Early exit in error callback from Wasm (#9)

* Early exit in error callback from Wasm

This avoids Wasm execution hanging due to problems such as wrong public
input.

Mimics circom_runtime behaviour with less detailed debug information.

See https://github.com/iden3/circom_runtime/blob/master/js/witness_calculator.js#L52-L64

Adds test for wrong public input. Without early exit, the test stalls.
With it, the Circom build step fails as expected.

* chore: clean up error handling

* ci: add caching

Co-authored-by: Georgios Konstantopoulos <me@gakonst.com>

src/witness/witness_calculator.rs

@@ -2,7 +2,7 @@ use color_eyre::Result;
 use num_bigint::BigInt;
 use num_traits::Zero;
 use std::cell::Cell;
-use wasmer::{imports, Function, Instance, Memory, MemoryType, Module, Store};
+use wasmer::{imports, Function, Instance, Memory, MemoryType, Module, RuntimeError, Store};
 
 use super::{fnv, SafeMemory, Wasm};
 
@@ -13,6 +13,12 @@ pub struct WitnessCalculator {
     pub n64: i32,
 }
 
+// Error type to signal end of execution.
+// From https://docs.wasmer.io/integrations/examples/exit-early
+#[derive(thiserror::Error, Debug, Clone, Copy)]
+#[error("{0}")]
+struct ExitCode(u32);
+
 impl WitnessCalculator {
     pub fn new(path: impl AsRef<std::path::Path>) -> Result<Self> {
         let store = Store::default();
@@ -144,7 +150,15 @@ mod runtime {
     pub fn error(store: &Store) -> Function {
         #[allow(unused)]
         #[allow(clippy::many_single_char_names)]
-        fn func(a: i32, b: i32, c: i32, d: i32, e: i32, f: i32) {}
+        fn func(a: i32, b: i32, c: i32, d: i32, e: i32, f: i32) {
+            // NOTE: We can also get more information why it is failing, see p2str etc here:
+            // https://github.com/iden3/circom_runtime/blob/master/js/witness_calculator.js#L52-L64
+            println!(
+                "runtime error, exiting early: {0} {1} {2} {3} {4} {5}",
+                a, b, c, d, e, f
+            );
+            RuntimeError::raise(Box::new(ExitCode(1)));
+        }
         Function::new_native(store, func)
     }