[feature] #2161: generate FFI functions for data_model

[mversic]

Signed-off-by: Marin Veršić marin.versic101@gmail.com

Description of the Change

You can find a description here: #1973

This PR introduces an ffi crate which holds a macro ffi_bindgen. This macro when attached to the impl block generates the equivalent ffi out of the methods listed in the impl block. The macro can also be attached to the structure definition where it integrates with getset to make ffi fn equivalents of the getters/setters produced by the getset.

Note:

1. All FFI functions return FfiResult return code. The idea is that the returned code can be used to query the error
2. Considering the previous points, all FFI functions take, as a last argument, a mutable pointer where the return value of the corresponding method is stored (the so called out-pointer pattern)

Limitations:

1. mutable references are not supported for input arguments, except for the self argument, i.e. handle

Issue

Closes #2161

Benefits

Possible Drawbacks

Usage Examples or Tests [optional]

#[derive(Getters)]
#[getset(get = "pub")]
struct Struct {
    elem: u32,
    #[getset(skip)]
    elems: Vec<u32>,
}

#[ffi_bindgen]
impl Struct {
    pub fn new(elem: u32) -> Self {
        Self {elem, elems: vec![]}
    }
    pub fn get_elems(&self) -> impl ExactSizeIterator<&u32> {
        self.elems.iter()
    }
}

fn main() {
    let s = struct_new(42);
    let elems = MaybeUninit::<*const *const u32>::uninit();
    let elems_len = MaybeUninit::<usize>::uninit();
    if FfiResult::Ok != unsafe {struct_elems(&s, elems.as_mut_ptr(), elems_len.as_mut_ptr())} {
        // TODO: Handle error
    }
    let elems_len = unsafe {elems_len.assume_init()};
    let elems = unsafe {elems.assume_init()};
}

Alternate Designs [optional]

List out ffi functions manually. This is tedious and highly error prone because we're operating in the unsafe space

[mversic]

the three functions above are just temporary placeholders. In the future smart contract will talk to host only through this module but there will be more meat in these functions. Namely, they'll convert opaque pointers received from the smart contract into scale encoded byte slice before passing them to the host for execution

[mversic]

this is potentially UB, though I think it's not

[appetrosyan]

While I'd err on the side of caution, I think you're right in it not being intrinsically UB.

[appetrosyan]

If you really want to be pedantic, maybe add an assert, that checks for conversion errors.

[appetrosyan]

While I'd err on the side of caution, I think you're right in it not being intrinsically UB.

[appetrosyan]

If you really want to be pedantic, maybe add an assert, that checks for conversion errors.

[appetrosyan]

Needs to be synchronised to the docs repo.

[codecov]

Codecov Report

Merging #2164 (48a27a3) into iroha2-dev (ea4f5e3) will increase coverage by 20.46%.
The diff coverage is 57.64%.

❗ Current head 48a27a3 differs from pull request most recent head f8d1fce. Consider uploading reports for the commit f8d1fce to get more accurate results

@@               Coverage Diff               @@
##           iroha2-dev    #2164       +/-   ##
===============================================
+ Coverage            0   20.46%   +20.46%     
===============================================
  Files               0      187      +187     
  Lines               0    26973    +26973     
===============================================
+ Hits                0     5519     +5519     
- Misses              0    21454    +21454     

Impacted Files	Coverage Δ
cli/src/lib.rs	0.59% <0.00%> (ø)
client/examples/million_accounts_genesis.rs	0.00% <0.00%> (ø)
client/src/config.rs	69.35% <0.00%> (ø)
client/tests/integration/add_asset.rs	0.00% <0.00%> (ø)
client/tests/integration/asset_propagation.rs	0.00% <0.00%> (ø)
...lient/tests/integration/multiple_blocks_created.rs	0.00% <0.00%> (ø)
client/tests/integration/multisignature_account.rs	0.00% <0.00%> (ø)
...nt/tests/integration/multisignature_transaction.rs	0.00% <0.00%> (ø)
client/tests/integration/non_mintable.rs	0.00% <0.00%> (ø)
client/tests/integration/pagination.rs	0.00% <0.00%> (ø)
... and 212 more

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