core/rawdb: fix head truncation below a diverged tail group

[rjl493456442]

Tail groups are pruned independently. Truncating the head below the tail of group can happen just in case the tails across the groups are not aligned.

Reset such table to empty at the new head instead, and refuse only truncations that fall below every group's tail.

Superseded #35536

[healthykim]

So we want to allow truncating below the head if there is at least one non-empty group ? But why do we want to prevent discarding all the data ? I think this is a valid fix, but I’m just a bit confused about the pruning.

[rjl493456442]

@healthykim

Before the BAL table was introduced, all tables were expected to remain strictly consistent: truncating from the chain head to a point below any table's tail was considered invalid. This was primarily a safeguard against programming errors, as go-ethereum has never had a use case for such an operation.

With the BAL table, however, the table set is divided into multiple groups whose tails may differ. Therefore, rewinding the chain head to a point below the BAL group's tail must be supported.