MB-62427: seeking the pointer correctly while reading segmentBase data

[Thejas-bhat]

v16 file format introduced the concept of sections in which as part of metadata the fieldsIndex part was replaced by a sectionsIndex which serves the same purpose of fieldsIndex of pointing to a file offset where the actual data of field's section is stored.

The PR tries to resolve the situation - where there was only one field's data being written out while creating the in-memory segmentBase and the process was crashing while initializing the fields related information in the loadFieldsNew() API at

zapx/segment.go

Line 323 in 0c7027f

numFields, sz := binary.Uvarint(s.mem[pos : pos+binary.MaxVarintLen64])

The reason for the crash is while writing out the data, because there is only one field metadata being written out at the very end of the buffer

zapx/write.go

Line 89 in 0c7027f

rv = uint64(w.Count())

is worth 9 bytes. However we were doing an out-of-bounds access over here

zapx/segment.go

Line 323 in 0c7027f

numFields, sz := binary.Uvarint(s.mem[pos : pos+binary.MaxVarintLen64])

because binary.MaxVarintLen64 = 10.

The solution proposed here is to guard this kind of access by keeping in mind the length of the backing buffer like how it was done in v15

zapx/segment.go

Line 268 in a72794c

fieldsIndexEnd := uint64(len(s.mem))

Pending one clarification