fs_core/

ffi.rs

//! C ABI for the block-device framework.
//!
//! Every sister crate (qcow2 reader, partition probe, fs-* drivers) speaks
//! through the [`FsCoreDevice`] handle defined here, so consumers (Swift
//! FSKit modules, Go callers, C programs) only learn one device-handle
//! type and one error convention.
//!
//! ## Conventions
//!
//! - Handles are opaque `*mut FsCoreDevice`. Allocate via a constructor in
//!   one of the sister crates (e.g. `qcow2_open` from rust-img-qcow2),
//!   free via [`fs_core_device_close`] regardless of which crate created
//!   it.
//! - Error reporting is errno-style: every fallible function returns an
//!   [`FsCoreErrorCode`] (0 = OK, non-zero = failure) and stashes a human
//!   message in a thread-local. Read it via
//!   [`fs_core_last_error_message`].
//! - Every entry point catches Rust panics with `catch_unwind` and maps
//!   them to [`FsCoreErrorCode::Panic`]. Crossing an FFI boundary while
//!   unwinding is UB; the catch-net is non-negotiable.
//! - Thread safety: handles wrap `Arc<dyn BlockDevice>`, which is
//!   `Send + Sync` by trait bound. Multiple threads can call read/write
//!   concurrently as long as the underlying device's locking permits it.

#![allow(clippy::missing_safety_doc)]

use crate::block::BlockDevice;
use crate::callback_device::CallbackDevice;
use crate::error::Error;
use std::cell::RefCell;
use std::ffi::{c_char, c_int, c_void, CString};
use std::io;
use std::panic::AssertUnwindSafe;
use std::ptr;
use std::slice;
use std::sync::Arc;

// ---------------------------------------------------------------------------
// Error codes — kept dense and stable so consumers can hard-code them.
// ---------------------------------------------------------------------------

/// Numeric error codes mirrored across every sister crate's C ABI.
///
/// `#[repr(i32)]` so the layout is identical to the matching C `enum`.
#[repr(i32)]
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum FsCoreErrorCode {
    /// Success.
    Ok = 0,
    /// Underlying I/O failed.
    Io = 1,
    /// A read the source could not satisfy in full — it ran out of data.
    /// What a file-backed handle returns for a read off the end of the
    /// file, and what a slice returns for a read past its own end.
    ShortRead = 2,
    /// Write attempted on a read-only device.
    ReadOnly = 3,
    /// A request refused up front because its range lies outside the
    /// device's declared size; nothing was transferred.
    ///
    /// This crate returns it only for a **write** past the end of an RW
    /// slice. It reaches **reads** from sister crates whose container
    /// declares a virtual size (the `img-*` readers), and from this
    /// crate's caching / read-only / slice wrappers when they forward
    /// such a parent's error. A C consumer that only wants to know "the
    /// read overran the device", and does not control which crate opened
    /// the handle, should accept this and `FS_CORE_SHORT_READ` alike —
    /// and should not treat the pair as exhaustive, since a
    /// callback-backed handle reports its host's refusal as
    /// `FS_CORE_IO`.
    OutOfBounds = 4,
    /// Driver-specific error — message in the thread-local last-error.
    Custom = 5,
    /// One of the input pointers was null.
    NullArg = 6,
    /// `catch_unwind` caught a panic crossing the FFI boundary.
    Panic = 7,
    /// Reserved. Never returned.
    ///
    /// It was meant for a path that is not valid UTF-8, but the one
    /// function that meets that case — `fs_core_open_file` — returns a
    /// POINTER, not a code, so it reports the failure as NULL plus a
    /// message and cannot return this. No other entry point takes a
    /// path.
    ///
    /// Kept rather than removed because the numbering is published in
    /// `include/fs_core.h` and a consumer may already switch on 8;
    /// renumbering the codes after it would be an ABI break for a
    /// tidiness gain. A future path-taking function that returns a code
    /// should use this rather than invent another.
    BadString = 8,
}

impl FsCoreErrorCode {
    fn from_error(e: &Error) -> Self {
        match e {
            Error::Io(_) => FsCoreErrorCode::Io,
            Error::ShortRead { .. } => FsCoreErrorCode::ShortRead,
            Error::ReadOnly => FsCoreErrorCode::ReadOnly,
            Error::OutOfBounds { .. } => FsCoreErrorCode::OutOfBounds,
            Error::Custom(_) => FsCoreErrorCode::Custom,
        }
    }
}

// ---------------------------------------------------------------------------
// Thread-local last-error — errno-style detail companion.
// ---------------------------------------------------------------------------

thread_local! {
    static LAST_ERROR: RefCell<Option<CString>> = const { RefCell::new(None) };
}

/// Stash a message in the thread-local, replacing any previous one. Public
/// to sister crates so they can populate it for their own error paths.
pub fn set_last_error(message: impl Into<String>) {
    let s = message.into();
    let cs = CString::new(s.replace('\0', "?")).expect("contains no NUL after replace");
    LAST_ERROR.with(|slot| {
        *slot.borrow_mut() = Some(cs);
    });
}

fn clear_last_error() {
    LAST_ERROR.with(|slot| {
        *slot.borrow_mut() = None;
    });
}

/// Return a pointer to the calling thread's most recent error message, or
/// NULL if there is none. The pointer is owned by the framework and remains
/// valid until the next FFI call on this thread.
#[unsafe(no_mangle)]
pub extern "C" fn fs_core_last_error_message() -> *const c_char {
    LAST_ERROR.with(|slot| {
        slot.borrow()
            .as_ref()
            .map(|cs| cs.as_ptr())
            .unwrap_or(ptr::null())
    })
}

/// Helper for sister crates: run `body`, catch panics, map errors to codes,
/// stash the message in the thread-local. Returns the error code.
pub fn ffi_guard<F>(body: F) -> FsCoreErrorCode
where
    F: FnOnce() -> Result<(), Error>,
{
    clear_last_error();
    match std::panic::catch_unwind(AssertUnwindSafe(body)) {
        Ok(Ok(())) => FsCoreErrorCode::Ok,
        Ok(Err(e)) => {
            let code = FsCoreErrorCode::from_error(&e);
            set_last_error(e.to_string());
            code
        }
        Err(panic) => {
            set_last_error(panic_message(&panic));
            FsCoreErrorCode::Panic
        }
    }
}

/// Run `body`, catching a panic and returning `fail` instead — and
/// recording the panic's message where a caller can read it.
///
/// # Why the message matters more than the fallback
///
/// Every fallback value here is also a legitimate answer. Zero is what
/// an empty device reports for its size; `false` is what a read-only
/// device reports for writability; a null pointer is what a failed open
/// returns. So a caller that only sees the fallback cannot tell an
/// ordinary answer from a driver that exploded computing it.
///
/// [`fs_core_last_error_message`] is what separates them, and a guard
/// that returns the fallback without setting it throws away the only
/// evidence there was.
///
/// # Why this is separate from [`ffi_guard`]
///
/// `ffi_guard` returns an [`FsCoreErrorCode`] and takes a body that
/// returns `Result<(), Error>`. That fits an entry point whose whole
/// answer is a status code, and fits nothing else — which is why the
/// eight entry points in this file that return a size, a flag or a
/// pointer each wrote `catch_unwind(AssertUnwindSafe(…)).unwrap_or(…)`
/// by hand instead, sixty lines below the helper.
///
/// Sister crates did the same: eleven of them re-roll one of these two
/// shapes rather than share either.
///
/// The error slot is cleared on entry, like [`ffi_guard`]: a call that
/// succeeds must not leave the previous call's message in place for a
/// caller to read and attribute to this one.
///
/// `AssertUnwindSafe` is used deliberately. The bodies here touch a
/// handle the caller owns and a thread-local error slot; a panic can
/// leave neither in a state another call can observe as inconsistent,
/// because the handle is not read again on this path and the slot is
/// overwritten whole.
pub fn ffi_guard_or<T, F>(fail: T, body: F) -> T
where
    F: FnOnce() -> T,
{
    clear_last_error();
    match std::panic::catch_unwind(AssertUnwindSafe(body)) {
        Ok(value) => value,
        Err(panic) => {
            set_last_error(panic_message(&panic));
            fail
        }
    }
}

fn panic_message(panic: &Box<dyn std::any::Any + Send>) -> String {
    if let Some(s) = panic.downcast_ref::<&'static str>() {
        return (*s).to_string();
    }
    if let Some(s) = panic.downcast_ref::<String>() {
        return s.clone();
    }
    "panic in FFI".to_string()
}

// ---------------------------------------------------------------------------
// Device handle — opaque to C callers, shared across crates.
// ---------------------------------------------------------------------------

/// Opaque handle wrapping an `Arc<dyn BlockDevice>`. Allocated by sister
/// crates' constructors and freed via [`fs_core_device_close`].
pub struct FsCoreDevice {
    inner: Arc<dyn BlockDevice>,
}

impl FsCoreDevice {
    /// Internal constructor — sister crates use this to wrap their own
    /// device types (Qcow2Reader, FileDevice, OwnedSlice, etc.) into the
    /// shared handle type. Returns a `Box::into_raw` pointer ready to hand
    /// across the FFI boundary.
    pub fn into_handle(inner: Arc<dyn BlockDevice>) -> *mut FsCoreDevice {
        Box::into_raw(Box::new(FsCoreDevice { inner }))
    }

    /// Borrow the inner device. `Arc::clone` it if you want shared
    /// ownership — e.g. when handing the device to a slice adapter while
    /// keeping the original handle alive.
    pub fn inner(&self) -> &Arc<dyn BlockDevice> {
        &self.inner
    }
}

/// Free a device handle. Safe to call with NULL (no-op).
#[unsafe(no_mangle)]
pub unsafe extern "C" fn fs_core_device_close(handle: *mut FsCoreDevice) {
    if handle.is_null() {
        return;
    }
    ffi_guard_or((), || unsafe {
        drop(Box::from_raw(handle));
    });
}

/// Total device size in bytes. Returns 0 if `handle` is NULL.
#[unsafe(no_mangle)]
pub unsafe extern "C" fn fs_core_device_size_bytes(handle: *const FsCoreDevice) -> u64 {
    if handle.is_null() {
        return 0;
    }
    ffi_guard_or(0, || unsafe { (*handle).inner.size_bytes() })
}

/// True if `write_at` is likely to succeed. Returns false on NULL.
#[unsafe(no_mangle)]
pub unsafe extern "C" fn fs_core_device_is_writable(handle: *const FsCoreDevice) -> bool {
    if handle.is_null() {
        return false;
    }
    ffi_guard_or(false, || unsafe { (*handle).inner.is_writable() })
}

/// Read exactly `len` bytes from `offset` into `buf`. `buf` must be at
/// least `len` bytes. Returns an `FsCoreErrorCode`.
#[unsafe(no_mangle)]
pub unsafe extern "C" fn fs_core_device_read_at(
    handle: *const FsCoreDevice,
    offset: u64,
    buf: *mut u8,
    len: usize,
) -> FsCoreErrorCode {
    // A null buffer is refused whatever the length. `from_raw_parts_mut`
    // requires a non-null, aligned pointer even for a zero-length slice,
    // so `(NULL, 0)` was undefined behaviour rather than the no-op it
    // looks like -- in a crate that otherwise denies
    // `unsafe_op_in_unsafe_fn`.
    if handle.is_null() || buf.is_null() {
        return FsCoreErrorCode::NullArg;
    }
    ffi_guard(|| {
        let slice_buf = unsafe { slice::from_raw_parts_mut(buf, len) };
        unsafe { (*handle).inner.read_at(offset, slice_buf) }
    })
}

/// Write exactly `len` bytes from `buf` to `offset`. Returns `ReadOnly`
/// for read-only devices.
#[unsafe(no_mangle)]
pub unsafe extern "C" fn fs_core_device_write_at(
    handle: *const FsCoreDevice,
    offset: u64,
    buf: *const u8,
    len: usize,
) -> FsCoreErrorCode {
    // Null is refused whatever the length; see `fs_core_device_read_at`.
    if handle.is_null() || buf.is_null() {
        return FsCoreErrorCode::NullArg;
    }
    ffi_guard(|| {
        let slice_buf = unsafe { slice::from_raw_parts(buf, len) };
        unsafe { (*handle).inner.write_at(offset, slice_buf) }
    })
}

/// Flush pending writes to stable storage.
#[unsafe(no_mangle)]
pub unsafe extern "C" fn fs_core_device_flush(handle: *const FsCoreDevice) -> FsCoreErrorCode {
    if handle.is_null() {
        return FsCoreErrorCode::NullArg;
    }
    ffi_guard(|| unsafe { (*handle).inner.flush() })
}

// ---------------------------------------------------------------------------
// Convenience: open a regular file as a device. Saves callers the trouble
// of building a Rust crate just to wrap `FileDevice`.
// ---------------------------------------------------------------------------

/// Open `path` (NUL-terminated UTF-8) as a `FileDevice` and return a
/// handle. Pass `writable=true` for RW. On failure returns NULL and the
/// thread-local last-error has detail.
#[unsafe(no_mangle)]
pub unsafe extern "C" fn fs_core_file_open(
    path: *const c_char,
    writable: bool,
) -> *mut FsCoreDevice {
    if path.is_null() {
        set_last_error("path is null");
        return ptr::null_mut();
    }
    ffi_guard_or(ptr::null_mut(), || {
        let cstr = unsafe { std::ffi::CStr::from_ptr(path) };
        let s = match cstr.to_str() {
            Ok(s) => s,
            Err(_) => {
                set_last_error("path is not valid UTF-8");
                return ptr::null_mut();
            }
        };
        let dev = if writable {
            crate::file_device::FileDevice::open_rw(s)
        } else {
            crate::file_device::FileDevice::open(s)
        };
        match dev {
            Ok(d) => FsCoreDevice::into_handle(Arc::new(d)),
            Err(e) => {
                set_last_error(e.to_string());
                ptr::null_mut()
            }
        }
    })
}

// ---------------------------------------------------------------------------
// Callback-backed device. Used when the caller already owns the underlying
// resource (FSKit FSBlockDeviceResource, Go file handle, C-side fd) and
// wants to expose it as an `FsCoreDevice` so it can be stacked under a
// container reader (qcow2, vhd, ...) before reaching a filesystem driver.
// ---------------------------------------------------------------------------

/// Read callback. Returns 0 on success, non-zero (errno-like) on failure.
/// Must fully fill `len` bytes — short reads are treated as I/O errors.
pub type FsCoreReadCb =
    Option<unsafe extern "C" fn(ctx: *mut c_void, offset: u64, buf: *mut u8, len: usize) -> c_int>;

/// Write callback. NULL → device is read-only.
pub type FsCoreWriteCb = Option<
    unsafe extern "C" fn(ctx: *mut c_void, offset: u64, buf: *const u8, len: usize) -> c_int,
>;

/// Flush/fsync callback. NULL → flush is a no-op.
pub type FsCoreFlushCb = Option<unsafe extern "C" fn(ctx: *mut c_void) -> c_int>;

/// Configuration passed to [`fs_core_device_from_callbacks`].
#[repr(C)]
pub struct FsCoreCallbackCfg {
    pub read: FsCoreReadCb,
    pub write: FsCoreWriteCb,
    pub flush: FsCoreFlushCb,
    pub ctx: *mut c_void,
    pub size: u64,
}

/// Turn a callback's non-zero return into an `io::Error`.
fn cb_io_err(rc: c_int, op: &str) -> io::Error {
    io::Error::other(format!("callback {op} returned {rc}"))
}

/// The host callback contract, in one place: **zero is success**.
///
/// All three adapters below wrapped a call in the same four lines —
/// invoke, compare against zero, `Ok(())` or `cb_io_err`. Three copies
/// of a convention is three chances to write `rc != 0` where the others
/// write `rc == 0`, and a caller would see reads succeed while writes
/// reported failure on the very same device.
///
/// `op` names the operation in the error, which is the only thing the
/// three genuinely differ in.
fn cb_result(rc: c_int, op: &'static str) -> io::Result<()> {
    if rc == 0 {
        Ok(())
    } else {
        Err(cb_io_err(rc, op))
    }
}

/// Build an [`FsCoreDevice`] backed by host-provided callbacks. Returns NULL
/// on failure (config null, read callback null, etc.) and stashes detail in
/// the thread-local last-error.
///
/// `cfg.ctx` is opaque to fs-core; it is passed back verbatim to every
/// callback invocation. The caller is responsible for ensuring it remains
/// valid until [`fs_core_device_close`] is called on the returned handle.
#[unsafe(no_mangle)]
pub unsafe extern "C" fn fs_core_device_from_callbacks(
    cfg: *const FsCoreCallbackCfg,
) -> *mut FsCoreDevice {
    if cfg.is_null() {
        set_last_error("cfg is null");
        return ptr::null_mut();
    }
    ffi_guard_or(ptr::null_mut(), || unsafe {
        let cfg = &*cfg;
        let read_fn = match cfg.read {
            Some(f) => f,
            None => {
                set_last_error("cfg.read is null");
                return ptr::null_mut();
            }
        };
        let write_fn = cfg.write;
        let flush_fn = cfg.flush;
        // `*mut c_void` is `!Send + !Sync` by default, and `unsafe impl
        // Send` on a newtype does not propagate cleanly through closure
        // auto-traits. Round-tripping the pointer through `usize` gives
        // something that is `Copy + Send + Sync`, and the callback
        // contract already puts the host on the hook for using `ctx`
        // safely across threads.
        let ctx_addr = cfg.ctx as usize;
        let size = cfg.size;

        let read_cb: crate::callback_device::ReadCb = Box::new(move |off, buf| {
            let ctx = ctx_addr as *mut c_void;
            cb_result(read_fn(ctx, off, buf.as_mut_ptr(), buf.len()), "read")
        });
        let write_cb: Option<crate::callback_device::WriteCb> = write_fn.map(|f| {
            Box::new(move |off, buf: &[u8]| {
                let ctx = ctx_addr as *mut c_void;
                cb_result(f(ctx, off, buf.as_ptr(), buf.len()), "write")
            }) as crate::callback_device::WriteCb
        });
        let flush_cb: Option<crate::callback_device::FlushCb> = flush_fn.map(|f| {
            Box::new(move || {
                let ctx = ctx_addr as *mut c_void;
                cb_result(f(ctx), "flush")
            }) as crate::callback_device::FlushCb
        });

        let dev = CallbackDevice {
            size,
            read: read_cb,
            write: write_cb,
            flush: flush_cb,
        };
        FsCoreDevice::into_handle(Arc::new(dev))
    })
}

// ---------------------------------------------------------------------------
// Slice constructor. Returns a child `FsCoreDevice` whose byte 0 maps to
// `start` of the parent and whose addressable range is `length` bytes.
// Useful for partition-table walkers that want to hand one partition to
// a filesystem driver without copying. The slice keeps an `Arc` to the
// parent, so closing the parent before the slice is fine.
// ---------------------------------------------------------------------------

/// Read-only slice. Writes via the returned handle return
/// `FS_CORE_READ_ONLY` regardless of the parent's writability.
#[unsafe(no_mangle)]
pub unsafe extern "C" fn fs_core_device_slice_ro(
    parent: *const FsCoreDevice,
    start: u64,
    length: u64,
) -> *mut FsCoreDevice {
    if parent.is_null() {
        set_last_error("parent is null");
        return ptr::null_mut();
    }
    ffi_guard_or(ptr::null_mut(), || unsafe {
        let parent_arc = (*parent).inner().clone();
        // OwnedSlice takes Arc<dyn BlockRead>; trait upcast from
        // BlockDevice -> BlockRead is supported in the pinned toolchain.
        let parent_read: Arc<dyn crate::block::BlockRead> = parent_arc;
        let slice = crate::slice::OwnedSlice::new(parent_read, start, length);
        FsCoreDevice::into_handle(Arc::new(slice))
    })
}

/// Read-write slice. Writes are forwarded to the parent at `start +
/// offset`; writes outside `[0, length)` return `FS_CORE_OUT_OF_BOUNDS`.
/// If the parent reports `is_writable() == false`, write attempts return
/// `FS_CORE_READ_ONLY`.
#[unsafe(no_mangle)]
pub unsafe extern "C" fn fs_core_device_slice_rw(
    parent: *const FsCoreDevice,
    start: u64,
    length: u64,
) -> *mut FsCoreDevice {
    if parent.is_null() {
        set_last_error("parent is null");
        return ptr::null_mut();
    }
    ffi_guard_or(ptr::null_mut(), || unsafe {
        let parent_arc = (*parent).inner().clone();
        let slice = crate::slice::OwnedRwSlice::new(parent_arc, start, length);
        FsCoreDevice::into_handle(Arc::new(slice))
    })
}

// ---------------------------------------------------------------------------
// Tests — exercise the FFI surface from Rust. The C side is verified by
// the consumer crates that use these functions through their own headers.
// ---------------------------------------------------------------------------

#[cfg(test)]
mod tests {
    use super::*;
    use std::fs::File;
    use std::io::Write;

    fn tmp_image(bytes: &[u8]) -> String {
        use std::sync::atomic::{AtomicU32, Ordering};
        static C: AtomicU32 = AtomicU32::new(0);
        let n = C.fetch_add(1, Ordering::Relaxed);
        let p = std::env::temp_dir()
            .join(format!("fs_core_ffi_{}_{n}.img", std::process::id()))
            .to_string_lossy()
            .into_owned();
        File::create(&p).unwrap().write_all(bytes).unwrap();
        p
    }

    #[test]
    fn open_read_close_round_trip() {
        let path = tmp_image(b"hello, fs-core ffi");
        let cpath = CString::new(path.as_str()).unwrap();
        let h = unsafe { fs_core_file_open(cpath.as_ptr(), false) };
        assert!(!h.is_null(), "open failed");

        unsafe {
            assert_eq!(fs_core_device_size_bytes(h), 18);
            assert!(!fs_core_device_is_writable(h));

            let mut buf = [0u8; 5];
            let rc = fs_core_device_read_at(h, 0, buf.as_mut_ptr(), buf.len());
            assert_eq!(rc, FsCoreErrorCode::Ok);
            assert_eq!(&buf, b"hello");

            // Write should fail with ReadOnly.
            let rc = fs_core_device_write_at(h, 0, b"x".as_ptr(), 1);
            assert_eq!(rc, FsCoreErrorCode::ReadOnly);

            fs_core_device_close(h);
        }
        let _ = std::fs::remove_file(&path);
    }

    #[test]
    fn null_args_return_null_arg() {
        let mut buf = [0u8; 4];
        let rc = unsafe { fs_core_device_read_at(ptr::null(), 0, buf.as_mut_ptr(), buf.len()) };
        assert_eq!(rc, FsCoreErrorCode::NullArg);
        let rc = unsafe { fs_core_device_flush(ptr::null()) };
        assert_eq!(rc, FsCoreErrorCode::NullArg);
    }

    #[test]
    fn last_error_populated_on_open_failure() {
        let cpath = CString::new("/path/that/does/not/exist/we/hope").unwrap();
        let h = unsafe { fs_core_file_open(cpath.as_ptr(), false) };
        assert!(h.is_null());
        let msg = fs_core_last_error_message();
        assert!(!msg.is_null());
        let s = unsafe { std::ffi::CStr::from_ptr(msg).to_string_lossy().into_owned() };
        assert!(!s.is_empty(), "expected an error message");
    }

    // ---- callback-backed device tests --------------------------------

    use std::sync::{Arc as StdArc, Mutex as StdMutex};

    struct CbState {
        data: Vec<u8>,
        flushed: u32,
    }

    /// Trampoline that pulls a `*mut CbState` out of the opaque ctx.
    unsafe extern "C" fn t_read(ctx: *mut c_void, offset: u64, buf: *mut u8, len: usize) -> c_int {
        let st = unsafe { &mut *(ctx as *mut CbState) };
        let off = offset as usize;
        if off + len > st.data.len() {
            return 5; // out of bounds
        }
        unsafe {
            std::ptr::copy_nonoverlapping(st.data.as_ptr().add(off), buf, len);
        }
        0
    }
    unsafe extern "C" fn t_write(
        ctx: *mut c_void,
        offset: u64,
        buf: *const u8,
        len: usize,
    ) -> c_int {
        let st = unsafe { &mut *(ctx as *mut CbState) };
        let off = offset as usize;
        if off + len > st.data.len() {
            return 5;
        }
        unsafe {
            std::ptr::copy_nonoverlapping(buf, st.data.as_mut_ptr().add(off), len);
        }
        0
    }
    unsafe extern "C" fn t_flush(ctx: *mut c_void) -> c_int {
        let st = unsafe { &mut *(ctx as *mut CbState) };
        st.flushed += 1;
        0
    }

    #[test]
    fn callback_device_round_trip_rw() {
        let mut st = Box::new(CbState {
            data: vec![0u8; 32],
            flushed: 0,
        });
        for (i, b) in st.data.iter_mut().enumerate() {
            *b = i as u8;
        }
        let ctx = &mut *st as *mut CbState as *mut c_void;

        let cfg = FsCoreCallbackCfg {
            read: Some(t_read),
            write: Some(t_write),
            flush: Some(t_flush),
            ctx,
            size: 32,
        };
        let h = unsafe { fs_core_device_from_callbacks(&cfg) };
        assert!(!h.is_null(), "device_from_callbacks returned NULL");

        unsafe {
            assert_eq!(fs_core_device_size_bytes(h), 32);
            assert!(fs_core_device_is_writable(h));

            let mut buf = [0u8; 4];
            let rc = fs_core_device_read_at(h, 4, buf.as_mut_ptr(), buf.len());
            assert_eq!(rc, FsCoreErrorCode::Ok);
            assert_eq!(buf, [4, 5, 6, 7]);

            let payload = [0xDE, 0xAD, 0xBE, 0xEF];
            let rc = fs_core_device_write_at(h, 8, payload.as_ptr(), payload.len());
            assert_eq!(rc, FsCoreErrorCode::Ok);

            let rc = fs_core_device_flush(h);
            assert_eq!(rc, FsCoreErrorCode::Ok);

            let mut readback = [0u8; 4];
            let rc = fs_core_device_read_at(h, 8, readback.as_mut_ptr(), readback.len());
            assert_eq!(rc, FsCoreErrorCode::Ok);
            assert_eq!(readback, payload);

            fs_core_device_close(h);
        }
        assert_eq!(st.flushed, 1);
        assert_eq!(&st.data[8..12], &[0xDE, 0xAD, 0xBE, 0xEF]);
    }

    #[test]
    fn callback_device_readonly_when_write_null() {
        let mut st = Box::new(CbState {
            data: vec![0xAAu8; 16],
            flushed: 0,
        });
        let ctx = &mut *st as *mut CbState as *mut c_void;
        let cfg = FsCoreCallbackCfg {
            read: Some(t_read),
            write: None,
            flush: None,
            ctx,
            size: 16,
        };
        let h = unsafe { fs_core_device_from_callbacks(&cfg) };
        assert!(!h.is_null());
        unsafe {
            assert!(!fs_core_device_is_writable(h));
            let rc = fs_core_device_write_at(h, 0, [1u8].as_ptr(), 1);
            assert_eq!(rc, FsCoreErrorCode::ReadOnly);
            // Flush is a no-op when callback is NULL.
            assert_eq!(fs_core_device_flush(h), FsCoreErrorCode::Ok);
            fs_core_device_close(h);
        }
        // suppress unused warning
        let _ = StdArc::new(StdMutex::new(0u8));
    }

    #[test]
    fn callback_device_null_cfg_returns_null() {
        let h = unsafe { fs_core_device_from_callbacks(ptr::null()) };
        assert!(h.is_null());
        let msg = fs_core_last_error_message();
        assert!(!msg.is_null());
    }
}

#[cfg(test)]
mod panic_message_tests {
    use super::*;
    use crate::block::{BlockDevice, BlockRead};

    /// A device whose every method panics.
    ///
    /// Not a hypothetical: a driver's `size_bytes` computes a geometry
    /// from on-disk fields, and an arithmetic overflow there panics.
    /// The FFI boundary is where that has to stop being a panic and
    /// start being a reportable error.
    struct Panicking;

    impl BlockRead for Panicking {
        fn read_at(&self, _offset: u64, _buf: &mut [u8]) -> Result<(), Error> {
            panic!("read_at exploded")
        }
        fn size_bytes(&self) -> u64 {
            panic!("size_bytes exploded")
        }
    }
    impl BlockDevice for Panicking {
        fn is_writable(&self) -> bool {
            panic!("is_writable exploded")
        }
    }

    fn handle() -> *mut FsCoreDevice {
        FsCoreDevice::into_handle(std::sync::Arc::new(Panicking))
    }

    fn last_error() -> Option<String> {
        let p = fs_core_last_error_message();
        if p.is_null() {
            return None;
        }
        Some(
            unsafe { std::ffi::CStr::from_ptr(p) }
                .to_string_lossy()
                .into_owned(),
        )
    }

    /// A panic caught at the boundary must leave a message behind.
    ///
    /// `fs_core_device_size_bytes` returns 0 on panic — and 0 is also
    /// what a legitimately empty device returns. Without a message the
    /// caller cannot tell "this device is empty" from "the driver
    /// exploded computing its size", which is the whole reason the
    /// thread-local error slot exists.
    #[test]
    fn a_panic_computing_the_size_is_reported_not_just_swallowed() {
        clear_last_error();
        let h = handle();
        let size = unsafe { fs_core_device_size_bytes(h) };
        assert_eq!(size, 0, "the fallback value is still returned");
        let msg = last_error().expect("a caught panic must leave a message");
        assert!(
            msg.contains("size_bytes exploded"),
            "the message should carry the panic's own text, got: {msg}"
        );
        unsafe { fs_core_device_close(h) };
    }

    /// Same for the writability probe, whose fallback is `false` — the
    /// answer a perfectly good read-only device gives.
    #[test]
    fn a_panic_probing_writability_is_reported() {
        clear_last_error();
        let h = handle();
        let writable = unsafe { fs_core_device_is_writable(h) };
        assert!(!writable, "the fallback value is still returned");
        assert!(
            last_error().is_some(),
            "a caught panic must leave a message"
        );
        unsafe { fs_core_device_close(h) };
    }

    /// A call that succeeds must not leave a stale message behind for
    /// the next one to pick up.
    #[test]
    fn a_successful_call_clears_the_previous_error() {
        let h = handle();
        let _ = unsafe { fs_core_device_size_bytes(h) };
        assert!(last_error().is_some(), "setup: an error is recorded");
        unsafe { fs_core_device_close(h) };

        struct Sixteen;
        impl BlockRead for Sixteen {
            fn read_at(&self, _offset: u64, _buf: &mut [u8]) -> Result<(), Error> {
                Ok(())
            }
            fn size_bytes(&self) -> u64 {
                16
            }
        }
        impl BlockDevice for Sixteen {}
        let h2 = FsCoreDevice::into_handle(std::sync::Arc::new(Sixteen));
        assert_eq!(unsafe { fs_core_device_size_bytes(h2) }, 16);
        assert!(
            last_error().is_none(),
            "a call that worked must not leave the previous panic's message in place"
        );
        unsafe { fs_core_device_close(h2) };
    }
}