[release-7.3] Clip read-ahead cache reads to file size for small/last blocks

[arnav-ag]

Backport of #13264 to release-7.3.

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.

The only difference vs. the main PR is the filename: in release-7.3 the header is AsyncFileReadAhead.actor.h; the surrounding code is identical.

[foundationdb-ci]

Result of foundationdb-pr-73 on Linux RHEL 9

• Commit ID: d12faa8
• Duration 0:58:00
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[foundationdb-ci]

Result of foundationdb-pr-cluster-tests-73 on Linux RHEL 9

• Commit ID: d12faa8
• Duration 1:00:38
• Result: ✅ SUCCEEDED
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[foundationdb-ci]

Result of foundationdb-pr-clang-73 on Linux RHEL 9

• Commit ID: d12faa8
• Duration 3:00:48
• Result: ❌ FAILED
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[foundationdb-ci]

Result of foundationdb-pr-clang-73 on Linux RHEL 9

• Commit ID: d12faa8
• Duration 0:36:14
• Result: ✅ SUCCEEDED
• Error: N/A
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