patterns/idioms/ffi/errors.md

Error Handling in FFI

Description

In foreign languages like C, errors are represented by return codes. However, Rust's type system allows much more rich error information to be captured and propogated through a full type.

This best practice shows different kinds of error codes, and how to expose them in a usable way:

1. Flat Enums should be converted to integers and returned as codes.
2. Structured Enums should be converted to an integer code with a string error message for detail.
3. Custom Error Types should become "transparent", with a C representation.

Code Example

Flat Enums

enum DatabaseError {
    IsReadOnly = 1, // user attempted a write operation
    IOError = 2, // user should read the C errno() for what it was
    FileCorrupted = 3, // user should run a repair tool to recover it
}

impl From<DatabaseError> for libc::c_int {
    fn from(e: DatabaseError) -> libc::c_int {
        (e as i8).into()
    }
}

Structured Enums

pub mod errors {
    enum DatabaseError {
        IsReadOnly,
        IOError(std::io::Error),
        FileCorrupted(String), // message describing the issue
    }

    impl From<DatabaseError> for libc::c_int {
        fn from(e: DatabaseError) -> libc::c_int {
            match e {
                DatabaseError::IsReadOnly => 1,
                DatabaseError::IOError(_) => 2,
                DatabaseError::FileCorrupted(_) => 3,
            }
        }
    }
}

pub mod c_api {
    use super::errors::DatabaseError;

    #[no_mangle]
    pub extern "C" fn db_error_description(
        e: *const DatabaseError
        ) -> *mut libc::c_char {

        let error: &DatabaseError = unsafe {
            // SAFETY: pointer lifetime is greater than the current stack frame
            &*e
        };

        let error_str: String = match error {
            DatabaseError::IsReadOnly => {
                format!("cannot write to read-only database");
            }
            DatabaseError::IOError(e) => {
                format!("I/O Error: {}", e);
            }
            DatabaseError::FileCorrupted(s) => {
                format!("File corrupted, run repair: {}", &s);
            }
        };

        let c_error = unsafe {
            // SAFETY: copying error_str to an allocated buffer with a NUL
            // character at the end
            let mut malloc: *mut u8 = libc::malloc(error_str.len() + 1) as *mut _;

            if malloc.is_null() {
                return std::ptr::null_mut();
            }

            let src = error_str.as_bytes().as_ptr();

            std::ptr::copy_nonoverlapping(src, malloc, error_str.len());

            std::ptr::write(malloc.add(error_str.len()), 0);

            malloc as *mut libc::c_char
        };

        c_error
    }
}

Custom Error Types

struct ParseError {
    expected: char,
    line: u32,
    ch: u16
}

impl ParseError { /* ... */ }

/* Create a second version which is exposed as a C structure */
#[repr(C)]
pub struct parse_error {
    pub expected: libc::c_char,
    pub line: u32,
    pub ch: u16
}

impl From<ParseError> for parse_error {
    fn from(e: ParseError) -> parse_error {
        let ParseError { expected, line, ch } = e;
        parse_error { expected, line, ch }
    }
}

Advantages

This ensures that the foreign language has clear access to error information while not compromising the Rust code's API at all.

Disadvantages

It's a lot of typing, and some types may not be able to be converted easily to C.