port post write callback

[tabokie]

Compared to https://github.com/BusyJay/tirocks/pull/24, it uses C assertion for memory safety.

Signed-off-by: tabokie xy.tao@outlook.com

[BusyJay]

I think you can copy the definition from tirocks and imiplement it the same way.

[tabokie]

@BusyJay In this repo the C bindings are generated by hand. C++ bindings are too complicated to be done this way.

[BusyJay]

What matters is just the align and size. The definition you need are just:

#[repr(C)]
pub struct rocksdb_PostWriteCallback__bindgen_vtable(libc::c_void);
#[repr(C)]
pub struct rocksdb_PostWriteCallback {
    pub vtable_: *const rocksdb_PostWriteCallback__bindgen_vtable,
}
#[repr(C)]
pub struct SimplePostWriteCallback {
    pub _base: rocksdb_PostWriteCallback,
    pub state: *mut libc::c_void,
    pub cb: ::std::option::Option<
        unsafe extern "C" fn(arg1: *mut libc::c_void, arg2: u64),
    >,
}
pub type on_post_write_callback_cb = ::std::option::Option<
    unsafe extern "C" fn(arg1: *mut libc::c_void, arg2: u64),
>;
extern "C" {
    pub fn crocksdb_simple_post_write_callback_init(
        callback: *mut SimplePostWriteCallback,
        state: *mut libc::c_void,
        cb: on_post_write_callback_cb,
    );
}

[tabokie]

No, I don't trust anyone to review this code without looking at the generated version from tirocks.

[BusyJay]

The code is just copied from tirocks.

[tabokie]

Exactly. The idea of using generated code from an unrelated repo as the review basis is unheard of. Let alone there's the potential hazard of extending this code pattern towards other more complex virtual classes that no one can tell if it's strictly correct without using tirocks to generate them first.

[BusyJay]

I'm not suggesting change all the codes but only this struct. The motivation is to reduce the cost before we actually landing tirocks. If you can prove this won't hurt performance, I'm also OK with this implementation.

[tabokie]

Added a stack version that uses runtime checks for correctness. Microbench shows the heap version has a stable 70ns overhead, no overhead from the stack version.