fix(c-api) Trap's messages are always null terminated.

`wasm_trap_new` expects a `wasm_message_t`. It's a type alias to
`wasm_name_t` with the exception that it represents a null-terminated
string.

When calling `wasm_trap_new`, no check was present to ensure the
string was well-formed. That's a first issue. But in the best
scenario, the string was correctly formed and was
null-terminated. This string was transformed to a Rust `String` —with
the null byte!— and passed to `RuntimeError`.

Then in `wasm_trap_message`, another null byte was pushed at the end
of the message. It's been introduced in
https://github.com/wasmerio/wasmer/pull/1947. It results in a
doubly-null-terminated string, which is incorrect.

This patch does the following:

1. It checks that the string given to `wasm_trap_new` contains a
   null-terminated string or not, and will act accordingly. Note that
   it's possible to pass a non-null-terminated string, and it will
   still work because this detail is vicious. The idea is to get a
   well-formed `RuntimeError` in anycase.

  * If no null byte is found, the string is passed to `RuntimeError`
    as a valid Rust string,

  * If a null byte is found at the end of the string, a new string is
    passed to `RuntimeError` but without the final null byte,

  * If a null byte is found but not at the end, it's considered as an
    error,

  * If the string contains invalid UTF-8 bytes, it's considered as an
    error.

2. It updates `wasm_trap_message` to always add a null byte at the end
   of the returned owned string.

3. It adds test cases when passing a null-terminated or a
   non-null-terminated string to `wasm_trap_new` and to compare the
   results to `wasm_trap_message`.

lib/c-api/src/wasm_c_api/trap.rs

@@ -1,6 +1,6 @@
 use super::store::wasm_store_t;
 use super::types::{wasm_byte_vec_t, wasm_frame_t, wasm_frame_vec_t, wasm_message_t};
-use std::str;
+use std::ffi::CString;
 use wasmer::RuntimeError;
 
 // opaque type which is a `RuntimeError`
@@ -15,14 +15,39 @@ impl From<RuntimeError> for wasm_trap_t {
     }
 }
 
+/// Create a new trap message.
+///
+/// Be careful, the message is typed with `wasm_message_t` which
+/// represents a null-terminated string.
 #[no_mangle]
 pub unsafe extern "C" fn wasm_trap_new(
     _store: &mut wasm_store_t,
     message: &wasm_message_t,
 ) -> Option<Box<wasm_trap_t>> {
     let message_bytes = message.into_slice()?;
-    let message_str = c_try!(str::from_utf8(message_bytes));
-    let runtime_error = RuntimeError::new(message_str);
+
+    // The trap message is typed with `wasm_message_t` which is a
+    // typeref to `wasm_name_t` with the exception that it's a
+    // null-terminated string. `RuntimeError` must contain a valid
+    // Rust `String` that doesn't contain a null byte. We must ensure
+    // this behavior.
+    let runtime_error = match CString::new(message_bytes) {
+        // The string is well-formed and doesn't contain a nul byte.
+        Ok(cstring) => RuntimeError::new(cstring.into_string().ok()?),
+
+        // The string is well-formed but is nul-terminated. Let's
+        // create a `String` which is null-terminated too.
+        Err(nul_error) if nul_error.nul_position() + 1 == message_bytes.len() => {
+            let mut vec = nul_error.into_vec();
+            vec.pop();
+
+            RuntimeError::new(String::from_utf8(vec).ok()?)
+        }
+
+        // The string not well-formed.
+        Err(_) => return None,
+    };
+
     let trap = runtime_error.into();
 
     Some(Box::new(trap))
@@ -39,7 +64,8 @@ pub unsafe extern "C" fn wasm_trap_message(
 ) {
     let message = trap.inner.message();
     let mut byte_vec = message.into_bytes();
-    byte_vec.push(0); // append NUL
+    byte_vec.push(0);
+
     let byte_vec: wasm_byte_vec_t = byte_vec.into();
 
     out.size = byte_vec.size;
@@ -63,3 +89,70 @@ pub unsafe extern "C" fn wasm_trap_trace(
     out.size = frame_vec.size;
     out.data = frame_vec.data;
 }
+
+#[cfg(test)]
+mod tests {
+    use inline_c::assert_c;
+
+    #[test]
+    fn test_trap_message_null_terminated() {
+        (assert_c! {
+            #include "tests/wasmer.h"
+
+            int main() {
+                wasm_engine_t* engine = wasm_engine_new();
+                wasm_store_t* store = wasm_store_new(engine);
+
+                wasm_message_t original_message;
+                wasm_name_new_from_string_nt(&original_message, "foobar");
+                assert(original_message.size == 7); // 6 for `foobar` + 1 for nul byte.
+
+                wasm_trap_t* trap = wasm_trap_new(store, &original_message);
+                assert(trap);
+
+                wasm_message_t retrieved_message;
+                wasm_trap_message(trap, &retrieved_message);
+                assert(retrieved_message.size == 7);
+
+                wasm_name_delete(&original_message);
+                wasm_trap_delete(trap);
+                wasm_store_delete(store);
+                wasm_engine_delete(engine);
+
+                return 0;
+            }
+        })
+        .success();
+    }
+
+    #[test]
+    fn test_trap_message_not_null_terminated() {
+        (assert_c! {
+            #include "tests/wasmer.h"
+
+            int main() {
+                wasm_engine_t* engine = wasm_engine_new();
+                wasm_store_t* store = wasm_store_new(engine);
+
+                wasm_message_t original_message;
+                wasm_name_new_from_string(&original_message, "foobar");
+                assert(original_message.size == 6); // 6 for `foobar` + 0 for nul byte.
+
+                wasm_trap_t* trap = wasm_trap_new(store, &original_message);
+                assert(trap);
+
+                wasm_message_t retrieved_message;
+                wasm_trap_message(trap, &retrieved_message);
+                assert(retrieved_message.size == 7);
+
+                wasm_name_delete(&original_message);
+                wasm_trap_delete(trap);
+                wasm_store_delete(store);
+                wasm_engine_delete(engine);
+
+                return 0;
+            }
+        })
+        .success();
+    }
+}