Meaning of entry_idx in log_offer

[anujkaliaiitd]

I was assuming that the log_offer callback always appends to the log. However, the callback has an entry_idx parameter. Does this mean that log_offer might require overwriting an old entry in the log, and writing beyond the current head of the log?

On a related note: are the newest/oldest log entries for log_pop/log_poll determined by entry_idx, or the order in which log_offer is called for entries?