dd: derive the count_bytes read budget from bytes read#13435
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dd: derive the count_bytes read budget from bytes read#13435relative23 wants to merge 7 commits into
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calc_loop_bsize computed the remaining byte budget for
count=N iflag=count_bytes from the bytes written so far, while
below_count_limit gates the copy loop on bytes read. A conversion that
writes more than it reads (conv=block record expansion, conv=sync
padding of partial reads) pushes the written total past the limit while
the read total is still below it, and the u128 subtraction underflowed:
a panic in debug builds ('attempt to subtract with overflow' in
calc_loop_bsize), and a wrapped huge value in release builds that
unlocked a full-sized read on every following iteration, silently
overshooting the count limit.
Derive the remaining budget from the bytes read instead, matching
below_count_limit and the documented semantics of count_bytes
(saturating, so an inconsistency degrades to a zero-sized read instead
of wrapping), and drop the now-unused WriteStat parameter.
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Merging this PR will not alter performance
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sylvestre
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Fixes #13434.
calc_loop_bsizeused bytes written to calculate the remaining input budget forcount=N iflag=count_bytes. Expanding conversions could make the subtraction underflow: debug builds panicked, while release builds read 1,024 input bytes forcount=1000.Use
rstat.bytes_totalwith saturating subtraction. The regression test verifies that only the first 1,000 input bytes are converted.Validation
cargo test -p uu_dd