lib/c-api: fix env_finalizer being ran everytime WrapperEnv was dropped

We would like to run the environment finalizer callback only when the
last reference to env pointer is dropped, which is when the Arc<_>
reference count is one.

Closes #2871

lib/c-api/src/wasm_c_api/externals/function.rs

@@ -6,7 +6,7 @@ use super::CApiExternTag;
 use std::convert::TryInto;
 use std::ffi::c_void;
 use std::mem::MaybeUninit;
-use std::sync::Arc;
+use std::sync::{Arc, Mutex};
 use wasmer_api::{Function, RuntimeError, Val};
 
 #[derive(Debug, Clone)]
@@ -110,7 +110,7 @@ pub unsafe extern "C" fn wasm_func_new_with_env(
     #[repr(C)]
     struct WrapperEnv {
         env: *mut c_void,
-        env_finalizer: Arc<Option<wasm_env_finalizer_t>>,
+        env_finalizer: Arc<Mutex<Option<wasm_env_finalizer_t>>>,
     }
 
     impl wasmer_api::WasmerEnv for WrapperEnv {}
@@ -122,8 +122,12 @@ pub unsafe extern "C" fn wasm_func_new_with_env(
 
     impl Drop for WrapperEnv {
         fn drop(&mut self) {
-            if let Some(env_finalizer) = *self.env_finalizer {
-                unsafe { (env_finalizer)(self.env as _) }
+            if Arc::strong_count(&self.env_finalizer) == 1 {
+                if let Ok(mut guard) = self.env_finalizer.lock() {
+                    if let Some(env_finalizer) = guard.take() {
+                        unsafe { (env_finalizer)(self.env as _) };
+                    }
+                }
             }
         }
     }
@@ -166,7 +170,7 @@ pub unsafe extern "C" fn wasm_func_new_with_env(
         func_sig,
         WrapperEnv {
             env,
-            env_finalizer: Arc::new(env_finalizer),
+            env_finalizer: Arc::new(Mutex::new(env_finalizer)),
         },
         trampoline,
     );