

[EliahKagan]

The cargo-zng script contained:

trap 'rm -rf "$tempdir"' 0 INT
tempdir="$(mktemp -d)"

That releases a resource that may occasionally not have been acquired.

In the rare case that the script would be interrupted before $tempdir was set, this would attempt to read the previous value of $tempdir. But if the calling environment had an environment variable of this name, then its value would have been inherited as the initial $tempdir in the script, and wrongly deleted.

The change made here puts the commands in the other order, so we only arrange for the deletion of $tempdir once the directory we want to delete exists and $tempdir will definitely expand to its name and not that of another directory.

In a sense, this trades one race condition for another, in that now the directory will not be deleted if the script is interrupted immediately after it is created. But this is not a problem, because it does not cause data loss, and the script already does not try to cover all ways the script can be terminated in the trap command.

[Byron]

Thanks so much for the fix, this is a critical one!

I never thought about this, but now once again realize that globals really are a problem.
This wouldn't have been possible if there was a function () {} that declares local tempdir, but it isn't very intuitive just for the 'local variable' feature.