fetch_ml/native/rust/common/src/lib.rs

//! Common FFI utilities for native libraries
//!
//! Provides safe wrappers for FFI boundary operations including:
//! - Panic recovery at FFI boundaries
//! - String conversion between C and Rust
//! - Error handling patterns

use std::ffi::{CStr, CString};
use std::os::raw::{c_char, c_int};
use std::ptr;

/// Recover from panics at FFI boundaries, returning a safe default
///
/// # Safety
/// The closure should not leak resources on panic
pub unsafe fn ffi_boundary<T>(f: impl FnOnce() -> T + std::panic::UnwindSafe) -> Option<T> {
    match std::panic::catch_unwind(f) {
        Ok(result) => Some(result),
        Err(_) => {
            eprintln!("FFI boundary panic caught and recovered");
            None
        }
    }
}

/// Convert C string to Rust String
/// 
/// # Safety
/// ptr must be a valid null-terminated UTF-8 string or null
pub unsafe fn c_str_to_string(ptr: *const c_char) -> Option<String> {
    if ptr.is_null() {
        return None;
    }
    
    CStr::from_ptr(ptr)
        .to_str()
        .ok()
        .map(|s| s.to_string())
}

/// Convert Rust String to C string (leaked, caller must free)
/// 
/// Returns null on error. On success, returns a pointer that must be freed with `free_string`.
pub fn string_to_c_str(s: &str) -> *mut c_char {
    match CString::new(s) {
        Ok(cstring) => cstring.into_raw(),
        Err(_) => ptr::null_mut(),
    }
}

/// Free a string previously created by `string_to_c_str`
/// 
/// # Safety
/// ptr must be a string previously returned by string_to_c_str, or null
pub unsafe fn free_string(ptr: *mut c_char) {
    if !ptr.is_null() {
        let _ = CString::from_raw(ptr);
    }
}

/// Set an error code and message
/// 
/// Returns -1 for error, caller should return this from FFI function
pub fn set_error(error_ptr: *mut *const c_char, msg: &str) -> c_int {
    if !error_ptr.is_null() {
        unsafe {
            *error_ptr = string_to_c_str(msg);
        }
    }
    -1
}

/// FFI-safe result type for boolean operations
pub type FfiResult = c_int;
pub const FFI_OK: FfiResult = 0;
pub const FFI_ERROR: FfiResult = -1;

/// Thread-local error storage for FFI boundaries
pub mod error {
    use std::cell::RefCell;
    
    thread_local! {
        static LAST_ERROR: RefCell<Option<String>> = RefCell::new(None);
    }
    
    /// Store an error message
    pub fn set_error(msg: impl Into<String>) {
        LAST_ERROR.with(|e| {
            *e.borrow_mut() = Some(msg.into());
        });
    }
    
    /// Get and clear the last error
    pub fn take_error() -> Option<String> {
        LAST_ERROR.with(|e| e.borrow_mut().take())
    }
    
    /// Peek at the last error without clearing
    pub fn peek_error() -> Option<String> {
        LAST_ERROR.with(|e| e.borrow().clone())
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    
    #[test]
    fn test_c_str_roundtrip() {
        let original = "hello world";
        let c_ptr = string_to_c_str(original);
        assert!(!c_ptr.is_null());
        
        unsafe {
            let recovered = c_str_to_string(c_ptr);
            assert_eq!(recovered, Some(original.to_string()));
            free_string(c_ptr);
        }
    }
    
    #[test]
    fn test_null_handling() {
        unsafe {
            assert_eq!(c_str_to_string(ptr::null()), None);
            free_string(ptr::null_mut()); // Should not panic
        }
    }
    
    #[test]
    fn test_ffi_boundary_recovery() {
        unsafe {
            // Normal case
            let result = ffi_boundary(|| 42);
            assert_eq!(result, Some(42));
            
            // Panic case - should recover
            let result = ffi_boundary(|| {
                panic!("test panic");
            });
            assert_eq!(result, None);
        }
    }
}