feat: add scatter write benchmark#92
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1. fopen O_APPEND bug: opening with 'a'/'a+' used fopen("ab+") which
enables O_APPEND, causing every fwrite to go to EOF regardless of
fseek. The library manages offsets via fseek+fwrite, so 'a' must
map to "rb+" (with "wb+" fallback for file creation).
2. btree is_readonly for 'r+': btree was constructed with
is_readonly=(mode_b & mode_bit::r), which is true for 'r+' since
both r and plus bits are set. read_child() then called
child.unmodify() preventing btree updates from being persisted to
disk — scatter writes silently lost their index updates.
Fix: is_readonly is true only for pure 'r' without '+', 'w', or 'a'.
3. defragmentation mid-flush: allocator::deallocate() triggers
maintenance() every 64 deallocs, which calls perform_defragmentation()
from within block::~block() during clear_cache(). At that point the btree index is only partially updated — remaining unflushed blocks
still have stale addresses. Defrag read garbage data and corrupted
the archive. Fix: pass perform_maintenance=false in the destructor.
Co-authored-by: mozhaa <mozhay2005@gmail.com>
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