Borg: Incorrect borg prune --stats output with --dry-run

[Arcovion]

With borg 1.1.7 on ubuntu 18.10:

borg prune <borg_dir> --dry-run --list --stats --keep-last 1 
...
                       Original size      Compressed size    Deduplicated size
Deleted data:                    0 B                  0 B                  0 B
All archives:              981.25 GB            793.40 GB            112.32 GB

                       Unique chunks         Total chunks
Chunk index:                  250054              2164226

borg prune <borg_dir> --list --stats --keep-last 1 
...
                       Original size      Compressed size    Deduplicated size
Deleted data:             -820.45 GB           -669.72 GB             -9.60 GB
All archives:              160.80 GB            123.69 GB            102.72 GB

                       Unique chunks         Total chunks
Chunk index:                  237044               371408

Pruning worked as I wanted it to, but the dry run stats do not give an accurate preview.

[ThomasWaldmann]

Likely, that is simply not implemented (just guessing, needs to be checked in the src).

[ThomasWaldmann]

I had the look at the src:

• the intended purpose of borg prune --dry-run is to see what archives would be deleted (pruned) and what would be kept. this is mostly to avoid calling it with wrong arguments and losing backups (if called w/o -dry-run).
• as a dry-run does not even call archive.delete() (and stats are updated there, for real runs), there is no way it can show accurate stats with --dry-run.
• due to deduplication, it is non-trivial and not fast to give accurate stats. it would have to emulate all the refcounting without actually deleting anything.

Proposed fix:
emit an error message saying --dry-run and --stats can't be used together.

[4rund3v]

If this is still available, could i work on this issue.

[ThomasWaldmann]

sure, go for it, make a PR.

[ThomasWaldmann]

@4rund3v did you have a look?

[user062]

@ThomasWaldmann

some tests do use --dry-run and --stats together

i think we could add a simple check in archiver.run would that be appropriate ?

[ThomasWaldmann]

@user062 yes. there already some such checks.

[demure]

Rather than exiting when someone uses --dry-run and --stats together, could we instead give then a warning and continue to run the dry-run while ignoring the stats?

My reasoning is that this current change breaks my wrapper script's current succinct of handling real/dry purges ( https://notabug.org/demure/scripts/src/master/borg_wrap.sh#L138-L150 ).

If the decision is to leave this as is, I'll just make my wrapper list two full commands I suppose, but this feel like an unintended roadblock so I figured I'd point it out.

ps. sorry I didn't notice this sooner, but I haven't pruned in a long time.

[ThomasWaldmann]

OK, guess we can do that, reopening...

[ThomasWaldmann]

Fixed in 1.1-maint, todo for master.