Mounted binaries

Due to the inherent limitations of Wasm modules, such as a lack of sockets, it may be necessary for a module to interact with its host to bridge such gaps, e.g. use a https transport provider like curl. In order for a Wasm module to use a host's curl capabilities, we need to provide access to the binary, which at the code level is achieved through the Rust extern block:

rust
use marine_rs_sdk::marine;
use marine_rs_sdk::MountedBinaryResult;
pub fn main() {}
#[marine]
pub fn curl_request(curl_cmd: Vec<String>) -> MountedBinaryResult {
    let response = curl(curl_cmd);
    response
}
#[marine]
#[host_import]
extern "C" {
    fn curl(cmd: Vec<String>) -> MountedBinaryResult;
}

rust
use marine_rs_sdk::marine;
use marine_rs_sdk::MountedBinaryResult;
pub fn main() {}
#[marine]
pub fn curl_request(curl_cmd: Vec<String>) -> MountedBinaryResult {
    let response = curl(curl_cmd);
    response
}
#[marine]
#[host_import]
extern "C" {
    fn curl(cmd: Vec<String>) -> MountedBinaryResult;
}

Note that to import functions from a host, not from other Wasm modules, the extern block must be marked by #[host_import] macro.

The code above creates a "curl adapter", i.e., a Wasm module that allows other Wasm modules to use the the curl_request function, which calls the imported curl binary in this case, to make http calls. Please note that we are wrapping the extern block with the #[marine] macro and introduce a Marine-native data structure MountedBinaryResult as the linked-function return value.

To play with it in the Marine REPL you need to specify a path to curl on your local system, it is described in more details here.