Clip read-ahead cache reads to file size for small/last blocks

[arnav-ag]

Problem

AsyncFileReadAheadCache::read_impl issues every block read with length = m_block_size, regardless of where the block sits in the file. readBlock then allocates a CacheBlock of exactly that size and calls m_f->read(..., m_block_size, offset).

For a file smaller than one block, or the trailing block of any file, this means:

• The cache over-allocates: e.g. a 100-byte file with m_block_size = 1 MiB allocates a full 1 MiB buffer.
• The underlying IAsyncFile::read is asked for bytes past EOF. Most backends tolerate this (they return a short read), but some, notably HTTP-range blob store reads, can reject or misbehave on a range that extends past the object's end, surfacing as a spurious read failure on small files.

The function already clips the overall request to fileSize - offset near the top, but the per-block reads still happen at the full m_block_size.

Solution

Clip each per-block read to the bytes remaining in the file, matching the blockStart/std::min pattern already used a few lines further down in the copy loop:

int64_t blockStart = (int64_t)blockNum * f->m_block_size;
int readLength = std::min<int64_t>(f->m_block_size, fileSize - blockStart);
fblock = readBlock(f.getPtr(), readLength, blockStart);

fileSize is captured at the top of the function and lastBlockToStart is already clamped to lastBlockNumInFile, so fileSize - blockStart is strictly positive within this loop. readLength is bounded above by m_block_size (int), so the narrowing into readBlock(..., int length, ...) is safe.

---

I would also like to backport this to 7.3 (and 7.4) if it is acceptable, since we have run into an issue with the blob store reads failing.

[Copilot]

Pull request overview

This PR fixes AsyncFileReadAheadCache::read_impl to avoid issuing per-block reads that extend past EOF, which previously caused oversized cache allocations for small/trailing blocks and could break backends that reject out-of-range reads (e.g., HTTP range reads).

Changes:

• Compute each block’s start offset and clip the read length to min(m_block_size, fileSize - blockStart).
• Use the clipped per-block length when invoking readBlock(...), preventing over-allocation and out-of-range reads.

[arnav-ag]

@akankshamahajan15 do we need to trigger the tests to run to get this merged?

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