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//! Module for System V ABI unwind registry.

use crate::SendSyncPtr;
use anyhow::Result;
use std::ptr::{self, NonNull};
use std::sync::atomic::{AtomicUsize, Ordering::Relaxed};

/// Represents a registration of function unwind information for System V ABI.
pub struct UnwindRegistration {
    registrations: Vec<SendSyncPtr<u8>>,
}

extern "C" {
    // libunwind import
    fn __register_frame(fde: *const u8);
    fn __deregister_frame(fde: *const u8);
}

/// There are two primary unwinders on Unix platforms: libunwind and libgcc.
///
/// Unfortunately their interface to `__register_frame` is different. The
/// libunwind library takes a pointer to an individual FDE while libgcc takes a
/// null-terminated list of FDEs. This means we need to know what unwinder
/// is being used at runtime.
///
/// This detection is done currently by looking for a libunwind-specific symbol.
/// This specific symbol was somewhat recommended by LLVM's
/// "RTDyldMemoryManager.cpp" file which says:
///
/// > We use the presence of __unw_add_dynamic_fde to detect libunwind.
///
/// I'll note that there's also a different libunwind project at
/// https://www.nongnu.org/libunwind/ but that doesn't appear to have
/// `__register_frame` so I don't think that interacts with this.
fn using_libunwind() -> bool {
    static USING_LIBUNWIND: AtomicUsize = AtomicUsize::new(LIBUNWIND_UNKNOWN);

    const LIBUNWIND_UNKNOWN: usize = 0;
    const LIBUNWIND_YES: usize = 1;
    const LIBUNWIND_NO: usize = 2;

    // On macOS the libgcc interface is never used so libunwind is always used.
    if cfg!(target_os = "macos") {
        return true;
    }

    // On other platforms the unwinder can vary. Sometimes the unwinder is
    // selected at build time and sometimes it differs at build time and runtime
    // (or at least I think that's possible). Fall back to a `libc::dlsym` to
    // figure out what we're using and branch based on that.
    //
    // Note that the result of `libc::dlsym` is cached to only look this up
    // once.
    match USING_LIBUNWIND.load(Relaxed) {
        LIBUNWIND_YES => true,
        LIBUNWIND_NO => false,
        LIBUNWIND_UNKNOWN => {
            let looks_like_libunwind = unsafe {
                !libc::dlsym(ptr::null_mut(), "__unw_add_dynamic_fde\0".as_ptr().cast()).is_null()
            };
            USING_LIBUNWIND.store(
                if looks_like_libunwind {
                    LIBUNWIND_YES
                } else {
                    LIBUNWIND_NO
                },
                Relaxed,
            );
            looks_like_libunwind
        }
        _ => unreachable!(),
    }
}

impl UnwindRegistration {
    #[allow(missing_docs)]
    pub const SECTION_NAME: &'static str = ".eh_frame";

    /// Registers precompiled unwinding information with the system.
    ///
    /// The `_base_address` field is ignored here (only used on other
    /// platforms), but the `unwind_info` and `unwind_len` parameters should
    /// describe an in-memory representation of a `.eh_frame` section. This is
    /// typically arranged for by the `wasmtime-obj` crate.
    pub unsafe fn new(
        _base_address: *const u8,
        unwind_info: *const u8,
        unwind_len: usize,
    ) -> Result<UnwindRegistration> {
        debug_assert_eq!(
            unwind_info as usize % crate::page_size(),
            0,
            "The unwind info must always be aligned to a page"
        );

        let mut registrations = Vec::new();
        if using_libunwind() {
            // For libunwind, `__register_frame` takes a pointer to a single
            // FDE. Note that we subtract 4 from the length of unwind info since
            // wasmtime-encode .eh_frame sections always have a trailing 32-bit
            // zero for the platforms above.
            let start = unwind_info;
            let end = start.add(unwind_len - 4);
            let mut current = start;

            // Walk all of the entries in the frame table and register them
            while current < end {
                let len = current.cast::<u32>().read_unaligned() as usize;

                // Skip over the CIE
                if current != start {
                    __register_frame(current);
                    let cur = NonNull::new(current.cast_mut()).unwrap();
                    registrations.push(SendSyncPtr::new(cur));
                }

                // Move to the next table entry (+4 because the length itself is
                // not inclusive)
                current = current.add(len + 4);
            }
        } else {
            // On gnu (libgcc), `__register_frame` will walk the FDEs until an
            // entry of length 0
            __register_frame(unwind_info);
            let info = NonNull::new(unwind_info.cast_mut()).unwrap();
            registrations.push(SendSyncPtr::new(info));
        }

        Ok(UnwindRegistration { registrations })
    }
}

impl Drop for UnwindRegistration {
    fn drop(&mut self) {
        unsafe {
            // libgcc stores the frame entries as a linked list in decreasing
            // sort order based on the PC value of the registered entry.
            //
            // As we store the registrations in increasing order, it would be
            // O(N^2) to deregister in that order.
            //
            // To ensure that we just pop off the first element in the list upon
            // every deregistration, walk our list of registrations backwards.
            for fde in self.registrations.iter().rev() {
                __deregister_frame(fde.as_ptr());
            }
        }
    }
}