Reduce boxing in callback structs

[yiwu-arbug]

RocksDB expose several APIs to allow client code to inject custom logic, such as compaction filter and table properties collector. rust-rocksdb usually implement such APIs in the follow way.

Assume rocksdb expose such a struct for callback

// C++
class Callback {
    virtual void SomeCallback() = 0;
}

rust-rocksdb usually provide the API in the following way:

// Rust
pub trait Callback {
    fn callback();
}

pub fn new_callback<C: Callback>(callback: C) -> *mut DBCallback {
    let boxed: Box<dyn Callback> = Box::new(callback) as _;
    unsafe {
        crocksdb_ffi::crocksdb_callback_create(
            Box::into_raw(Box::new(boxed)),
            callback_fn,
        )
    }
}

extern "C" fn callback_fn(ctx: *mut c_void) {
    let callback = &*(ctx as *mut Box<dyn Callback>);
    callback.callback();
}

Note here ctx is a Box<Box<dyn Callback>>. The inner box is to hold the trait object. The outer box is needed since C code cannot hold the raw pointer of Box<dyn Callback> natively since the pointer is of 128-bits. However, the extra boxing can be reduced by using generic.

pub fn new_callback<C: Callback>(callback: C) -> *mut DBCallback {
    let boxed: Box<C> = Box::new(callback);
    unsafe {
        crocksdb_ffi::crocksdb_callback_create(
            Box::into_raw(boxed),
            callback_fn::<C>,
        )
    }
}

extern "C" fn callback_fn<C: Callback>(ctx: *mut c_void) {
    let callback = &*(ctx as *mut C);
    callback.callback();
}

The following APIs need to be updated:

• CompactionFilter
• TablePropertiesCollector
• SliceTransform
• EventListener
• Logger
• TableFilter

[tabokie]

Reopen this, #663 missed the following types:

• CompationFilter, CompactionFilterFactory
• SliceTransform