Add inspect message

xtc/Cargo.toml

@@ -9,6 +9,7 @@ xtc-history = {path="../xtc-history/xtc-history"}
 xtc-history-common = {path= "../xtc-history/xtc-history-common" }
 serde_bytes = "0.11"
 ic-kit = "0.4.2"
+ic-kit-sys = "0.1.1"
 ic-cdk = "0.3.1"
 serde = { version="1.0.130", features = ["derive"] }
 derive_builder = "0.10.2"

xtc/src/lib.rs

@@ -14,6 +14,50 @@ mod utils;
 #[cfg(test)]
 mod tests;
 
+// TODO(qti3e): Fix this mess when kit v0.5 is out. there shouldn't be any unsafe code.
+use crate::fee::compute_fee;
+use crate::ledger::Ledger;
+use ic_kit::ic;
+use ic_kit::macros::inspect_message;
+use ic_kit_sys::ic0;
+
+#[inspect_message]
+fn inspect_message() {
+    let message_size = unsafe { ic0::msg_arg_data_size() as usize };
+
+    // based on our candid the largest fixed size payload we can accept is only 2 principals
+    // encode(opt principal, opt principal) -> 83 bytes
+    // encode(opt principal, opt principal, opt principal) -> 119 bytes
+    // the only unsized type used is our good friend `nat`, which is dynamically sized
+    // but supposing a u128 is only legal, it is 16 bytes, for nat.
+    // (nat, nat) -> ~40bytes
+    // here we allow an error margin, and reject messages larger than 500bytes.
+    if message_size <= 500 {
+        ic_cdk::api::call::accept_message();
+        return;
+    }
+
+    let method_name = ic_cdk::api::call::method_name();
+
+    // Exception: wallet_call should actually accept large payload, but only if user has
+    // the fee to pay.
+    if method_name == "wallet_call" {
+        // The fee is currently constant, but we assume the call is gonna spend 100B cycles.
+        let fee = compute_fee(100_000_000_000);
+        let ledger = ic::get_mut::<Ledger>();
+        let caller = ic_cdk::caller();
+        let balance = ledger.balance(&caller);
+
+        // Accept the message if the user does have the balance to pay the fee.
+        if balance > fee {
+            ic_cdk::api::call::accept_message();
+            return;
+        }
+    }
+}
+
+// inspect message mess ->
+
 /// Perform only one pending async task, returns whether an async call was performed
 /// as the result of calling this method or not.
 /// This method should only be called from updates.