storage: report success from rollback and commit

[Poseidonas]

Fixes #597.

mtda-cli storage rollback prints could not rollback changes made to shared storage! and exits non-zero after a rollback which did exactly what it was asked to, so the reporter has to write || : around it.

Where the false failure comes from

usbf.rollback()                 no return statement          -> None
main.storage_rollback()         result = self.storage...     -> None
grpc/servicer._bool_response()  BoolResponse(value=bool(v))  -> False
mtda-cli storage_rollback()     if status is False           -> prints, exits 1

bool(None) is where it turns into a failure. The same applies to commit, and to both the usbf and qemu controllers.

The fix

return True on the paths that completed, and on the ignore_missing path where there was nothing to do.

This is not a new convention: every other storage method already returns a value.

	rollback	commit	to_host	to_target	mount	open	close
before	no	no	yes	yes	yes	yes	yes

I checked the other callers before changing the signature — helpers/image.py:72, usbf.py:286 and qemu.py:64 all discard the return value, so only the CLI path behaves differently.

On ignore_missing

I chose True there. The only caller is helpers/image._close(), which ignores the result, so nothing depends on it; True seemed the better reading of "there was no copy-on-write device, so there was nothing to roll back" — False would report a failure that did not happen. Happy to flip it if you read it the other way.

What I verified, and what I could not

Verified by driving both controllers with the subprocess calls patched out: all four methods return True on the normal path and on ignore_missing. flake8 is clean with the exclusions from tox.ini.

I could not run scripts/test-using-docker here: the archlinux image has no linux/arm64 manifest, and mtda-service imports systemd, which is not installable on macOS. So this rests on reading the call chain and on the isolated check above rather than on the integration suite — worth a second look on that account.

I did not add a test. The suite is integration-based against a live service, and the docker backend has no rollback to exercise. If you would like one, I am glad to add it in whatever shape fits.

[chombourger]

thank you @Poseidonas - changes LGTM. I am running our CI even though I understand that your changes will not be exercised. that's more of a checklist type of things than anything else.

[chombourger]

I also changed the target branch from main to next. we should probably update our contributing guide for this recent policy change