Debug io regression

[ValentaTomas]

No description provided.

[cursor]

PR Summary

Low Risk
Low risk change that only reorders validation to fail fast for oversized cache allocations, avoiding creating/truncating an invalidly large file before mmap.

Overview
Fixes an IO regression in block.NewCache by validating size against math.MaxInt before truncating the backing file, so oversized cache requests fail fast without attempting to allocate/truncate a huge sparse file prior to mmap mapping.

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[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

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[claude]

Bug: raw unix PROT flags conflict with mmap-go own flag encoding, silently switching MAP_SHARED to MAP_PRIVATE.

mmap-go defines its own constants (not POSIX PROT_* values):

• mmap.RDONLY = 0
• mmap.RDWR = 1
• mmap.COPY = 2 (triggers MAP_PRIVATE)
• mmap.EXEC = 4

unix.PROT_READ|unix.PROT_WRITE = 0x1|0x2 = 3, which mmap-go interprets as RDWR|COPY (1|2=3). In mmap_unix.go the COPY branch fires first in the switch:

case inprot&COPY != 0:  // 3 & 2 = 2, non-zero -> TRUE
    prot |= syscall.PROT_WRITE
    flags = syscall.MAP_PRIVATE  // writes never reach the file

With MAP_PRIVATE, all writes to the mmap stay in process memory and are never flushed to the backing file. This breaks ExportToDiff which reads dirty blocks from the file via unix.CopyFileRange/unix.SyncFileRange -- those reads will see stale/zero data.

The original mmap.RDWR (= 1) correctly produced MAP_SHARED. This line should be reverted to mmap.RDWR.