bug#30875: Garbage collector may leave empty files

[Marius Bakke <mbakke@fastmail.com>]

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Hello,

Recently I've seen a couple of instances like these:

exporting path `/gnu/store/mi1rbw8fdsbi4bc4pndbb6smq39722vm-baobab-3.26.1.tar.xz'
guix offload: error: build failed: hash of path `/gnu/store/mi1rbw8fdsbi4bc4pndbb6smq39722vm-baobab-3.26.1.tar.xz' has changed from `4223b4813b2253e68ea2b824d0d0e284ff75714ad0faf5a33d65a78df6b65915' to `77ac62e2629d8e45f624589c0c8bf99e24b3a722349bf1e79bc186008534e246'!
cannot build derivation `/gnu/store/jbx6a4n1d55dpydr87d11m7xwapm3pcx-gnome-3.24.3.drv': 1 dependencies couldn't be built

Without the build hook, it manifests as:

starting phase `unpack'
tar: This does not look like a tar archive
xz: (stdin): File format not recognized
tar: Child returned status 1
tar: Error is not recoverable: exiting now

There was another instance on help-guix recently with a user having
empty .drv files in the store.

There is a bug lurking here somewhere, but I'm not sure where to start
looking.

$ find /gnu/store/ -maxdepth 1 -size 0 | grep -v '\.lock$' | wc -l
24

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