finding 2: use JS-provided throw_or_abort_impl for WASM error recovery

In WASM builds, BB_NO_EXCEPTIONS is defined. The C++ definition of
throw_or_abort_impl calls std::abort(), killing the WASM process on
any error. Meanwhile, the header declares it as WASM_IMPORT from JS,
where it throws a catchable Error. But the local C++ definition
shadows the import.

Fix: guard out the C++ definition with #ifndef __wasm__ so the JS
import is used in WASM builds. For wasmtime (non-JS WASM runtime
used in CI tests), add -Wunknown-imports-trap=y so the import is
stubbed with a trap (correct behavior: errors in wasmtime should
still abort).

Before: any throw_or_abort in WASM kills the process, PXE crashes.
After: the JS caller gets a catchable Error("actual error message").

Author: johnathan79717

barretenberg/cpp/scripts/wasmtime.sh

@@ -5,6 +5,7 @@ set -eu
 export WASMTIME_BACKTRACE_DETAILS=1
 exec wasmtime run \
   -Wthreads=y \
+  -Wunknown-imports-trap=y \
   -Sthreads=y \
   ${HARDWARE_CONCURRENCY:+--env HARDWARE_CONCURRENCY} \
   --env HOME \

barretenberg/cpp/src/barretenberg/env/throw_or_abort_impl.cpp

@@ -1,3 +1,14 @@
+// In WASM builds, throw_or_abort_impl is provided by the JavaScript environment
+// (declared as WASM_IMPORT in throw_or_abort_impl.hpp). The JS implementation
+// throws a catchable JS Error with the error message.
+//
+// For non-JS WASM runtimes (wasmtime), the import must also be provided.
+// See barretenberg/cpp/scripts/wasmtime.sh for the wasmtime configuration.
+//
+// We guard out the C++ definition so the JS import is used instead of being
+// shadowed by the local definition (which would call std::abort).
+#ifndef __wasm__
+
 #include "barretenberg/common/log.hpp"
 #include "barretenberg/common/wasm_export.hpp"
 #include <stdexcept>
@@ -27,3 +38,5 @@ WASM_EXPORT void throw_or_abort_impl [[noreturn]] (const char* err)
     abort_with_message(err);
 #endif
 }
+
+#endif // !__wasm__