Fix `d.getStagingDir("")`

[mtrmac]

Since #1784 , https://github.com/containers/storage/blob/10981abafe3e3ece4e0cc872761f7624cd1bef0e/drivers/overlay/overlay.go#L2188 is calling getStagingDir(""), i.e. dir2 with a layer ID "".

Just from reading the code, that happens to work as expected because it is not using the primary return value of dir2, but that’s extremely non-obvious and very likely to be broken during future maintenance. To the extent the cod is only looking for the “homeDir”, I think that should be modeled explicitly and not use the id value at all. Alternatively, if the id value mattered, then this needs to actually be implemented.

Cc: @giuseppe

[giuseppe]

opened:

• https://github.com/containers/storage/pull/2107