Add support for --sparse to only create holes in files that had holes in the original

[textshell]

• Add a is-sparse metadata bool that is true if the original file had any holes.
• Use this flag on extraction to use sparse extraction mode if the original was sparse or normal extraction mode if it was not.
• maybe add new option to select between never using sparse files and this mode. (always using sparse files already has an option)

[ThomasWaldmann]

note: one needs to be careful with determining sparsity, see the unit tests how to do that (at least until we use SEEK_HOLE/SEEK_DATA and know for sure then by that).

[ThomasWaldmann]

btw, this is no black-and-white thing. a file could have large runs of sparse zeros and also large runs of non-sparse zeros. VM software might do clever tricks here for performance reasons.

[textshell]

Maybe there is some VM software that is really smart about it. Is there any example of one?
Some cases might be served better by perfect restoring of the holes like in #14.
This idea is easier to implement, but it might not be worth it, if the full sparse file replication is implemented.

[enkore]

I dunno, the layout of the holes depends on what the allocator of the FS decided to do and you can't force it. So I'm not even sure if that's (#14) really a feasible thing to do, or why it would make a difference (unless some software decides to track it's sparse files internally and breaks if that changes?!)

(this really belongs into #14)

[textshell]

I think everything that has encodes semantics into holes is really broken. I can somehow understand that an advanced application might do smart things with performance vs storage requirement trade ofs. But i wonder if some application is really smart about this if it doesn‘t need an "reoptimize" feature anyway, because files sometimes need to be copied anyway. Depending on a special copy procedure that fatefully reproduces holes seems not like something that would be popular.

[enkore]

I'm closing this due to the complexities involved and that there doesn't seem to be any real use to this, but if someone really wants this -> reopen