Feature Request: don't sync if it would result in more than NUM deletions.

[Daniel Mare]

As a safety feature, I would like to see a feature that would prevent rsync from syncing when the sync, if it were to go ahead, would result in more than a certain number of files being deleted from the destination.

A similar feature, --max-delete, does exist, but does not prevent rsync from doing a partial run when --max-delete entries would be exceeded - it simply deletes up to that amount of files.

In scheduled scripts, this would result in damage being done a bit at a time at each run, so not really a safety feature and additionally, even if problem caught after only one run, it still means the clone is in an unknown state (at least without processing thousands of log delete entries).

A workaround I considered was doing a dry-run first, then only proceeding with the rsync when the dry-run doesn't exit with exit codes 25, but this is extremely inefficient - completely impractical for e.g. syncing large data stores overnight (4Tb in my case).

I suspect there would be reasonable demand for and use of this feature by others.