So we can't completely remove xl_prev field, without giving up some functionality. But we don't really need to store the 8-byte previous WAL pointer in order to detect torn pages. Something else which can tell us that the WAL record does not belong to current WAL segno would be enough as well. I propose that we replace it with a much smaller 2-byte field (let's call it xl_walid). The "xl_walid" (or whatever we decide to call it) is the low order 16-bits of the WAL segno to which the WAL record belongs. While reading WAL, we always match that the "xl_walid" value stored in the WAL record matches with the current WAL segno's lower order 16-bits and if not, then consider that as the end of the stream.
For this to work, we must ensure that WAL files are either recycled in such a way that the "xl_walid" of the previous (to be recycled) WAL differs from the new WAL or we zero-out the new WAL file. Seems quite easy to do with the existing infrastructure.
Or, you can use the lower 16-bits of the previous record's CRC
