Restore backup never reports load_keys progress of empty backup

[ajbura]

Before restoreKeyBackup starts the process of importing key backup, it reports load_keys without total, successes and failures as progress, indicating the progress is shifted from fetch_keys.

After that it reports progress with keys import data which client can use to track key restore progress, but in the case where backup doesn't have any keys (0 keys) it doesn't report with

opts?.progressCallback?.({
    total: 0,
    successes: 0,
    stage: "load_keys",
    failures: 0,
});

and tricks the client thinking forever that import progress is yet to happen

this should contain actual total number of keys with successes and failures set to 0

matrix-js-sdk/src/rust-crypto/backup.ts

Lines 615 to 617 in 30d9b05

	opts?.progressCallback?.({
	stage: "load_keys",
	});

[richvdh]

The documentation on ImportRoomKeyProgressData is poor, but for the record, I believe the intended flow is:

1. {stage: "fetch"}: it is downloading the keys
2. {stage: "load_keys"}: it is starting to import the keys
3. {stage: "load_keys", total: X, successes: Y, failures: Z }: import is proceeding or complete.

I agree, it looks like there is no final callback in the case of an empty backup.

this should contain actual total number of keys with successes and failures set to 0

Yes, that would seem sensible, though note that the call would need to be moved into importKeyBackup so as to have access to totalKeyCount. Would you make a PR to improve this?

It might also be good to have another stage which reports completion of the import, so that the client doesn't need to check the key counts. If that would be useful to you, feel free to make another PR.