feat: default AUFS -> OverlayFS whiteout conversion for squashfs #26
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By default, replace AUFS whiteout markers with OverlayFS whiteout markers when converting a layer from TAR to SquashFS. It is expected that SquashFS layers will be used via direct mount, rather than extraction to disk. Therefore, the whiteout markers must be in a format that supports assembling multiple mounted layers into a rootfs, using OverlayFS. A `.wh.file` marker indicating a whiteout of `file` is replaced by `file` as a 0:0 character device. A `dir/.wh..wh..opq` marker indicating an opaque directory is replaced with the `trusted.overlay.opaque="y"` xattr on `dir`. A two-pass approach is required. AUFS directory opaque markers are not required to be adjacent to the directory to which they apply, in the tar stream. As `tar2sqfs` and `sqfstar` do not support tar files with two instances of the same dir/file, we cannot append an additional tar entry if we come across an opaque marker distant from the target directory entry. A new option `OptNoConvertWhiteout` will disable whiteout conversion. This can be employed by callers to avoid unneccessary processing where the layer is part of a squashed / single-layer image that will not have any whiteout markers. Fixes sylabs#25
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By default, replace AUFS whiteout markers with OverlayFS whiteout markers when converting a layer from TAR to SquashFS.
It is expected that SquashFS layers will be used via direct mount, rather than extraction to disk. Therefore, the whiteout markers must be in a format that supports assembling multiple mounted layers into a rootfs, using OverlayFS.
A
.wh.filemarker indicating a whiteout offileis replaced byfileas a 0:0 character device.A
dir/.wh..wh..opqmarker indicating an opaque directory is replaced with thetrusted.overlay.opaque="y"xattr ondir.A two-pass approach is required. AUFS directory opaque markers are not required to be adjacent to the directory to which they apply, in the tar stream. As
tar2sqfsandsqfstardo not support tar files with two instances of the same dir/file, we cannot append an additional tar entry if we come across an opaque marker distant from the target directory entry.A new option
OptNoConvertWhiteoutwill disable whiteout conversion. This can be employed by callers to avoid unneccessary processing where the layer is part of a squashed / single-layer image that will not have any whiteout markers.Fixes #25