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annotate rust/hg-cpython/src/pybytes_deref.rs @ 47982:4afd6cc447b9

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rust: Make OwningDirstateMap generic and move it into hg-core This will enable using it in rhg too. The `OwningDirstateMap::new_empty` constructor is generic and accepts a value of any type that gives acces to a bytes buffer. That buffer must stay valid as long as the value hasn?t been dropped, and must keep its memory address even if the value is moved. The `StableDeref` marker trait encodes those constraints. Previously no trait was needed because the value was always of type `PyBytes` which we know satisfies those constraints. The buffer type is ereased in the struct itself through boxing and dynamic dispatch, in order to simplify other signatures that mention `OwningDirstateMap`. Differential Revision: https://phab.mercurial-scm.org/D11396
author Simon Sapin <simon.sapin@octobus.net>
date Thu, 09 Sep 2021 18:07:40 +0200
parents 8f031a274cd6
children be3b545c5cff
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1 use cpython::{PyBytes, Python};
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2 use stable_deref_trait::StableDeref;
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3
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4 /// Safe abstraction over a `PyBytes` together with the `&[u8]` slice
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5 /// that borrows it. Implements `Deref<Target = [u8]>`.
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6 ///
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7 /// Calling `PyBytes::data` requires a GIL marker but we want to access the
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8 /// data in a thread that (ideally) does not need to acquire the GIL.
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9 /// This type allows separating the call an the use.
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10 ///
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11 /// It also enables using a (wrapped) `PyBytes` in GIL-unaware generic code.
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12 pub struct PyBytesDeref {
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13 #[allow(unused)]
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14 keep_alive: PyBytes,
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15
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16 /// Borrows the buffer inside `self.keep_alive`,
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17 /// but the borrow-checker cannot express self-referential structs.
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18 data: *const [u8],
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19 }
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20
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21 impl PyBytesDeref {
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22 pub fn new(py: Python, bytes: PyBytes) -> Self {
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23 Self {
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24 data: bytes.data(py),
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25 keep_alive: bytes,
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26 }
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27 }
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29 pub fn unwrap(self) -> PyBytes {
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30 self.keep_alive
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31 }
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32 }
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33
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34 impl std::ops::Deref for PyBytesDeref {
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35 type Target = [u8];
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36
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37 fn deref(&self) -> &[u8] {
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38 // Safety: the raw pointer is valid as long as the PyBytes is still
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39 // alive, and the returned slice borrows `self`.
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40 unsafe { &*self.data }
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41 }
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42 }
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43
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44 unsafe impl StableDeref for PyBytesDeref {}
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45
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46 fn require_send<T: Send>() {}
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47
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48 #[allow(unused)]
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49 fn static_assert_pybytes_is_send() {
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50 require_send::<PyBytes>;
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51 }
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52
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53 // Safety: PyBytes is Send. Raw pointers are not by default,
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54 // but here sending one to another thread is fine since we ensure it stays
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55 // valid.
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56 unsafe impl Send for PyBytesDeref {}