lib-index: Simplified writing to dovecot.index.cache file. The old method was basically: - write max. 32 kB to internal buffer - flush it by writing to reserved areas (with no locks) The reserved areas were acquired by doing (whenever needed): - lock dovecot.index.cache - reserve data from dovecot.index.cache for writing, potentially increasing the file size by writing 0 bytes. the reserved area size varies. - unlock dovecot.index.cache This worked, but if multiple processes were writing to the cache file it could have left incomplete reserved areas as holes. The holes were attempted to be filled if they were large enough. The new method is: - write max. 256 kB to internal buffer - lock dovecot.index.cache - append the buffer to dovecot.index.cache - unlock dovecot.index.cache No reserved areas, holes or anything else weird going on. Ideally no data would be overwritten in the dovecot.index.cache file, only appended. Unfortunately currently some data is still overwritten: - mail_cache_header.{deleted_space,continued_record_count} - mail_cache_header_fields.next_offset when writing a new one - mail_cache_header_fields.{last_used,decision} - mail_cache_record.prev_offset The changing headers could eventually be moved to dovecot.index. This however is a backwards-incompatible change. The record's prev_offset could maybe simply just not be written in those (somewhat rare) problematic situations.