Fix race condition scheduling $tempdir deletion#216
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Byron merged 1 commit intorust-lang:mainfrom Sep 6, 2024
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The `cargo-zng` script contained:
trap 'rm -rf "$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
approved these changes
Sep 6, 2024
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Byron
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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.
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The
cargo-zngscript contained:That releases a resource that may occasionally not have been acquired.
In the rare case that the script would be interrupted before
$tempdirwas set, this would attempt to read the previous value of$tempdir. But if the calling environment had an environment variable of this name, then its value would have been inherited as the initial$tempdirin the script, and wrongly deleted.The change made here puts the commands in the other order, so we only arrange for the deletion of
$tempdironce the directory we want to delete exists and$tempdirwill 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
trapcommand.