nixide/rust/nix-util/src/context.rs

use anyhow::{bail, Result};
use nix_c_raw as raw;
use std::ptr::null_mut;
use std::ptr::NonNull;

pub struct Context {
    inner: NonNull<raw::c_context>,
}

impl Context {
    pub fn new() -> Self {
        let ctx = unsafe { raw::c_context_create() };
        if ctx.is_null() {
            // We've failed to allocate a (relatively small) Context struct.
            // We're almost certainly going to crash anyways.
            panic!("nix_c_context_create returned a null pointer");
        }
        let ctx = Context {
            inner: NonNull::new(ctx).unwrap(),
        };
        ctx
    }
    pub fn ptr(&self) -> *mut raw::c_context {
        self.inner.as_ptr()
    }
    pub fn check_err(&self) -> Result<()> {
        let err = unsafe { raw::err_code(self.inner.as_ptr()) };
        if err != raw::NIX_OK.try_into().unwrap() {
            // msgp is a borrowed pointer (pointing into the context), so we don't need to free it
            let msgp = unsafe { raw::err_msg(null_mut(), self.inner.as_ptr(), null_mut()) };
            // Turn the i8 pointer into a Rust string by copying
            let msg: &str = unsafe { core::ffi::CStr::from_ptr(msgp).to_str()? };
            bail!("{}", msg);
        }
        Ok(())
    }

    pub fn check_err_and_clear(&self) -> Result<()> {
        let r = self.check_err();
        if r.is_err() {
            unsafe {
                // TODO (https://github.com/NixOS/nix/pull/10910): raw::err_clear
                raw::set_err_msg(
                    self.inner.as_ptr(),
                    raw::NIX_OK.try_into().unwrap(),
                    b"\0".as_ptr() as *const i8,
                );
            }
        }
        r
    }

    /// Run the function, and check the error, then reset the error.
    /// Make at most one call to a Nix function in `f`.
    /// Do not use if the context isn't fresh or cleared (e.g. with `check_err_and_clear`).
    pub fn check_one_call<T, F: FnOnce(*mut raw::c_context) -> T>(&self, f: F) -> Result<T> {
        let t = f(self.ptr());
        self.check_err_and_clear()?;
        Ok(t)
    }

    /// NIX_ERR_KEY is returned when e.g. an attribute is missing. Return true if the error is of this type.
    pub fn is_key_error(&self) -> bool {
        unsafe { raw::err_code(self.inner.as_ptr()) == raw::NIX_ERR_KEY }
    }
}

impl Drop for Context {
    fn drop(&mut self) {
        unsafe {
            raw::c_context_free(self.inner.as_ptr());
        }
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn context_new_and_drop() {
        // don't crash
        let _c = Context::new();
    }
}