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annotate rust/hg-core/src/dirstate_tree/owning.rs @ 48605:a809f1465a76

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filemerge: set default labels a little earlier By setting the default labels a little earlier, we can rely on them always being set, as far as I can tell. It may actually even be fine to rely on that even if we don't set them earlier, but it makes more sense to me to do it. Differential Revision: https://phab.mercurial-scm.org/D12015
author Martin von Zweigbergk <martinvonz@google.com>
date Thu, 20 Jan 2022 13:43:43 -0800
parents d1210d56008b
children cfd270d83169
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1 use super::dirstate_map::DirstateMap;
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2 use stable_deref_trait::StableDeref;
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3 use std::ops::Deref;
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4
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5 /// Keep a `DirstateMap<'on_disk>` next to the `on_disk` buffer that it
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6 /// borrows.
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7 ///
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8 /// This is similar to [`OwningRef`] which is more limited because it
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9 /// represents exactly one `&T` reference next to the value it borrows, as
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10 /// opposed to a struct that may contain an arbitrary number of references in
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11 /// arbitrarily-nested data structures.
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12 ///
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13 /// [`OwningRef`]: https://docs.rs/owning_ref/0.4.1/owning_ref/struct.OwningRef.html
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14 pub struct OwningDirstateMap {
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15 /// Owned handle to a bytes buffer with a stable address.
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16 ///
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17 /// See <https://docs.rs/owning_ref/0.4.1/owning_ref/trait.StableAddress.html>.
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18 on_disk: Box<dyn Deref<Target = [u8]> + Send>,
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19
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20 /// Pointer for `Box<DirstateMap<'on_disk>>`, typed-erased because the
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21 /// language cannot represent a lifetime referencing a sibling field.
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22 /// This is not quite a self-referencial struct (moving this struct is not
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23 /// a problem as it doesn’t change the address of the bytes buffer owned
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24 /// by `on_disk`) but touches similar borrow-checker limitations.
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25 ptr: *mut (),
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26 }
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27
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28 impl OwningDirstateMap {
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29 pub fn new_empty<OnDisk>(on_disk: OnDisk) -> Self
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30 where
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31 OnDisk: Deref<Target = [u8]> + StableDeref + Send + 'static,
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32 {
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33 let on_disk = Box::new(on_disk);
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34 let bytes: &'_ [u8] = &on_disk;
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35 let map = DirstateMap::empty(bytes);
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36
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37 // Like in `bytes` above, this `'_` lifetime parameter borrows from
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38 // the bytes buffer owned by `on_disk`.
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39 let ptr: *mut DirstateMap<'_> = Box::into_raw(Box::new(map));
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40
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41 // Erase the pointed type entirely in order to erase the lifetime.
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42 let ptr: *mut () = ptr.cast();
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43
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44 Self { on_disk, ptr }
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45 }
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46
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47 pub fn get_pair_mut<'a>(
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48 &'a mut self,
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49 ) -> (&'a [u8], &'a mut DirstateMap<'a>) {
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50 // SAFETY: We cast the type-erased pointer back to the same type it had
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51 // in `new`, except with a different lifetime parameter. This time we
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52 // connect the lifetime to that of `self`. This cast is valid because
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53 // `self` owns the same `on_disk` whose buffer `DirstateMap`
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54 // references. That buffer has a stable memory address because our
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55 // `Self::new_empty` counstructor requires `StableDeref`.
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56 let ptr: *mut DirstateMap<'a> = self.ptr.cast();
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57 // SAFETY: we dereference that pointer, connecting the lifetime of the
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58 // new `&mut` to that of `self`. This is valid because the
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59 // raw pointer is to a boxed value, and `self` owns that box.
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60 (&self.on_disk, unsafe { &mut *ptr })
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61 }
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62
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63 pub fn get_map_mut<'a>(&'a mut self) -> &'a mut DirstateMap<'a> {
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64 self.get_pair_mut().1
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65 }
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66
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67 pub fn get_map<'a>(&'a self) -> &'a DirstateMap<'a> {
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68 // SAFETY: same reasoning as in `get_pair_mut` above.
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69 let ptr: *mut DirstateMap<'a> = self.ptr.cast();
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70 unsafe { &*ptr }
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71 }
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72
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73 pub fn on_disk<'a>(&'a self) -> &'a [u8] {
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74 &self.on_disk
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75 }
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76 }
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77
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78 impl Drop for OwningDirstateMap {
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79 fn drop(&mut self) {
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80 // Silence a "field is never read" warning, and demonstrate that this
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81 // value is still alive.
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82 let _: &Box<dyn Deref<Target = [u8]> + Send> = &self.on_disk;
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83 // SAFETY: this cast is the same as in `get_mut`, and is valid for the
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84 // same reason. `self.on_disk` still exists at this point, drop glue
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85 // will drop it implicitly after this `drop` method returns.
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86 let ptr: *mut DirstateMap<'_> = self.ptr.cast();
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87 // SAFETY: `Box::from_raw` takes ownership of the box away from `self`.
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88 // This is fine because drop glue does nothing for `*mut ()` and we’re
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89 // in `drop`, so `get` and `get_mut` cannot be called again.
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90 unsafe { drop(Box::from_raw(ptr)) }
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91 }
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92 }
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93
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94 fn _static_assert_is_send<T: Send>() {}
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95
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96 fn _static_assert_fields_are_send() {
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97 _static_assert_is_send::<Box<DirstateMap<'_>>>();
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98 }
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99
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100 // SAFETY: we don’t get this impl implicitly because `*mut (): !Send` because
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101 // thread-safety of raw pointers is unknown in the general case. However this
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102 // particular raw pointer represents a `Box<DirstateMap<'on_disk>>` that we
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103 // own. Since that `Box` is `Send` as shown in above, it is sound to mark
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104 // this struct as `Send` too.
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105 unsafe impl Send for OwningDirstateMap {}