1 // cindex.rs

     2 //

     3 // Copyright 2018 Georges Racinet <gracinet@anybox.fr>

     4 //

     5 // This software may be used and distributed according to the terms of the

     6 // GNU General Public License version 2 or any later version.

     7 

     8 //! Bindings to use the Index defined by the parsers C extension

     9 //!

    10 //! Ideally, we should use an Index entirely implemented in Rust,

    11 //! but this will take some time to get there.

    12 #[cfg(feature = "python27")]

    13 extern crate python27_sys as python_sys;

    14 #[cfg(feature = "python3")]

    15 extern crate python3_sys as python_sys;

    16 

    17 use self::python_sys::PyCapsule_Import;

    18 use cpython::{PyErr, PyObject, PyResult, Python};

    19 use hg::{Graph, GraphError, Revision};

    20 use libc::c_int;

    21 use std::ffi::CStr;

    22 use std::mem::transmute;

    23 

    24 type IndexParentsFn = unsafe extern "C" fn(

    25     index: *mut python_sys::PyObject,

    26     rev: c_int,

    27     ps: *mut [c_int; 2],

    28 ) -> c_int;

    29 

    30 /// A `Graph` backed up by objects and functions from revlog.c

    31 ///

    32 /// This implementation of the `Graph` trait, relies on (pointers to)

    33 /// - the C index object (`index` member)

    34 /// - the `index_get_parents()` function (`parents` member)

    35 ///

    36 /// # Safety

    37 ///

    38 /// The C index itself is mutable, and this Rust exposition is **not

    39 /// protected by the GIL**, meaning that this construct isn't safe with respect

    40 /// to Python threads.

    41 ///

    42 /// All callers of this `Index` must acquire the GIL and must not release it

    43 /// while working.

    44 ///

    45 /// # TODO find a solution to make it GIL safe again.

    46 ///

    47 /// This is non trivial, and can wait until we have a clearer picture with

    48 /// more Rust Mercurial constructs.

    49 ///

    50 /// One possibility would be to a `GILProtectedIndex` wrapper enclosing

    51 /// a `Python<'p>` marker and have it be the one implementing the

    52 /// `Graph` trait, but this would mean the `Graph` implementor would become

    53 /// likely to change between subsequent method invocations of the `hg-core`

    54 /// objects (a serious change of the `hg-core` API):

    55 /// either exposing ways to mutate the `Graph`, or making it a non persistent

    56 /// parameter in the relevant methods that need one.

    57 ///

    58 /// Another possibility would be to introduce an abstract lock handle into

    59 /// the core API, that would be tied to `GILGuard` / `Python<'p>`

    60 /// in the case of the `cpython` crate bindings yet could leave room for other

    61 /// mechanisms in other contexts.

    62 

    63 pub struct Index {

    64     index: PyObject,

    65     parents: IndexParentsFn,

    66 }

    67 

    68 impl Index {

    69     pub fn new(py: Python, index: PyObject) -> PyResult<Self> {

    70         Ok(Index {

    71             index: index,

    72             parents: decapsule_parents_fn(py)?,

    73         })

    74     }

    75 }

    76 

    77 impl Graph for Index {

    78     /// wrap a call to the C extern parents function

    79     fn parents(&self, rev: Revision) -> Result<[Revision; 2], GraphError> {

    80         let mut res: [c_int; 2] = [0; 2];

    81         let code = unsafe {

    82             (self.parents)(

    83                 self.index.as_ptr(),

    84                 rev as c_int,

    85                 &mut res as *mut [c_int; 2],

    86             )

    87         };

    88         match code {

    89             0 => Ok(res),

    90             _ => Err(GraphError::ParentOutOfRange(rev)),

    91         }

    92     }

    93 }

    94 

    95 /// Return the `index_get_parents` function of the parsers C Extension module.

    96 ///

    97 /// A pointer to the function is stored in the `parsers` module as a

    98 /// standard [Python capsule](https://docs.python.org/2/c-api/capsule.html).

    99 ///

   100 /// This function retrieves the capsule and casts the function pointer

   101 ///

   102 /// Casting function pointers is one of the rare cases of

   103 /// legitimate use cases of `mem::transmute()` (see

   104 /// https://doc.rust-lang.org/std/mem/fn.transmute.html of

   105 /// `mem::transmute()`.

   106 /// It is inappropriate for architectures where

   107 /// function and data pointer sizes differ (so-called "Harvard

   108 /// architectures"), but these are nowadays mostly DSPs

   109 /// and microcontrollers, hence out of our scope.

   110 fn decapsule_parents_fn(py: Python) -> PyResult<IndexParentsFn> {

   111     unsafe {

   112         let caps_name = CStr::from_bytes_with_nul_unchecked(

   113             b"mercurial.cext.parsers.index_get_parents_CAPI\0",

   114         );

   115         let from_caps = PyCapsule_Import(caps_name.as_ptr(), 0);

   116         if from_caps.is_null() {

   117             return Err(PyErr::fetch(py));

   118         }

   119         Ok(transmute(from_caps))

   120     }

   121 }