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rust-index: add a struct wrapping the C index Implementing the full index logic in one go is journey larger than we would like. To achieve a smoother transition, we start with a simple Rust wrapper that delegates allwork to the current C implementation. Once we will have a fully working index object in Rust, we can easily start using more and more Rust Code with it. The object in this patch is functional and tested. However, multiple of the currently existing rust (in the `hg-cpython` crate) requires a `Graph`. Right now we build this `Graph` (as cindex::Index) using the C index passed as a PyObject. They will have to be updated to be made compatible. Differential Revision: https://phab.mercurial-scm.org/D7655
author Georges Racinet <georges.racinet@octobus.net>
date Mon, 23 Dec 2019 10:02:50 -0800
parents ab3fd8077f5e
children 2728fcb8127c
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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
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13 use cpython::{PyClone, PyObject, PyResult, Python};
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14 use hg::{Graph, GraphError, Revision, WORKING_DIRECTORY_REVISION};
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15 use libc::c_int;
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16
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17 #[repr(C)]
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18 pub struct Revlog_CAPI {
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19 index_parents: unsafe extern "C" fn(
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20 index: *mut revlog_capi::RawPyObject,
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21 rev: c_int,
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22 ps: *mut [c_int; 2],
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23 ) -> c_int,
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24 }
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25
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26 py_capsule!(
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27 from mercurial.cext.parsers import revlog_CAPI
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28 as revlog_capi for Revlog_CAPI);
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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 capi: &'static Revlog_CAPI,
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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 capi: unsafe { revlog_capi::retrieve(py)? },
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72 })
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73 }
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74
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75 /// return a reference to the CPython Index object in this Struct
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76 pub fn inner(&self) -> &PyObject {
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77 &self.index
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78 }
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79 }
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80
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81 impl Clone for Index {
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82 fn clone(&self) -> Self {
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83 let guard = Python::acquire_gil();
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84 Index {
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85 index: self.index.clone_ref(guard.python()),
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86 capi: self.capi,
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87 }
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88 }
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89 }
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90
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91 impl Graph for Index {
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92 /// wrap a call to the C extern parents function
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93 fn parents(&self, rev: Revision) -> Result<[Revision; 2], GraphError> {
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94 if rev == WORKING_DIRECTORY_REVISION {
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95 return Err(GraphError::WorkingDirectoryUnsupported);
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96 }
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97 let mut res: [c_int; 2] = [0; 2];
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98 let code = unsafe {
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99 (self.capi.index_parents)(
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100 self.index.as_ptr(),
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101 rev as c_int,
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102 &mut res as *mut [c_int; 2],
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103 )
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104 };
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105 match code {
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106 0 => Ok(res),
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107 _ => Err(GraphError::ParentOutOfRange(rev)),
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108 }
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109 }
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110 }