when receiving a large file, "exporting" takes significant additional time (few minutes)

[belkka]

Issue

when receiving a ~4G file there was a an "export " stage at receiving side that took multiple minutes, after the sender finished the transaction on their side

Possible cause

@matheus23 pointed out that sendme uses ExportMode::Copy from iroh-blobs, instead of other existing option, ExportMode::TryReference.
https://docs.rs/iroh-blobs/latest/iroh_blobs/api/blobs/enum.ExportMode.html

This leads to file being copied from "internal store" to target location (instead of being moved).

To confirm, I measured that on receiving file system copying the 4G file took around 7.5 minutes, which matches the previously observed delay.

[belkka]

Possible solution

Could ExportMode::TryReference be used instead of ExportMode::Copy?

Some supporting thoughts:

• ExportMode::TryReference was used previously, but changed to ExportMode::Copy in this commit. The commit message doesn't explain this specific change though, so, perhaps, it might've been accidental?
• This comment in a bug report mentions that using ExportMode::Copy might be an immediate workaround for the bug. this fix for iroh-blobs claims to fix [BUG] directory contains files with the same hash value #103, which, perhaps, is a "real" solution, making the workaround unnecessary (?)