iilyak commented on code in PR #5611:
URL: https://github.com/apache/couchdb/pull/5611#discussion_r2248619020
##########
src/couch/src/couch_bt_engine_header.erl:
##########
@@ -204,17 +204,16 @@ upgrade_tuple(Old) when is_record(Old, db_header) ->
Old;
upgrade_tuple(Old) when is_tuple(Old) ->
NewSize = record_info(size, db_header),
- if
- tuple_size(Old) < NewSize -> ok;
- true -> erlang:error({invalid_header_size, Old})
- end,
- {_, New} = lists:foldl(
- fun(Val, {Idx, Hdr}) ->
- {Idx + 1, setelement(Idx, Hdr, Val)}
+ Upgrade = tuple_size(Old) < NewSize,
+ ProhibitDowngrade = config:get_boolean("couchdb", "prohibit_downgrade",
true),
+ OldKVs =
+ case {Upgrade, ProhibitDowngrade} of
+ {true, AnyBool} when is_boolean(AnyBool) -> tuple_to_list(Old);
+ {false, true} -> error({invalid_header_size, Old});
+ {false, false} -> lists:sublist(tuple_to_list(Old), NewSize)
Review Comment:
I think this only works if field was added at the end. How about the case
when we remove one of the fields in the middle? The downgrade in general is not
solvable problem if we don't maintain fields order for a previous version. We
can try something like:
```
record_info(1) ->
[
disk_version, update_seq, ...
];
...
record_info(7) ->
[
disk_version, update_seq, unused, id_tree_state,
seq_tree_state, local_tree_state, purge_tree_state,
purge_seq_tree_state, security_ptr, revs_limit, uuid,
epochs, compacted_seq, purge_infos_limit, props_ptr
];
record_info(?LATEST_DISK_VERSION) ->
record_info(fields, db_header).
```
If we would maintain such info we should be able to figure out what has been
changed.
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