perf: reuse buffer when hashing source files for checksums - #2925
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io.CopyBuffer ignored the pre-allocated buffer because *os.File implements WriterTo: it took the (*os.File).WriteTo path, which allocates a fresh 32KiB buffer per file via os.genericWriteTo. Hashing many small source files thus allocated ~32KiB per file (~640MB for 20,000 files). Wrapping the file in a plain io.Reader forces io.CopyBuffer to reuse the caller's buffer, keeping the loop allocation-free. The checksum value is unchanged.
vmaerten
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August 3, 2026 20:25
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Summary
io.CopyBufferignored the pre-allocated buffer because*os.FileimplementsWriterTo: it took the(*os.File).WriteTopath, which allocates a fresh 32KiB buffer per file via os.genericWriteTo. Hashing many small source files thus allocated ~32KiB per file (~640MB for 20,000 files).Wrapping the file in a plain
io.Readerforcesio.CopyBufferto reuse the caller's buffer, keeping the loop allocation-free. The checksum value is unchanged.