fix: remove the unsound atexit cleanup (eclipse-zenoh#1015)

* fix: remove the unsound atexit cleanup

* fix: refine the unsafe function

ci/valgrind-check/src/pub_sub/bin/z_pub_sub.rs

@@ -17,6 +17,7 @@ use zenoh::prelude::r#async::*;
 
 #[tokio::main]
 async fn main() {
+    let _z = zenoh_runtime::ZRuntimePoolGuard;
     zenoh_util::init_log_test();
 
     let pub_key_expr = KeyExpr::try_from("test/valgrind/data").unwrap();

ci/valgrind-check/src/queryable_get/bin/z_queryable_get.rs

@@ -18,6 +18,7 @@ use zenoh::prelude::r#async::*;
 
 #[tokio::main]
 async fn main() {
+    let _z = zenoh_runtime::ZRuntimePoolGuard;
     zenoh_util::init_log_test();
 
     let queryable_key_expr = KeyExpr::try_from("test/valgrind/data").unwrap();

commons/zenoh-runtime/src/lib.rs

@@ -145,24 +145,24 @@ lazy_static! {
         .collect();
 }
 
-// To drop the data mannually since Rust does not drop static variables.
-pub extern "C" fn cleanup() {
-    unsafe {
-        std::mem::drop((ZRUNTIME_POOL.deref() as *const ZRuntimePool).read());
-        std::mem::drop((ZRUNTIME_INDEX.deref() as *const HashMap<ZRuntime, AtomicUsize>).read());
+// A runtime guard used to explicitly drop the static variables that Rust doesn't drop by default
+pub struct ZRuntimePoolGuard;
+
+impl Drop for ZRuntimePoolGuard {
+    fn drop(&mut self) {
+        unsafe {
+            std::mem::drop((ZRUNTIME_POOL.deref() as *const ZRuntimePool).read());
+            std::mem::drop(
+                (ZRUNTIME_INDEX.deref() as *const HashMap<ZRuntime, AtomicUsize>).read(),
+            );
+        }
     }
 }
 
 pub struct ZRuntimePool(HashMap<ZRuntime, OnceLock<Runtime>>);
 
 impl ZRuntimePool {
     fn new() -> Self {
-        // It has been recognized that using atexit within Windows DLL is problematic
-        #[cfg(not(target_os = "windows"))]
-        // Register a callback to clean the static variables.
-        unsafe {
-            libc::atexit(cleanup);
-        }
         Self(ZRuntime::iter().map(|zrt| (zrt, OnceLock::new())).collect())
     }