LogMemFD hands off the shared-memory and that back the double-buffered log data. fd1 and fd2 can be mapped; but must be dup(2)'d before use. size is the max length (in bytes) of each region the receiver should mmap. slotsize is the size of each slot/line within the shared-memory region. Return a Log
(fd1, fd2, size, slotsize int)
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