EXCEPTION!!!: UnknownHashId (HashId 526)

[aryairani]

While debugging #1918, I started getting this different error instead. It's crashing on line 382, which seems to suggest that there are some bad ids in the LocalIds:

unison/codebase2/codebase-sqlite/U/Codebase/Sqlite/Sync22.hs

Lines 371 to 388 in ca7b32d

	syncWatch :: WK.WatchKind -> Sqlite.Reference.IdH -> m Bool
	syncWatch wk r = do
	r' <- traverse syncHashLiteral r
	doneKinds <- runDest (Q.loadWatchKindsByReference r')
	if (wk `elem` doneKinds) then do
	runSrc (Q.loadWatch wk r) >>= traverse \blob -> do
	(L.LocalIds tIds hIds, termBytes) <-
	case runGetS S.decomposeWatchResult blob of
	Right x -> pure x
	Left s -> throwError $ DecodeError ErrWatchResult blob s
	tIds' <- traverse syncTextLiteral tIds
	hIds' <- traverse syncHashLiteral hIds
	when debug $ traceM $ "LocalIds for Source watch result " ++ show r ++ ": " ++ show (tIds, hIds)
	when debug $ traceM $ "LocalIds for Dest watch result " ++ show r' ++ ": " ++ show (tIds', hIds')
	let blob' = runPutS (S.recomposeWatchResult (L.LocalIds tIds' hIds', termBytes))
	runDest (Q.saveWatch wk r' blob')
	pure True
	else pure False

[aryairani]

Hypothesis: Some spots deserializing watch results (e.g. line 378) are not expecting/handling the format tag (which was a late addition to serialized watch results), so there's an off-by-one byte issue and it's a miracle that anything works.