nix_bindings_expr/
primop.rs

1use crate::eval_state::{EvalState, EvalStateWeak};
2use crate::value::Value;
3use anyhow::Result;
4use nix_bindings_expr_sys as raw;
5use nix_bindings_util::check_call;
6use nix_bindings_util_sys as raw_util;
7use std::ffi::{c_int, c_void, CStr, CString};
8use std::mem::ManuallyDrop;
9use std::ptr::{null, null_mut};
10
11/// Metadata for a primop, used with `PrimOp::new`.
12pub struct PrimOpMeta<'a, const N: usize> {
13    /// Name of the primop. Note that primops do not have to be registered as
14    /// builtins. Nonetheless, a name is required for documentation purposes, e.g.
15    /// :doc in the repl.
16    pub name: &'a CStr,
17
18    /// Documentation for the primop. This is displayed in the repl when using
19    /// :doc. The format is markdown.
20    pub doc: &'a CStr,
21
22    /// The number of arguments the function takes, as well as names for the
23    /// arguments, to be presented in the documentation (if applicable, e.g.
24    /// :doc in the repl).
25    pub args: [&'a CStr; N],
26}
27
28pub struct PrimOp {
29    pub(crate) ptr: *mut raw::PrimOp,
30}
31impl Drop for PrimOp {
32    fn drop(&mut self) {
33        unsafe {
34            raw::gc_decref(null_mut(), self.ptr as *mut c_void);
35        }
36    }
37}
38impl PrimOp {
39    pub fn new<const N: usize>(
40        eval_state: &mut EvalState,
41        meta: PrimOpMeta<N>,
42        f: Box<dyn Fn(&mut EvalState, &[Value; N]) -> Result<Value>>,
43    ) -> Result<PrimOp> {
44        assert!(N != 0);
45
46        let mut args = Vec::new();
47        for arg in meta.args {
48            args.push(arg.as_ptr());
49        }
50        args.push(null());
51
52        // Primops weren't meant to be dynamically created, as of writing.
53        // This leaks, and so do the primop fields in Nix internally.
54        let user_data = {
55            // We'll be leaking this Box.
56            // TODO: Use the GC with finalizer, if possible.
57            let user_data = ManuallyDrop::new(Box::new(PrimOpContext {
58                arity: N,
59                function: Box::new(move |eval_state, args| f(eval_state, args.try_into().unwrap())),
60                eval_state: eval_state.weak_ref(),
61            }));
62            user_data.as_ref() as *const PrimOpContext as *mut c_void
63        };
64        let op = unsafe {
65            check_call!(raw::alloc_primop(
66                &mut eval_state.context,
67                FUNCTION_ADAPTER,
68                N as c_int,
69                meta.name.as_ptr(),
70                args.as_mut_ptr(), /* TODO add an extra const to bindings to avoid mut here. */
71                meta.doc.as_ptr(),
72                user_data
73            ))?
74        };
75        Ok(PrimOp { ptr: op })
76    }
77}
78
79/// The user_data for our Nix primops
80struct PrimOpContext {
81    arity: usize,
82    function: Box<dyn Fn(&mut EvalState, &[Value]) -> Result<Value>>,
83    eval_state: EvalStateWeak,
84}
85
86unsafe extern "C" fn function_adapter(
87    user_data: *mut ::std::os::raw::c_void,
88    context_out: *mut raw_util::c_context,
89    _state: *mut raw::EvalState,
90    args: *mut *mut raw::Value,
91    ret: *mut raw::Value,
92) {
93    let primop_info = (user_data as *const PrimOpContext).as_ref().unwrap();
94    let mut eval_state = primop_info.eval_state.upgrade().unwrap_or_else(|| {
95        panic!("Nix primop called after EvalState was dropped");
96    });
97    let args_raw_slice = unsafe { std::slice::from_raw_parts(args, primop_info.arity) };
98    let args_vec: Vec<Value> = args_raw_slice
99        .iter()
100        .map(|v| Value::new_borrowed(*v))
101        .collect();
102    let args_slice = args_vec.as_slice();
103
104    let r = primop_info.function.as_ref()(&mut eval_state, args_slice);
105
106    match r {
107        Ok(v) => unsafe {
108            raw::copy_value(context_out, ret, v.raw_ptr());
109        },
110        Err(e) => unsafe {
111            let cstr = CString::new(e.to_string()).unwrap_or_else(|_e| {
112                CString::new("<rust nix-expr application error message contained null byte>")
113                    .unwrap()
114            });
115            raw_util::set_err_msg(context_out, raw_util::err_NIX_ERR_UNKNOWN, cstr.as_ptr());
116        },
117    }
118}
119
120static FUNCTION_ADAPTER: raw::PrimOpFun = Some(function_adapter);
nix_bindings_expr/primop.rs

nix_bindings_expr/
primop.rs
