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annotate rust/hg-cpython/src/cindex.rs @ 41054:ef54bd33b476

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rust: core implementation for lazyancestors Once exposed through appropriate bindings, this should be able to replace ancestor.lazyancestors entirely. Differential Revision: https://phab.mercurial-scm.org/D5440
author Georges Racinet <gracinet@anybox.fr>
date Tue, 04 Dec 2018 11:05:06 +0100
parents 4c25038c112c
children ab0d762d89ef
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1 // cindex.rs
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2 //
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3 // Copyright 2018 Georges Racinet <gracinet@anybox.fr>
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4 //
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5 // This software may be used and distributed according to the terms of the
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6 // GNU General Public License version 2 or any later version.
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7
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8 //! Bindings to use the Index defined by the parsers C extension
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9 //!
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10 //! Ideally, we should use an Index entirely implemented in Rust,
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11 //! but this will take some time to get there.
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12 #[cfg(feature = "python27")]
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13 extern crate python27_sys as python_sys;
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14 #[cfg(feature = "python3")]
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15 extern crate python3_sys as python_sys;
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16
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17 use self::python_sys::PyCapsule_Import;
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18 use cpython::{PyClone, PyErr, PyObject, PyResult, Python};
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19 use hg::{Graph, GraphError, Revision};
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20 use libc::c_int;
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21 use std::ffi::CStr;
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22 use std::mem::transmute;
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23
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24 type IndexParentsFn = unsafe extern "C" fn(
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25 index: *mut python_sys::PyObject,
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26 rev: c_int,
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27 ps: *mut [c_int; 2],
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28 ) -> c_int;
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29
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30 /// A `Graph` backed up by objects and functions from revlog.c
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31 ///
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32 /// This implementation of the `Graph` trait, relies on (pointers to)
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33 /// - the C index object (`index` member)
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34 /// - the `index_get_parents()` function (`parents` member)
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35 ///
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36 /// # Safety
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37 ///
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38 /// The C index itself is mutable, and this Rust exposition is **not
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39 /// protected by the GIL**, meaning that this construct isn't safe with respect
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40 /// to Python threads.
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41 ///
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42 /// All callers of this `Index` must acquire the GIL and must not release it
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43 /// while working.
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44 ///
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45 /// # TODO find a solution to make it GIL safe again.
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46 ///
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47 /// This is non trivial, and can wait until we have a clearer picture with
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48 /// more Rust Mercurial constructs.
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49 ///
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50 /// One possibility would be to a `GILProtectedIndex` wrapper enclosing
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51 /// a `Python<'p>` marker and have it be the one implementing the
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52 /// `Graph` trait, but this would mean the `Graph` implementor would become
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53 /// likely to change between subsequent method invocations of the `hg-core`
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54 /// objects (a serious change of the `hg-core` API):
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55 /// either exposing ways to mutate the `Graph`, or making it a non persistent
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56 /// parameter in the relevant methods that need one.
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57 ///
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58 /// Another possibility would be to introduce an abstract lock handle into
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59 /// the core API, that would be tied to `GILGuard` / `Python<'p>`
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60 /// in the case of the `cpython` crate bindings yet could leave room for other
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61 /// mechanisms in other contexts.
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62 pub struct Index {
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63 index: PyObject,
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64 parents: IndexParentsFn,
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65 }
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66
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67 impl Index {
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68 pub fn new(py: Python, index: PyObject) -> PyResult<Self> {
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69 Ok(Index {
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70 index: index,
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71 parents: decapsule_parents_fn(py)?,
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72 })
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73 }
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74 }
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75
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76 impl Clone for Index {
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77 fn clone(&self) -> Self {
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78 let guard = Python::acquire_gil();
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79 Index {
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80 index: self.index.clone_ref(guard.python()),
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81 parents: self.parents.clone(),
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82 }
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83 }
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84 }
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85
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86 impl Graph for Index {
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87 /// wrap a call to the C extern parents function
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88 fn parents(&self, rev: Revision) -> Result<[Revision; 2], GraphError> {
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89 let mut res: [c_int; 2] = [0; 2];
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90 let code = unsafe {
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91 (self.parents)(
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92 self.index.as_ptr(),
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93 rev as c_int,
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94 &mut res as *mut [c_int; 2],
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95 )
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96 };
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97 match code {
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98 0 => Ok(res),
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99 _ => Err(GraphError::ParentOutOfRange(rev)),
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100 }
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101 }
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102 }
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103
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104 /// Return the `index_get_parents` function of the parsers C Extension module.
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105 ///
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106 /// A pointer to the function is stored in the `parsers` module as a
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107 /// standard [Python capsule](https://docs.python.org/2/c-api/capsule.html).
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108 ///
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109 /// This function retrieves the capsule and casts the function pointer
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110 ///
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111 /// Casting function pointers is one of the rare cases of
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112 /// legitimate use cases of `mem::transmute()` (see
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113 /// https://doc.rust-lang.org/std/mem/fn.transmute.html of
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114 /// `mem::transmute()`.
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115 /// It is inappropriate for architectures where
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116 /// function and data pointer sizes differ (so-called "Harvard
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117 /// architectures"), but these are nowadays mostly DSPs
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118 /// and microcontrollers, hence out of our scope.
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119 fn decapsule_parents_fn(py: Python) -> PyResult<IndexParentsFn> {
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120 unsafe {
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121 let caps_name = CStr::from_bytes_with_nul_unchecked(
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122 b"mercurial.cext.parsers.index_get_parents_CAPI\0",
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123 );
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124 let from_caps = PyCapsule_Import(caps_name.as_ptr(), 0);
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125 if from_caps.is_null() {
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126 return Err(PyErr::fetch(py));
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127 }
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128 Ok(transmute(from_caps))
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129 }
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130 }