avoid unnecessary reservations in std::io::Take::read_to_end #63216
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r? @bluss (rust_highfive has picked a reviewer for you, use r? to override) |
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☔ The latest upstream changes (presumably #62987) made this pull request unmergeable. Please resolve the merge conflicts. |
Prevously the `read_to_end` implementation for `std::io::Take` used its own `limit` as a cap on the `reservation_size`. However, that could still result in an over-allocation like this: 1. Call `reader.take(5).read_to_end(&mut vec)`. 2. `read_to_end_with_reservation` reserves 5 bytes and calls `read`. 3. `read` writes 5 bytes. 4. `read_to_end_with_reservation` reserves 5 bytes and calls `read`. 5. `read` writes 0 bytes. 6. The read loop ends with `vec` having length 5 and capacity 10. The reservation of 5 bytes was correct for the read at step 2 but unnecessary for the read at step 4. By that second read, `Take::limit` is 0, but the `read_to_end_with_reservation` loop is still using the same `reservation_size` it started with. Solve this by having `read_to_end_with_reservation` take a closure, which lets it get a fresh `reservation_size` for each read. This is an implementation detail which doesn't affect any public API.
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@bluss let me know if I should grab a different reviewer? |
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r? @sfackler |
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@bors r+ |
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…kler avoid unnecessary reservations in std::io::Take::read_to_end Prevously the `read_to_end` implementation for `std::io::Take` used its own `limit` as a cap on the `reservation_size`. However, that could still result in an over-allocation like this: 1. Call `reader.take(5).read_to_end(&mut vec)`. 2. `read_to_end_with_reservation` reserves 5 bytes and calls `read`. 3. `read` writes 5 bytes. 4. `read_to_end_with_reservation` reserves 5 bytes and calls `read`. 5. `read` writes 0 bytes. 6. The read loop ends with `vec` having length 5 and capacity 10. The reservation of 5 bytes was correct for the read at step 2 but unnecessary for the read at step 4. By that second read, `Take::limit` is 0, but the `read_to_end_with_reservation` loop is still using the same `reservation_size` it started with. Solve this by having `read_to_end_with_reservation` take a closure, which lets it get a fresh `reservation_size` for each read. This is an implementation detail which doesn't affect any public API.
Centril
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…kler avoid unnecessary reservations in std::io::Take::read_to_end Prevously the `read_to_end` implementation for `std::io::Take` used its own `limit` as a cap on the `reservation_size`. However, that could still result in an over-allocation like this: 1. Call `reader.take(5).read_to_end(&mut vec)`. 2. `read_to_end_with_reservation` reserves 5 bytes and calls `read`. 3. `read` writes 5 bytes. 4. `read_to_end_with_reservation` reserves 5 bytes and calls `read`. 5. `read` writes 0 bytes. 6. The read loop ends with `vec` having length 5 and capacity 10. The reservation of 5 bytes was correct for the read at step 2 but unnecessary for the read at step 4. By that second read, `Take::limit` is 0, but the `read_to_end_with_reservation` loop is still using the same `reservation_size` it started with. Solve this by having `read_to_end_with_reservation` take a closure, which lets it get a fresh `reservation_size` for each read. This is an implementation detail which doesn't affect any public API.
Centril
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Aug 20, 2019
…kler avoid unnecessary reservations in std::io::Take::read_to_end Prevously the `read_to_end` implementation for `std::io::Take` used its own `limit` as a cap on the `reservation_size`. However, that could still result in an over-allocation like this: 1. Call `reader.take(5).read_to_end(&mut vec)`. 2. `read_to_end_with_reservation` reserves 5 bytes and calls `read`. 3. `read` writes 5 bytes. 4. `read_to_end_with_reservation` reserves 5 bytes and calls `read`. 5. `read` writes 0 bytes. 6. The read loop ends with `vec` having length 5 and capacity 10. The reservation of 5 bytes was correct for the read at step 2 but unnecessary for the read at step 4. By that second read, `Take::limit` is 0, but the `read_to_end_with_reservation` loop is still using the same `reservation_size` it started with. Solve this by having `read_to_end_with_reservation` take a closure, which lets it get a fresh `reservation_size` for each read. This is an implementation detail which doesn't affect any public API.
Centril
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Aug 20, 2019
…kler avoid unnecessary reservations in std::io::Take::read_to_end Prevously the `read_to_end` implementation for `std::io::Take` used its own `limit` as a cap on the `reservation_size`. However, that could still result in an over-allocation like this: 1. Call `reader.take(5).read_to_end(&mut vec)`. 2. `read_to_end_with_reservation` reserves 5 bytes and calls `read`. 3. `read` writes 5 bytes. 4. `read_to_end_with_reservation` reserves 5 bytes and calls `read`. 5. `read` writes 0 bytes. 6. The read loop ends with `vec` having length 5 and capacity 10. The reservation of 5 bytes was correct for the read at step 2 but unnecessary for the read at step 4. By that second read, `Take::limit` is 0, but the `read_to_end_with_reservation` loop is still using the same `reservation_size` it started with. Solve this by having `read_to_end_with_reservation` take a closure, which lets it get a fresh `reservation_size` for each read. This is an implementation detail which doesn't affect any public API.
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Rollup of 7 pull requests Successful merges: - #63216 (avoid unnecessary reservations in std::io::Take::read_to_end) - #63265 (Implement `nth_back` for ChunksExactMut) - #63691 (Fix bug in iter::Chain::size_hint) - #63722 (Don't use stage naming in RUSTFLAGS environment variables) - #63723 (Consolidate sigemptyset workarounds) - #63736 (Restore the rustc_plugin crate in the sysroot) - #63743 (Allow git to merge `Cargo.lock`) Failed merges: r? @ghost
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Prevously the
read_to_endimplementation forstd::io::Takeused itsown
limitas a cap on thereservation_size. However, that couldstill result in an over-allocation like this:
reader.take(5).read_to_end(&mut vec).read_to_end_with_reservationreserves 5 bytes and callsread.readwrites 5 bytes.read_to_end_with_reservationreserves 5 bytes and callsread.readwrites 0 bytes.vechaving length 5 and capacity 10.The reservation of 5 bytes was correct for the read at step 2 but
unnecessary for the read at step 4. By that second read,
Take::limitis 0, but the
read_to_end_with_reservationloop is still using thesame
reservation_sizeit started with.Solve this by having
read_to_end_with_reservationtake a closure,which lets it get a fresh
reservation_sizefor each read. This is animplementation detail which doesn't affect any public API.