Allow making direct RPC queries from the C API (#8588)

parity-clib/parity.h

@@ -86,6 +86,22 @@ int parity_start(const ParityParams* params, void** out);
 ///					must not call this function.
 void parity_destroy(void* parity);
 
+/// Performs an RPC request.
+///
+/// Blocks the current thread until the request is finished. You are therefore encouraged to spawn
+/// a new thread for each RPC request that requires accessing the blockchain.
+///
+/// - `rpc` and `len` must contain the JSON string representing the RPC request.
+/// - `out_str` and `out_len` point to a buffer where the output JSON result will be stored. If the
+///	  buffer is not large enough, the function fails.
+/// - `out_len` will receive the final length of the string.
+/// - On success, the function returns 0. On failure, it returns 1.
+///
+/// **Important**: Keep in mind that this function doesn't write any null terminator on the output
+///                string.
+///
+int parity_rpc(void* parity, const char* rpc, size_t len, char* out_str, size_t* out_len);
+
 #ifdef __cplusplus
 }
 #endif

parity-clib/src/lib.rs

@@ -23,6 +23,7 @@ use std::os::raw::{c_char, c_void, c_int};
 use std::panic;
 use std::ptr;
 use std::slice;
+use std::str;
 
 #[repr(C)]
 pub struct ParityParams {
@@ -131,3 +132,33 @@ pub extern fn parity_destroy(client: *mut c_void) {
 		});
 	}
 }
+
+#[no_mangle]
+pub extern fn parity_rpc(client: *mut c_void, query: *const char, len: usize, out_str: *mut c_char, out_len: *mut usize) -> c_int {
+	unsafe {
+		panic::catch_unwind(|| {
+			let client: &mut parity::RunningClient = &mut *(client as *mut parity::RunningClient);
+
+			let query_str = {
+				let string = slice::from_raw_parts(query as *const u8, len);
+				match str::from_utf8(string) {
+					Ok(a) => a,
+					Err(_) => return 1,
+				}
+			};
+
+			if let Some(output) = client.rpc_query_sync(query_str) {
+				let q_out_len = output.as_bytes().len();
+				if *out_len < q_out_len {
+					return 1;
+				}
+
+				ptr::copy_nonoverlapping(output.as_bytes().as_ptr(), out_str as *mut u8, q_out_len);
+				*out_len = q_out_len;
+				0
+			} else {
+				1
+			}
+		}).unwrap_or(1)
+	}
+}