Skip fuzz run after encountering the first non-determinism (#1262)

* fix fuzz run after first non-determinism

The problem is that now multiple function could be run. If one function displays non-deterministic behavior between Wasmi and the other oracle, following function calls can be influenced by this via global state, for example global variable state. Thus a run needs to stop immediately after encountering non-determinism.

* update comment

fuzz/fuzz_targets/differential.rs

@@ -38,7 +38,6 @@ fuzz_target!(|seed: &[u8]| {
     let exports = wasmi_oracle.exports();
     let mut params = Vec::new();
     // True as long as differential execution is deterministic between both oracles.
-    let mut deterministic = true;
     for (name, func_type) in exports.funcs() {
         params.clear();
         params.extend(
@@ -52,18 +51,17 @@ fuzz_target!(|seed: &[u8]| {
         let params = &params[..];
         let result_wasmi = wasmi_oracle.call(name, params);
         let result_oracle = chosen_oracle.call(name, params);
-        // Note: If either of the oracles returns a non-deterministic error we ignore it
-        //       to avoid having to deal with non-deterministic behavior between oracles.
+        // Note: If either of the oracles returns a non-deterministic error we skip the
+        //       entire fuzz run since following function executions could be affected by
+        //       this non-determinism due to shared global state, such as global variables.
         if let Err(wasmi_err) = &result_wasmi {
             if wasmi_err.is_non_deterministic() {
-                deterministic = false;
-                continue;
+                return;
             }
         }
         if let Err(oracle_err) = &result_oracle {
             if oracle_err.is_non_deterministic() {
-                deterministic = false;
-                continue;
+                return;
             }
         }
         let wasmi_name = wasmi_oracle.name();
@@ -129,12 +127,6 @@ fuzz_target!(|seed: &[u8]| {
             }
         }
     }
-    if !deterministic {
-        // We bail out and do not check global state since potential non-determinism
-        // has been detected previously which could have led to non-deterministic changes
-        // to Wasm global state.
-        return;
-    }
     for name in exports.globals() {
         let wasmi_val = wasmi_oracle.get_global(name);
         let oracle_val = chosen_oracle.get_global(name);