feat(rust): expose hedera_subscribe via ffi

sdk/c/include/hedera.h

@@ -235,6 +235,15 @@ enum HederaError hedera_schedule_id_from_string(const char *s, struct HederaSche
  */
 struct HederaSigner *hedera_signer_private_key(struct HederaPrivateKey *key);
 
+/**
+ * Subscribe with this request against the provided client of the Hedera network.
+ * On successful completion, calls `callback` with `ERROR_OK` and a `NULL` `message`.
+ */
+enum HederaError hedera_subscribe(const struct HederaClient *client,
+                                  const char *request,
+                                  const void *context,
+                                  void (*callback)(const void *context, enum HederaError err, const char *message));
+
 /**
  * Parse a Hedera `TokenId` from the passed string.
  */

sdk/rust/src/ffi/callback.rs

@@ -16,7 +16,7 @@ impl<T> Callback<T> {
         Self { handle, context }
     }
 
-    pub(super) fn call(self, err: Error, value: T) {
+    pub(super) fn call(&self, err: Error, value: T) {
         (self.handle)(self.context, err, value)
     }
 }

sdk/rust/src/ffi/error.rs

@@ -26,7 +26,7 @@ pub(crate) fn set_last_error(error: crate::Error) {
 }
 
 /// Represents any possible result from a fallible function in the Hedera SDK.
-#[derive(Debug)]
+#[derive(Debug, PartialEq, Eq, Copy, Clone)]
 #[repr(C)]
 pub enum Error {
     Ok,

sdk/rust/src/ffi/execute.rs

@@ -3,19 +3,13 @@ use std::ffi::{c_void, CString};
 use std::os::raw::c_char;
 use std::ptr::null;
 
-use once_cell::sync::Lazy;
-use tokio::runtime::{self, Runtime};
-
 use crate::ffi::callback::Callback;
 use crate::ffi::error::Error;
+use crate::ffi::runtime::RUNTIME;
 use crate::ffi::util::cstr_from_ptr;
 use crate::transaction::AnyTransaction;
 use crate::{AnyMirrorQuery, AnyQuery, Client};
 
-static RUNTIME: Lazy<Runtime> = Lazy::new(|| {
-    runtime::Builder::new_multi_thread().enable_all().max_blocking_threads(8).build().unwrap()
-});
-
 thread_local! {
     static EXECUTE_RESPONSE: RefCell<CString> = RefCell::new(CString::new("").unwrap());
 }

sdk/rust/src/ffi/mod.rs

@@ -9,8 +9,10 @@ mod execute;
 mod file_id;
 mod private_key;
 mod public_key;
+mod runtime;
 mod schedule_id;
 mod signer;
+mod subscribe;
 mod token_id;
 mod topic_id;
 mod util;

sdk/rust/src/ffi/runtime.rs

@@ -0,0 +1,7 @@
+use once_cell::sync::Lazy;
+use tokio::runtime;
+use tokio::runtime::Runtime;
+
+pub(super) static RUNTIME: Lazy<Runtime> = Lazy::new(|| {
+    runtime::Builder::new_multi_thread().enable_all().max_blocking_threads(8).build().unwrap()
+});

sdk/rust/src/ffi/subscribe.rs

@@ -0,0 +1,67 @@
+use std::cell::RefCell;
+use std::ffi::{c_void, CString};
+use std::os::raw::c_char;
+use std::ptr::null;
+
+use futures_util::StreamExt;
+
+use crate::ffi::callback::Callback;
+use crate::ffi::error::Error;
+use crate::ffi::runtime::RUNTIME;
+use crate::ffi::util::cstr_from_ptr;
+use crate::{AnyMirrorQuery, Client};
+
+thread_local! {
+    static SUBSCRIBE_MESSAGE: RefCell<CString> = RefCell::new(CString::new("").unwrap());
+}
+
+/// Subscribe with this request against the provided client of the Hedera network.
+/// On successful completion, calls `callback` with `ERROR_OK` and a `NULL` `message`.
+#[no_mangle]
+pub extern "C" fn hedera_subscribe(
+    client: *const Client,
+    request: *const c_char,
+    context: *const c_void,
+    callback: extern "C" fn(context: *const c_void, err: Error, message: *const c_char),
+) -> Error {
+    assert!(!client.is_null());
+
+    let client = unsafe { &*client };
+    let request = unsafe { cstr_from_ptr(request) };
+
+    let request: AnyMirrorQuery =
+        ffi_try!(serde_json::from_str(&request).map_err(crate::Error::request_parse));
+
+    let callback = Callback::new(context, callback);
+
+    RUNTIME.spawn(async move {
+        let mut stream = request.subscribe(client);
+
+        while let Some(message) = stream.next().await {
+            let message = message.map(|message| {
+                let message = serde_json::to_string(&message).unwrap();
+
+                SUBSCRIBE_MESSAGE.with(|message_text| {
+                    *message_text.borrow_mut() = CString::new(message).unwrap();
+
+                    message_text.borrow().as_ptr()
+                })
+            });
+
+            let (err, message) = match message {
+                Ok(message) => (Error::Ok, message),
+                Err(error) => (Error::new(error), null()),
+            };
+
+            callback.call(err, message);
+
+            if err != Error::Ok {
+                return;
+            }
+        }
+
+        callback.call(Error::Ok, null());
+    });
+
+    Error::Ok
+}